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Primary and secondary sources of gasphase organic acids from diesel exhaust Beth Friedman, Michael F. Link, S. Ryan Fulgham, Patrick Brophy, Abril A. Galang, William H. Brune, Shantanu H Jathar, and Delphine K. Farmer Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01169 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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Environmental Science & Technology

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Primary and secondary sources of gas-phase organic

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acids from diesel exhaust

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Beth Friedmanǂ, Michael F. Linkǂ, S. Ryan Fulghamǂ, Patrick Brophyǂ, Abril Galang¶,William H.

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Bruneǁ, Shantanu H. Jathar¶, Delphine K. Farmer*ǂ

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ǂ Department of Chemistry, Colorado State University, Fort Collins, CO, USA 80523-1872

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¶ Department of Mechanical Engineering, Colorado State University, Fort Collins, CO, USA

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80523-1872

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