Secondary Organic Aerosol Formation over Coastal Ocean: Inferences

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Secondary organic aerosol formation over Coastal Ocean: Inferences from atmospheric water-soluble low molecular weight organic compounds Srinivas Bikkina, Kimitaka Kawamura, and Manmohan Sarin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05986 • Publication Date (Web): 29 Mar 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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Environmental Science & Technology

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Secondary organic aerosol formation over Coastal Ocean: Inferences from atmospheric

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water-soluble low molecular weight organic compounds

3 4 5 6 7 8 9 Srinivas Bikkina1,3,4, Kimitaka Kawamura1,2*, and Manmohan Sarin4

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Institute of Low Temperature Science, Hokkaido University, Sapporo, 060-0819, Japan

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Now at Chubu Institute for Advanced Studies, Chubu University, Kasugai, 487-8501, Japan 3

Now at Dept. of Environmental Sciences and Analytical Chemistry, Stockholm University,

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10691, Stockholm, Sweden 4

Physical Research Laboratory, Navrangpura, Ahmedabad – 380 009, India

16 17 18 19 20 21 22 23 24 25 26 27 28 29

*Corresponding author Prof. Kimitaka Kawamura E-mail: [email protected]

ACS Paragon Plus Environment


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Art figure

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ABSTRACT

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A lack of consensus on the distributions and formation pathways of secondary

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organic aerosols (SOA) oceanic regions downwind of pollution sources limits our ability to

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assess their climate impact globally. As a case study, we report here on water-soluble SOA

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components such as dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls in the

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continental outflows from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA) to the Bay

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of Bengal. Oxalic acid (C2) is the dominant species followed by succinic (C4) and glyoxylic

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acids (ωC2) in the outflow. Non-sea-salt SO42- also dominates (~70%) total water-soluble

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inorganic constituents and correlates well with aerosol liquid water content (LWC) and C2,

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indicating their production through aqueous phase photochemical reactions. Furthermore,

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mass ratios of dicarboxylic acids (C2/C4, C2/ωC2), and their relative abundances in water-

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soluble organic carbon and total organic carbon are quite similar between the two continental

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(IGP and SEA) outflows, indicating the formation of SOA through aqueous phase

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photochemical reactions in LWC-enriched aerosols, largely controlled by anthropogenic SO42-

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.

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INTRODUCTION

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The ubiquitous presence of fine mode organic aerosols (OAs) and their dominant

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role via direct (scattering and absorption of incoming/outgoing solar radiation) and indirect

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effects (aerosol-cloud interactions) are increasingly recognized in climate forcing. It is thus

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important to study their formation and distribution in the ambient atmosphere1-3. Although

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chemical characterization of OAs over continental sites have been comparatively well studied,

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sparse observations from the marine regions limit our understanding on their sources,

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transport and transformation pathways1. Over the oceanic regions, both primary (e.g., oceanic

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emissions and continentally transported) and secondary sources (i.e., gas-to-particle

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conversion processes) contribute significantly to atmospheric OAs.

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Among the large spectrum of OAs, water-soluble and oxygenated organic

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compounds have gained a considerable scientific interest due to the in-cloud formation

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process and consequent climate effects. Water-soluble organic compounds as a pivotal

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component of OAs provide crucial insights regarding the photochemical aging processes.

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Away from the emission sources, organic compounds can be secondarily formed in the

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atmosphere, as referred by secondary organic aerosols (SOAs), most of which are water-

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soluble4, through photochemical processes involving anthropogenic/biogenic precursor

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volatile organic compounds (VOCs) and oxidants. Indeed, SOAs account for a significant

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fraction of submicron particles during long-range transport5-7. SOAs are produced in the

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atmosphere by the photochemical oxidation of semi-volatile organic precursors with oxidants

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such as ozone and hydroxyl or nitrate radicals. However, global models often underestimate

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the SOA mass, mainly due to the incorporation of photochemical formation only through gas

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phase processes8, 9.

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Growing consensus from the laboratory and field studies highlight the significance of

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SOA formation via the aqueous phase photochemical oxidation of anthropogenic and/or

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biogenic water-soluble semi volatile organic compounds in clouds or LWC-enriched aerosols

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4, 10-15

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lacking from field studies particularly in polluted continental outflows (e.g., South and

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Southeast Asian outflow to the Indian Ocean). Here, we present a case study on SOA

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formation and its controlling factors in the Atmospheric Brown Clouds (aka ABCs), a haze

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pollution that usually develops over the northern India and spreads downwards over the

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Indian Ocean during the late northeast-monsoon (January-April)16. Because our winter cruise

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in the Bay of Bengal (eastern rim of the tropical Indian Ocean) overlaps with the time of

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occurrence of ABCs, this study will provide some information on SOA formation and

. However, such detailed insights into SOA formation over the marine regions are


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governing processes occurring in the marine atmospheric boundary layer (MABL). More

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importantly, aqueous phase reactions in clouds (mimic dilute solutions) are somewhat

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different from those in LWC-enriched aerosols (resemble highly concentrated solutions).

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However, similar mechanisms may operate in aqueous aerosols (e.g., proposed scheme of

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aqueous phase oxidation of α-dicarbonyls in cloud/fog waters may also work for wet

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aerosols4, 17-20). In this study, we have investigated the interrelationship between water-soluble

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organic compounds (i.e., dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls; widely

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studied SOAs and their precursors) and other chemical composition data in marine aerosols to

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understand their various formation pathways through aqueous phase chemistry.

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MATERIALS AND METHODS

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Sampling location and Meteorology

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The Bay of Bengal (BoB), a semi-closed basin, located in the north-west Indian

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Ocean, influenced by the continental outflows from the Indo-Gangetic Plain (IGP) and

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Southeast Asia (SEA) during late northeast-monsoon (January-April)21,

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strength of continental impact decreases from winter to spring intermonsoon (March-April) 22,

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23, 24

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contribution of marine VOCs to atmospheric OAs during the study period is expected to be

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insignificant. However, this study investigates the relative importance of atmospheric water-

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soluble organic compounds from the ocean surface vis-à-vis continental outflow.

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Collection of PM2.5 and supporting chemical parameters

22

. Moreover, the

. The BoB is often considered as low nutrient low chlorophyll region25 and, hence,

Ambient aerosols, PM2.5 (N = 31), were collected on board ORV Sagar Kanya during

116 th

December 2008 - 26th January 2009 along the cruise track depicted in Figure S1.

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27

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Relevant details of sampling protocol and chemical composition data on water-soluble

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inorganic constituents (WSIC: Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3- and SO42-) and

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carbonaceous components (elemental carbon: EC, organic carbon: OC, and water-soluble

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organic carbon: WSOC) are provided in the supplementary information as well as in our

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earlier publications21-23.

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Quantification of dicarboxylic acids and related compounds

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To measure water-soluble dicarboxylic acids (DCAs), ω-oxocarboxylic acids (or

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oxoacids) and α-dicarbonyls in PM2.5, we used the analytical protocol described by

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Kawamura and Ikushima26 and later modified for better recovery27. An aliquot of quartz filter

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was extracted with Milli-Q water (specific resistivity >18.2 MΩ cm) by ultrasonic agitation (3



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x 10 ml). The aqueous extract was then passed through a precleaned Pasteur pipette packed

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with quartz wool in order to remove the filter fibres and particles. Subsequently, pH of the

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water-extract was adjusted to 8.5–9.0 by adding 0.05 M potassium hydroxide solution. This

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step essentially minimizes the evaporation of low molecular weight DCAs such as oxalic

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acid27. Thereafter, the water-extract was concentrated and derivatized with 14% BF3 in n-

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butanol at 100ºC for 1 hour. This process results in the derivatization of carboxyl groups to

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dibutyl esters whereas keto and aldehyde groups to dibutoxy acetals. After derivatization,

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products were extracted with n-hexane and determined using a capillary gas chromatograph

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(GC) equipped with a split/splitless injector, fused silica capillary column (HP-5, 0.2 mm I.D.

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x 25 m long x 0.5 µm film thickness) and flame ionization detector (FID). The concentrations

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of DCAs, oxoacids and α-dicarbonyls were determined with authentic standard solution

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containing dibutyl esters of oxalic, malonic, succinic, adipic, glutaric, phthalic, and azelaic

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acids. In addition, peak identification was confirmed by comparing the mass spectra obtained

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by GC-mass spectrometer.

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To check the extraction efficiency, known amounts of DCAs were spiked on the blank

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filter and extracted, derivatized and analyzed by GC-FID in a way similar to real aerosol

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samples as described above. The recoveries of all DCAs measured in this study are better than

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85 % except for oxalic acid, which showed a modest recovery of 80 %. The overall analytical

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uncertainty in the DCA measurement is within 10% based on the repeat analyses of replicate

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samples. Statistical analyses were also carried out using two-tailed t-test, wherever necessary

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comparisons were made.

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Air mass back trajectories, fire count data, aerosol liquid water-content

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To examine the contribution of OA from various geographical sources to the PM2.5

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over the BoB, we computed seven day isentropic air mass back trajectories (AMBTs) using

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Hybrid Single Particle Lagrangian integrated trajectory model (version 428). We used the

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archived

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Prediction/National Oceanic Aeronautic Administration (NCEP/NOAA) as input files in

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HYSPLIT. AMBTs were computed at an arrival height of 100 m for all sampling days at the

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midpoint of cruise track for each PM2.5 collected onboard ORV Sagar Kanya (Figure S1;

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obtained from Bikkina et al.29). From the AMBT analyses, we ascertained that PM2.5 over the

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North BoB (27th December, 2008 – 10th January 2009) were influenced by atmospheric

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outflow from the IGP (hereafter referred as the IGP-outflow21). Alternatively, PM2.5 over the

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South BoB (11–26 January 2009) were associated with AMBTs from SEA (SEA-outflow21).

meteorological

data

sets

from

National

Center


for

Environmental

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We have also examined the MODIS fire count data from South and South-east Asia for the

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sampling days to assess the possible contribution of biomass burning emissions (Figure S1).

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To ascertain the significance of aqueous phase reactions in the formation of DCAs and

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related organic compounds, we estimated the aerosol liquid water content (LWC) using a

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thermodynamic equilibrium model, ISORROPIA-II30; http://nenes.eas.gatech.edu/ISORROPIA/.

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We used the concentration data of water-soluble inorganic ions and meteorological

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parameters (relative humidity and ambient temperature) as input parameters for assessing the

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aerosol LWC in ISORROPIA-II (as a reverse problem).

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RESULTS AND DISCUSSIONS

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Mass Concentrations

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Over the Indo-Gangetic Plain, the radiocarbon source apportionment of WSOC and

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total carbon (TC) revealed a significant contribution (~80%) from contemporary biomass

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carbon31, 32 (i.e., crop-residue/wood burning or biogenic emissions contribute to VOCs33, 34).

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Additionally, fossil-fuel combustion (vehicular emissions and coal fired thermal power plant

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emissions) is a significant source of various organic compounds (e.g., benzene, toluene,

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ethylbenzene and xylene or BTEX compounds)35-37 in the IGP-outflow. Alternately, the

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composition of OAs over the South BoB is influenced by contribution from forest fires in

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South-east Asia38. Growing evidences from the IGP based on OC/EC and WSOC/OC ratios,

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molecular markers and chemical mass balance (CMB) technique suggested that SOAs from

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both biomass burning and fossil fuel sources contribute to WSOC in winter period24, 39, 40;

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SOA contribution to OC increases with distance from the north-western IGP to central IGP.

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Such processes would become more important if mixing of biogenic VOCs with

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anthropogenic pollutants could enhance the SOA formation8, 41. However, it is difficult to

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ascertain what fraction of these anthropogenic/biogenic VOCs contributes to SOA formation

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in the ambient atmosphere given the multiple reaction pathways with myriad formation of

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different organic compounds.

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All these emissions from the IGP- and SEA-outflows contribute to atmospheric OAs

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over the BoB32, 38, 42. This is evident from large temporal variability in the concentrations of

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DCAs (aliphatic, branched chain, unsaturated, and multifunctional), oxocarboxylic acids and

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α-dicarbonyls over the BoB during January 2009 (Figure S2 and Table 1). These temporal

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variations are also consistent with those of major anthropogenic water-soluble inorganic

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species such as non-sea-salt (nss) SO42- 21, NO3- and NH4+ 22 as well as OC, EC and WSOC23.

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The two prominent peaks in Figure S2 refer to the occurrence of high concentrations of



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measured water-soluble organic compounds in the IGP-outflow during 29 December 2008 – 3

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January 2009 and 5 – 9 January 2009, which can be attributed to proximity of these sampling

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cruise tracks to pollution sources in South Asia. However, significantly lower concentrations

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of DCAs in the SEA-outflow are due to farther distance from the continental sources (less

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anthropogenic influence) in the southern BoB.

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Effect of aerosol LWC – aqueous phase SOA formation

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On a global scale, the mass of aerosol liquid water content (LWC) is 2-3 times higher

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than the ambient dry particulate mass concentration of hygroscopic particles (i.e., total mass

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of NH4+, SO42-, and NO3-)43-45. However, climate models often ignore considering this

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parameter and its immense potential to influence the partitioning of gaseous organic

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precursors to form SOAs through aqueous phase photochemical reactions, consequently

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leading to large uncertainties in assessing the climate and health effects of OAs. Aerosol

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LWC is a key parameter in controlling the aqueous phase formation of most abundant oxalic

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acid during long range transport46,

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partitioning of water-soluble semi-volatile organic compounds (e.g., α-dicarbonyls) and thus

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significantly contributes to SOA formation48. Recent studies have documented the

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significance of anthropogenic acidic inorganic species (e.g., SO42-, NO3-) in controlling

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aerosol LWC and governing the transfer of water-soluble gaseous organic precursors to

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deliquescent aerosol particle surface and/or to cloud water49. For instance, a recent modelling

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study highlighted the importance of anthropogenic SO42- over the south-eastern USA directly

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related to aerosol LWC50. Likewise, study from Po Valley, Italy emphasized the role of

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pollution source-derived NO3- in controlling the aerosol LWC48. Therefore, we have estimated

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aerosol LWC in PM2.5 over the BoB.

47

. Furthermore, aerosol LWC greatly influences the

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The aerosol-LWC varied in a range of 0.12 – 7.0 x 10-5 g m-3 (av. 1.7 x 10-5 g m-3) in

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the IGP-outflow and 0.37 – 3.5 x 10-5 g m-3 (1.5 x 10-5 g m-3) in the SEA-outflow.

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Furthermore, we observe a significant linear relationship of aerosol LWC with nss-SO42-

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(Figure 1a) as well as total anthropogenic water-soluble inorganic mass (WSIManth = nss-K+ +

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NH4+ + nss-SO42- + NO3-; Figure S3). However, there is no significant difference in the slope

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of linear regression lines between IGP- and SEA-outflows. Likewise, a significant linear

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relationship is noteworthy between molar abundances of nss-SO42- and oxalic acid (Figure 1b)

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for the IGP-outflow (slope = 0.021; R2 = 0.64; p < 0.05) as well as for the SEA-outflow (slope

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= 0.019; R2 = 0.69; p < 0.05). Based on the large number of observations from East Asia, Yu

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et al.51 had suggested that linear relationship of nss-SO42- with oxalate in atmospheric aerosols


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is due to their secondary production by in-cloud oxidation processes. Interestingly, the slope

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of the linear regression lines between nss-SO42- and oxalic acid over the BoB is similar to

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those documented over East Asia (Hong Kong: 0.034; Guangzhou: 0.033 and nearby coastal

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aerosols: 0.050; Nanjing: 0.029; ACE-Asia: 0.050; Beijing: 0.019 and 0.0046 for summer and

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winter, respectively)51. All these observations indicate that in-cloud formation processes of

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oxalic acid may also be applicable for LWC-enriched aerosols.

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The amount of particle bound water in the MABL is, however, much lower than those

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of cloud droplets. Consequently, ionic strength, pH ( C4 ≈ ωC2) and more or less comparable concentrations of

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ωC2 with C4 (Table 1) over the BoB, the formation of both low and high molecular weight

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organic compounds (i.e., relative to precursor MeGly having three carbons) during the

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aqueous phase photochemical oxidation of α-dicarbonyls are equally likely in marine

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aerosols.

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We also observe a very strong linear relationship between NH4+ and SO42- in PM2.5,

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suggesting the formation of ammonium sulphate or bisulphate aerosols. The RH% over the

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BoB (range: 49 – 80%, av. 64±8%, median: 66%) is more than efflorescence relative

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humidity (i.e., 35%) of (NH4)2SO4. Consequently, the PM2.5 collected over the BoB has

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substantial water content that allows the exchange of semi volatile organic precursors between

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gas and aerosol water and subsequent aqueous phase reactions to form oxalic acid and other

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dicarboxylic acids14, 17. Therefore, observed linear relationship of nss-SO42- concentrations

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with both aerosol-LWC and oxalic acid thus suggests a strong interplay between water-

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soluble organic precursors and oxidants in gaseous phase and deliquescent particle/cloud

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water. The subsequent aqueous phase photochemical oxidation process results in the

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formation of oxalic acid.

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The contribution of individual water-soluble inorganic ions (WSIC) to their total mass

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concentration (ΣWSIC) is not significantly different between the IGP- and SEA-outflows



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(Figure 1c). In both IGP- and SEA-outflows, nss-SO42- concentration (>99% of total SO42-)

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dominates (~68%) the ΣWSIC followed by NH4+ (~20%) and nss-K+ (~4%) and NO3- (~2%)

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(Figure 1c). WSIManth accounts for ~94±5% of ΣWSIC. These observations indicate a strong

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control of aerosol LWC by anthropogenic SO42- and other water-soluble inorganic ions over

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the BoB during winter. LMW dicarboxylic acids (C2-C4), glyoxylic acid (ωC2), pyruvic acid

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(Pyr) and methylglyoxal (MeGly) are SOA compounds that eventually form through the

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aqueous-phase photochemical reactions.

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A close similarity in the temporal variations of WSIManth, aerosol LWC, water-soluble

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SOA tracer compounds (C2, C3) and their precursors (MeGly, Pyr, ωC2, C4) as well as WSOC

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over the BoB (Figure S4), suggest their formation through aqueous phase photochemistry.

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This inference is further supported by similarity in the mass ratios of DCAs (ascertained

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through unpaired two-tailed t-test results, p > 0.05) and related water-soluble compounds

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(e.g., C2/C4, C3/C4, C2/ωC2, C2/Pyr, and C2/MeGly) between IGP- and SEA-outflows

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influenced by different emissions (fossil fuel combustion/wood burning in the IGP vs. forest

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fires in SEA). Because oxalic acid (C2) is a major or ultimate product of aqueous phase

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photochemical reactions involving MeGly, Pyr, ωC2 and C419, 20, 52, we have used the mass

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ratios of product to precursor to better understand the formation processes of dicarboxylic

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acids and related polar compounds. Several previous studies have demonstrated the usefulness

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of the mass ratios of dicarboxylic acids in understanding their potential pathways27. For

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instance, the diagnostic ratios of dicarboxylic acids (e.g., C2/C4, C2/ωC2, C2/Pyr and

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C2/MeGly) have been used for the long term cruise observations from the North Pacific to

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unveil the seasonal differences in the sources (winter/spring: East Asian outflow and

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summer/autumn: oceanic source)27.

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The relative abundances of individual aliphatic DCAs in their total mass concentration

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(Σ(C2-C12)) are also similar between the IGP- and SEA-outflows (Figure 1d). In particular,

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remarkable consistency is observed with respect to the relative abundances of major

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dicarboxylic acids (C2: ~80%; C3: ~4%; C4: ~7%). Higher relative abundance of azelaic acid

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(C9), which is a photochemical oxidation product of biogenic unsaturated fatty acids (e.g.,

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oleic acid) emitted from the ocean surface or from the continent, in the SEA-outflow than the

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IGP-outflow is a conspicuous feature of the data. The simultaneous collection of PM10 during

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the study period revealed dominant occurrence of sea-salts in the SEA-outflow53, as evident

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through higher relative abundances of water-soluble Na+ and the closeness of its weight ratio

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to Mg2+ (0.18, seawater value). Although we observe no significant differences in the


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abundances of oleic acid between the IGP- (1.1 – 13.7 ng m-3, av. 4.6 ng m-3) and SEA- (0.3 –

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64.2 ng m-3, av. 11.5 ng m-3) outflows, the median concentration is twice higher over the

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southern BoB. Under this scenario, it is likely that photochemical oxidation of oleic acid and

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other unsaturated fatty acids emitted from the ocean surface contribute to C9. Forest canopies

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also contribute to atmospheric azelaic acid54, 55. In such case, higher atmospheric abundances

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of azelaic acid can be explained by the contribution of biogenic unsaturated fatty acids in the

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SEA-outflow.

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Relative abundances in WSOC and TC

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Dicarboxylic acids accounted for 1.7 – 4.0% (2.8±0.6%) and 1.6 – 4.9% (2.5±0.8%) of

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WSOC in the IGP- and SEA-outflows, respectively. Likewise, DCAs accounted for 1.1 –

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2.2% (1.5±0.3%) of TC in the IGP-outflow and 0.6 – 2.0% (1.3±0.4%) of TC in the SEA-

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outflow. These results indicate that dicarboxylic acids and related compounds in the

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continental outflows make a significant contribution to water-soluble OAs over the BoB. We

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observed relatively high concentrations of dicarboxylic acids (aliphatic, branched-chain,

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unsaturated, multifunctional), oxocarboxylic acids and α-dicarbonyls in the IGP-outflow than

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SEA-outflow (Figure 2a). This is because IGP-outflow samples are strongly influenced by the

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pollution sources (close to continent) rather than those sampled in the SEA-outflow. Although

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OAs over the BoB are influenced by different sources (wood burning/fossil-fuel combustion

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in the IGP-outflow vs. forest fires in the SEA-outflow), the relative abundances of

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dicarboxylic acids and related polar compounds in WSOC (t-score =1.5, df = 29 and p > 0.05)

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and TC (t-score =1.4, df = 29 and p > 0.05) overlap within the spread of the data for both the

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outflows (Figure 2b and 2c). This observation reveals the similar photochemical oxidation or

315

formation processes through aqueous phase photochemistry within the MABL.

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Formation pathways of dicarboxylic acids

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Oxalic acid accounts for 49 – 91% (77±8%) of total DCA mass over the BoB.

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Moreover, C2 together with C4 and ωC2 dominates (~72±8%) water-soluble dicarboxylic

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acids and related compounds measured (DCAs + oxoacids + α-dicarbonyls). Several studies

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suggested the formation of oxalic acid from succinic acid through malonic acid as an

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intermediate oxidation product, based on the filed observations and laboratory photooxidation

322

experiments27. Based on the laboratory experiments, Carlton et al.46 have documented that

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aqueous phase oxidation of glyoxylic and pyruvic acids results in the formation of oxalic acid.

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Therefore, we have investigated the interrelationships between certain diagnostic mass ratios



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(e.g., C2/C4, C2/ωC2, C2/Pyr, and C2/MeGly) and the relative abundances of oxalic acid in

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total DCA mass over the BoB.

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A close examination of linear regression analysis has revealed that C2/C4 ratios are

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positively correlated with relative abundance of C2 in total DCA mass (Figure 3a). We also

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observed an inversed correlation between relative abundances of C2 and C4 in total DCA mass

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(Figure 3b). These significant relationships indicate the formation of oxalic acid via the

331

photochemical breakdown of succinic acid. Furthermore, C2/C4 and C3/C4 ratios are very

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similar between the IGP- and SEA-outflow samples (Table S1), although they are influenced

333

by different sources of OAs (wood/fossil fuel combustion vs. forest fires, respectively)38.

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LMW-dicarboxylic acids further showed a good correlation with aerosol LWC (Figure S5).

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Combining all these results, we infer that oxalic acid is formed via the aqueous phase

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oxidation of succinic acid in marine aerosols over the BoB during winter.

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A significant positive linear relationship is noteworthy between relative abundances of

338

oxalic acid and its mass ratio with ωC2, Pyr (Figures 3c and 3d) and, to some extent, MeGly

339

(figure not shown here). Atmospheric oxidation of biogenic VOCs such as isoprene with

340

ozone and/or hydroxyl radicals leads to the formation of semi-volatile gaseous organic

341

precursors (e.g., MeGly and Gly). These α-dicarbonyls usually partition into LWC-enriched

342

aerosols and/or cloud/fog water and the subsequent aqueous phase photochemical oxidation

343

results in Pyr and ωC2 that are further oxidized to produce oxalic acid19, 47. Therefore, several

344

studies used C2/ωC2, C2/Pyr, and C2/MeGly ratios to examine the significance of secondary

345

sources of oxalic acid in the atmosphere56-58.

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We observe no significant differences for C2/ωC2, C2/Pyr and C2/MeGly ratios

347

between the IGP- and SEA-outflow samples (Table S1). Although aerosol samples are

348

influenced by two different wind regimes (IGP vs. SEA-outflow), this observation indicates

349

their similar aqueous phase photochemical reactions in the MABL. Because ωC2 and Pyr are

350

the SOA components formed by the aqueous phase oxidation46, 47, 59, a linear relationship

351

between these diagnostic ratios and relative abundance of oxalic acid, thus, suggest the

352

formation of these species through aqueous-phase oxidation pathway. Therefore, it is likely

353

that the aqueous phase and ambient photochemical reactions involving these SOA precursors

354

could serve as major source of oxalic acid in the continental outflows over the tropical Indian

355

Ocean. Overall, these results combined with the dominance of nss-SO42- in WSIC and its

356

significant linear relationship with aerosol-LWC altogether corroborate the hypothesis that

357

anthropogenic sulphate-driven SOA formation (i.e., LMW-dicarboxylic acids, ωC2, Pyr)


Page 13 of 26


13 358

occurs via aqueous phase reactions over the ocean that is downwind region of pollution

359

sources (as depicted in art figure).

360

Based on the field observations27, laboratory experiments46, 47 and modelling studies60,

361

numerous researchers suggest that aqueous phase oxidation of α-dicarbonyls in clouds or

362

aqueous aerosols is a major source of semi-volatile organic acids such as ωC2, Pyr, and

363

LMW-diacids4,

17, 49

. For instance, MeGly and Gly contribute significantly to the SOA

18

364

formation globally . Due to their highly reactive nature and high solubility products (high

365

Henry’s law constants12), both MeGly and Gly partition into cloud waters and/or aerosol-

366

water (i.e., they become hydrated in the aqueous media because of carbonyl groups) in the

367

polluted continental outflows to form Pyr, which eventually oxidizes to form ωC2 and then

368

oxalic acid49, 61. Evaporation of cloud water results in the formation of oxalic acid in the

369

particulate phase.

370

The water samples from stratocumulus/cumulus clouds and below cloud aerosols

371

collected off the coast of Monterey (CA) revealed that both nss-SO42- and oxalate reside in the

372

size range between 0.26µm and 0.44 µm (i.e., droplet mode)62. Their occurrence in similar

373

size bin for below-cloud aerosols has led to hypothesize their formation through incloud

374

processing and the subsequent cloud evaporation results in particulate phase oxalate.

375

Likewise, Sorooshian et al.63 also observed elevated concentrations of oxalic acid in above

376

cloud aerosols, collected as part of Gulf of Mexico Atmospheric Composition and Climate

377

(GoMACCS) and Marine Stratus/Stratocumulus Experiment (MASE) conducted in southern

378

Texas and off the coast of northern California, respectively. These studies proposed that in-

379

cloud oxidation of semi-volatile gaseous precursors (e.g., Gly and MeGly) results in the

380

formation of oxalic acid in aerosols above/below cloud. Therefore, we hypothesize that

381

similar process may work for marine aerosols over the BoB. The global emissions of Gly and MeGly are estimated to be 45 Tg yr-1 and 140 Tg yr-

382 383

1

, respectively12. The magnitude of their incloud oxidation to SOA components is comparable

384

to their formation from other BVOCs and aromatic hydrocarbons12. These α-dicarbonyls

385

oxidize through aqueous phase oxidation to form particulate ωC2 and Pyr, which are further

386

oxidized to produce oxalic acid11,

387

(2.0±1.2 ng m-3) in PM2.5 dominate (~3.3 times) the Gly mass (0.6±0.5 ng m-3). We observed

388

significant linear relationships between MeGly and its SOA oxidation products (ωC2, Pyr and

389

C2) in both IGP- and SEA-outflow samples. Interestingly, we found no significant differences

390

between the slopes of linear regression analyses (Figure 3) for IGP- and SEA-outflows.

43

. Over the BoB, atmospheric abundances of MeGly



Page 14 of 26

14 391

However, no such significant correlations between Gly and its SOA oxidation products were

392

evident in the SEA-outflow samples.

393

Besides the formation of major oxidation products such as ωC2, Pyr and C2, aqueous

394

phase reactions of α-dicarbonyls with hydroxyl radicals can also produce other SOA

395

components such as organosulfates and oligomers in wet aerosols under acidic conditions15, 17,

396

18, 20, 64-67

397

processes involving organic radical-radical reactions, acid catalysed esterification,

398

acetal/hemiacetal formation, self oligomerization during evaporation of water droplets,

399

ammonia catalysed aldol condensation14,

400

accounts for a significant fraction of PM2.5 over the BoB (av. 31%; range: 10 – 63%).

401

Interestingly, we observe lower equivalent NH4+/SO42- ratios (0.77±0.22; median: 0.76) over

402

the southern BoB than those found over the northern BoB (0.92±0.15; median: 0.94),

403

indicating an increased acidity of fine mode aerosols in the SEA-outflow. Therefore, although

404

small, contribution from organosulfates and/or other SOAs formed by oligomerization could

405

be substantial over the BoB due to prevailing mild acidic conditions in the marine aerosols.

. These oligomers can be formed through a variety of complex aqueous phase

17-20, 68-70

. In this study, anthropogenic nss-SO42-

406

In both the IGP- and SEA-outflows, succinic acid (C4) is strongly correlated with

407

MeGly, Pyr (Figure 3e), glutaric acid (C5), malonic acid (C3) and malic acid (or

408

hydroxysuccinic acid, hC4; i.e. observed for the IGP-outflow samples but not evident in the

409

SEA-outflow) (Figure 3f). No differences in the slopes of these linear regression analyses,

410

indicate similar SOA formation processes governing their abundances in the MABL.

411

However, concentrations of hC4 over the BoB are rather low compared to C3 probably due to

412

the low yield for the former species in the OH radical reaction with C4 (~0.3% and 6%,

413

respectively71). It has been suggested that C4 in ambient aerosols further oxidizes to C2

414

through hC4 and C3 as intermediates57, 72, 73, which is also consistent with the photooxidation

415

experiments of C4 with hydroxyl radical that result in C3, hC4, ωC2 and C268, 71, 74.

416

According to these studies56,

60, 63

, C4 undergoes a hydrogen abstraction from α-

417

position followed by the addition of oxygen to form a peroxy radical, which then participates

418

in a bimolecular reaction to yield a tetroxide68,

419

decomposition of this tetroxide result in two alkoxy radicals followed by β-fragmentations to

420

form 3-oxopropanoic acid (ωC3) and ωC2, which eventually react with hydroxyl radicals to

421

yield C3 and C2, respectively. Alternately, Altieri et al.68 argued that tetroxide undergoes a

422

disproportionation reaction to form malic acid and 3-hydroxypropanoic acid, which is also

74

. Charbouillot et al.74 proposed that


Page 15 of 26


15 423

consistent with the study by Yang et al71. Therefore, observed linear relationships of hC4 and

424

C3 with C4 indicate their formation via the photochemical oxidation in marine aerosols.

425

Succinic (C4) and glutaric (C5) acids can also be formed as major oxidation products

426

in the aqueous phase reaction of azelaic acid (C9, a photochemical oxidation of oleic acid27)

427

with OH radical27, 71. Yang et al.71 suggested that increased frontier electron density in the

428

middle of carbon chain length of C9 for hydrogen abstraction (i.e., preferentially occur at

429

either β- and γ- position) by hydroxyl radical during photooxidation rather than the carbon

430

atoms nearby carboxyl group (due to its electron withdrawing nature)71. Consequently, the

431

prevalent linear relationship between C4 and C5 diacids over the BoB could be due to their

432

common source (i.e., either photochemical oxidation of C9 or formed as oligomers by the

433

oxidation of OH radical with MeGly). However, indistinguishable and overlapping

434

abundances of fractional mass contribution of nss-SO42- in ΣWSIC, aerosol-LWC and

435

diagnostic mass ratios of DCA and related compounds (C2/C4, C3/C4, C2/ωC2, C2/Pyr, and

436

C2/MeGly) altogether suggest a significance of SOA formation driven by aqueous phase

437

oxidation in the continental outflows over the tropical Indian Ocean.

438

In summary, this study highlights a potential role of anthropogenic SO42- driven SOA

439

formation in the polluted continental outflows through aqueous phase photochemical

440

reactions. In addition, the major SOA precursors that eventually oxidize to form oxalic acid

441

over the tropical Indian Ocean are succinic, glyoxylic and pyruvic acids and methylglyoxal.

442

The previous field studies, documenting a strong linear correlation between nss-SO42- and

443

oxalate in aerosols over East Asia51, suggested their formation through incloud oxidation

444

processes. The chamber based studies suggested that internally/externally mixed aerosol

445

particles of LMW-dicarboxylic acids and ammonium sulfate act as an efficient cloud

446

condensation nuclei (CCN)75. Subsequent studies emphasized the significance of

447

biogenic/anthropogenic VOCs to form oxalic acid through their partitioning into clouds and

448

oxidation by aqueous phase photochemical reactions4, 10, 17, 46, 47. Aerosol-LWC is crucial for

449

the gas-to-liquid transfer of α-dicarbonyls (MeGly and Gly). We observe very strong linear

450

relationships between MeGly and its oxidation products (e.g., Pyr, ωC2, and C4) as well as

451

those between succinic acid and its oxidation products (e.g., Pyr, ωC2, C3, hC4 and C2).

452

However, given the source variability of precursor anthropogenic/biogenic VOCs between the

453

IGP- and SEA-outflows, no significant differences in the mass ratios (C2/C4, C3/C4, C2/ωC2,

454

C2/Pyr, and C2/MeGly), and linear regressions were obtained between precursor compounds

455

and oxalic acid, indicating the similar formation pathways in aqueous aerosols. In addition,



Page 16 of 26

16 456

observed significant linear relationships of nss-SO42- with aerosol LWC and C2 hint their

457

formation through aqueous phase photochemical oxidation processes in the aerosol-associated

458

water in the MABL. Higher abundances of oxalic acid (31 – 661 ng m-3) in the ABCs over the

459

open ocean could influence the CCN activity, affecting cloud albedo and lifetime. These

460

results should be taken into account when evaluating aerosol-cloud interactions and are

461

necessary for improving our understanding of SOA formation.

462 463

Acknowledgements

464

We thank the Dr. Krishnamoorthy and Dr. C.B.S. Dutt for their logistic support

465

during ICARB-W field campaign. We also acknowledge the financial aid from ISRO-GBP for

466

the collection and analysis of chemical composition in aerosols used in this study. This study

467

was also supported by the Japan Society for the Promotion of Science (JSPS) through Grant-

468

in-Aid No. 24221001. All the chemical composition data of dicarboxylic acids will be

469

available upon a request to Prof. Kimitaka Kawamura ([email protected]).

470 471

Supporting Information Available: We provided additional brief description about the

472

analysis of chemical composition of PM2.5 over the Bay of Bengal. We also provided six

473

supporting figures and one table to understand the data presented in the manuscript. related to

474

cruise track with air mass back trajectories and fire count data for the study period (Figure

475

S1), temporal variability of diacids, oxoacids and α-dicarbonyls as well as anthropogenic

476

water-soluble inorganic constituent mass, aerosol-LWC and relative humidity (Figure S2, S4

477

and S6), linear regression analysis of aerosol-LWC with water-soluble inorganic ions (Figure

478

S3) and LMW-dicarboxylic acids (Figure S5).


Page 17 of 26


17 479

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Bikkina, S.; Kawamura, K.; Miyazaki, Y.; Fu, P., High abundances of oxalic, azelaic, and glyoxylic acids and methylglyoxal in the open ocean with high biological activity: Implication for secondary OA formation from isoprene. Geophysical Research Letters 2014, 41, (10), 3649-3657. Lim, H. J.; Carlton, A. G.; Turpin, B. J., Isoprene forms secondary organic aerosol through cloud processing: Model simulations. Environmental science & technology 2005, 39, (12), 4441-4446. Myriokefalitakis, S.; Tsigaridis, K.; Mihalopoulos, N.; Sciare, J.; Nenes, A.; Kawamura, K.; Segers, A.; Kanakidou, M., In-cloud oxalate formation in the global troposphere: a 3-D modeling study. Atmos. Chem. Phys. 2011, 11, (12), 5761-5782. Wren, S. N.; Gordon, B. P.; Valley, N. A.; McWilliams, L. E.; Richmond, G. L., Hydration, Orientation, and Conformation of Methylglyoxal at the Air–Water Interface. The Journal of Physical Chemistry A 2015, 119, (24), 6391-6403. Crahan, K. K.; Hegg, D.; Covert, D. S.; Jonsson, H., An exploration of aqueous oxalic acid production in the coastal marine atmosphere. Atmospheric Environment 2004, 38, (23), 3757-3764. Sorooshian, A.; Lu, M.-L.; Brechtel, F. J.; Jonsson, H.; Feingold, G.; Flagan, R. C.; Seinfeld, J. H., On the Source of Organic Acid Aerosol Layers above Clouds. Environmental Science & Technology 2007, 41, (13), 4647-4654. Yu, G.; Bayer, A. R.; Galloway, M. M.; Korshavn, K. J.; Fry, C. G.; Keutsch, F. N., Glyoxal in aqueous ammonium sulfate solutions: products, kinetics and hydration effects. Environmental science & technology 2011, 45, (15), 6336-6342. Zhang, H.; Surratt, J.; Lin, Y.; Bapat, J.; Kamens, R., Effect of relative humidity on SOA formation from isoprene/NO photooxidation: enhancement of 2-methylglyceric acid and its corresponding oligoesters under dry conditions. Atmospheric Chemistry and Physics 2011, 11, (13), 6411-6424. Gómez‐González, Y.; Surratt, J. D.; Cuyckens, F.; Szmigielski, R.; Vermeylen, R.; Jaoui, M.; Lewandowski, M.; Offenberg, J. H.; Kleindienst, T. E.; Edney, E. O., Characterization of organosulfates from the photooxidation of isoprene and unsaturated fatty acids in ambient aerosol using liquid chromatography/(−) electrospray ionization mass spectrometry. Journal of Mass Spectrometry 2008, 43, (3), 371-382. Galloway, M. M.; Chhabra, P. S.; Chan, A. W. H.; Surratt, J. D.; Flagan, R. C.; Seinfeld, J. H.; Keutsch, F. N., Glyoxal uptake on ammonium sulphate seed aerosol: reaction products and reversibility of uptake under dark and irradiated conditions. Atmos. Chem. Phys. 2009, 9, (10), 3331-3345. Altieri, K.; Seitzinger, S.; Carlton, A.; Turpin, B. J.; Klein, G.; Marshall, A., Oligomers formed through in-cloud methylglyoxal reactions: Chemical composition, properties, and mechanisms investigated by ultra-high resolution FT-ICR mass spectrometry. Atmospheric Environment 2008, 42, (7), 1476-1490. Loeffler, K. W.; Koehler, C. A.; Paul, N. M.; De Haan, D. O., Oligomer formation in evaporating aqueous glyoxal and methyl glyoxal solutions. Environmental science & technology 2006, 40, (20), 6318-6323. De Haan, D. O.; Corrigan, A. L.; Tolbert, M. A.; Jimenez, J. L.; Wood, S. E.; Turley, J. J., Secondary organic aerosol formation by self-reactions of methylglyoxal and glyoxal in evaporating droplets. Environmental science & technology 2009, 43, (21), 8184-8190. Yang, L.; Ray, M. B.; Yu, L. E., Photooxidation of dicarboxylic acids—Part II: Kinetics, intermediates and field observations. Atmospheric Environment 2008, 42, (5), 868-880.



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23 744

Table 1. Statistical description of mass concentrations of water-soluble dicarboxylic acids,

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oxocarboxylic acids and α-dicarbonyls in PM2.5 sampled from the Indo-Gangetic Plain (IGP) outflow

746

and South-east Asian (SEA-) outflow over the Bay of Bengal during a winter cruise (27th Dec’2008 –

747

26th Jan’2009). -3

Rang (Av±Sd, Median), ng m

Parameter Diacids

IGP-outflow (N = 15)

SEA-outflow (N = 16)

Oxalic, C2

104 - 661 (320 ± 193, 213)

31 - 232 (116±65, 112)

Malonic, C3 Succinic, C4

5.2 - 25.4 (13.4±6.3, 12.0) 9.3 - 51.5 (25.3±13.9, 21.2)

1.0 - 13.6 (6.5±4.2, 6.6) 1.2 - 22.9 (10.7±6.7, 10.5)

Glutaric, C5 Adipic, C6

1.7 - 12.5 (5.7±3.4, 4.2) 0.1 - 5.0 (2.1±1.9, 1.4)

0.2 -5.5 (2.3 ± 1.7, 2.0) 0.2 - 3.2 (1.2±1.1, 0.9)

Pimelic, C7 Suberic, C8

0.3 - 4.1(1.5±1.1, 1.2) 0.2 - 1.1 (0.6±0.3, 0.6)

nd-2.0 (0.7±0.5, 0.4) 0.04 - 1.5 (0.6±0.4, 0.5)

Azelaic, C9 Sebacic, C10

0.2-4.9(2.7±1.5, 2.6) 0.2-1.2(0.6±0.3, 0.6)

1.7-24.2(7.0±5.6, 6.1) 0.1-0.7(0.3±0.2, 0.3)

Undecanedioic, C11 Dodecanedioic, C12

0.6-2.7(1.7±0.8, 1.4) 0.5-3.6(1.6±1.1, 1.1)

0.1-3.8(1.0±0.9, 0.7) n.d.-2.5(0.7±0.6, 0.7)

Σ(C2-C12) Methylmalonic, iC4

122-768(374±220, 253)

Methylsuccinic, iC5 Methylglutaric, iC6 Maleic, M Furmaric, F Methylmaleic, mM Phthalic, Ph

Oxoacids

α-Dicarbonyls

39-281(145±77, 142) 0.32 − 1.31 (0.70±0.30, 0.58) 0.11 - 1.10 (0.38±0.26, 0.33) 1.0 - 5.8 (2.7±1.5, 2.4) 0.2 - 2.2 (1.0±0.7, 0.9) 0.1 - 0.7 (0.3±0.2, 0.3) n.d. - 0.3 (0.2±0.1, 0.2) 0.5 -1.1 (0.7±0.2, 0.7) 0.4 -2.3 (0.9±0.6, 0.7)

n.d. - 1.9 (0.4±0.5, 0.1) n.d. - 1.0 (0.4±0.3, 0.4)

n.d. - 0.55 (0.34 ±0.13, 0.38) 0.06 - 0.89 (0.26±0.20, 0.23) n.d. - 16.4 (7.6±4.8, 6.4) n.d. - 5.9 (2.6±2.2, 2.0)

Isophthalic, iPh Terephthalic acid, tPh

0.4 - 2.3 (1.0±0.6, 0.8) 2.0 - 13.4 (7.0±4.3, 5.1)

0.05 - 0.8 (0.3±0.2, 0.3) 0.1 - 9.5 (4.4±3.2, 4.9)

Malic, hC4 Ketomalonic, kC3

0.1 - 0.6 (0.3±0.2, 0.2) 1.2 - 7.2 (3.3±2.0, 2.9)

n.d. -1.2 (0.2±0.3, 0.1) 0.04 - 3.9 (1.7±1.1, 1.6)

Ketopimelic, kC7 ΣDiacids

2.1 - 13.1 (6.1±3.9, 5.1) 130 - 830 (405±237, 280)

0.23 - 7.4 (2.5±1.8, 2.5) 39 - 298 (154±84, 154)

Glyoxylic acid, ωC2 3-Oxopropanoic, ωC3

4.0 - 49.5 (22.9±13.5, 20.5) 1.5 - 8.4 (4.5±2.3, 4.1)

1.8-27.5(11.4±7.4, 10.2) 0.4 - 4.7 (2.0±1.2, 1.9)

4-Oxobutanoic, ωC4

2.3 - 14.0 (7.0±3.9, 5.7)

0.3 - 7.0 (2.5±1.8, 2.4)

5-Oxopentanoic, ωC5

0.6 - 2.4 (1.3±0.6, 1.2)

0.4 - 1.8 (0.8±0.4, 0.7)

7-Oxoheptanoic, ωC7

4.8 - 26.6 (12.8±6.6, 13.5)

1.7 - 16.2 (10.2±4.3, 11.3)

8-Oxooctanoic, ωC8

7.1 - 37.3 (18.5±10.1, 19.6)

1.8 - 27.5 (11.5±5.5, 11.8)

9-Oxononanoic, ωC9 ΣOxoacids Pyruvic, Pyr Glyoxal, Gly

0.6 - 2.6 (1.5±0.6, 1.4) 26 - 137 (68±35, 70)

0.1 - 1.9 (0.8±0.5, 0.8) 7 - 85 (39±19, 37)

3.3 - 13.2 (6.9±3.1, 6.2) n.d. - 0.6 (0.3±0.2, 0.2)

0.4 - 9.5 (3.6±2.3, 3.4) n.d. - 2.3 (0.8±0.6, 0.6)

0.7 - 3.9 (2.4±1.1, 2.3) 0.9 - 3.8 (2.6±1.0, 2.7)

n.d. - 4.2 (1.5±1.1, 1.4) 0.3 - 5.7 (2.0±1.3, 1.8)

Methylglyoxal, MeGly

Σα-Dicarbonyls



Page 24 of 26

24 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767

Figure 1. Linear regression analysis between mass concentrations of (a) non-sea-salt SO42-

768

and aerosol liquid water content (LWC), estimated using ISORROPIA-II (b) nss- SO42- and

769

oxalic acid. Pie charts, showing the relative abundances of (c) water-soluble inorganic

770

constituents (WSIC) in their total mass concentration, (d) relative abundances of individual

771

aliphatic dicarboxylic acids from C2 to C12 in their total mass concentration (ΣC2-C12) of

772

PM2.5 sampled in the IGP- and SEA-outflows.


Page 25 of 26


25 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801

Figure 2. Box-whiskers, showing total mass concentrations and relative abundances of aliphatic (ΣAli-DCA), branched chain (ΣBrc-DCA), and unsaturated (ΣUns-DCA) and multifunctional (ΣMf-DCA) dicarboxylic acids, also oxocarboxylic acids and α-dicarbonyls in the mass concentrations of water-soluble organic carbon (WSOC) and total carbon (TC) in fine mode aerosols sampled over the Bay of Bengal for IGP- and SEA-outflows. The lower and upper edges of the boxes correspond to 25 and 75 percentile data, respectively. The downside and upside whiskers represent 5 and 95 percentile data, respectively, whereas the black dots outside this whiskers corresponds outliers that fall below 5 percentile and above 95 percentile. Likewise, the solid and dotted lines in each box refer to median and mean concentrations, respectively.



Page 26 of 26

26 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822

Figure 3. Linear regression analysis between relative abundances of oxalic acid in total

823

dicarboxylic acid mass (i.e., C2/ΣDCA(%)) and (a) oxalic to succinic acid ratio (C2/C4), (b)

824

relative abundances of succinic acid (C4/ΣDCA(%)), (c) oxalic to glyoxylic acid (C2/ωC2), (d)

825

oxalic to pyruvic acid (C2/Pyr), (e) methylglyoxal and pyruvic acid versus succinic acid

826

(MeGly, Pyr vs. C4), and (f) glutaric, malonic and malic acids versus succinic acid (i.e., C5,

827

C3, hC4 vs. C4) for the PM2.5 collected over the Bay of Bengal during a winter cruise (27

828

December, 2008 – 26 January, 2009).


Secondary Organic Aerosol Formation over Coastal Ocean: Inferences

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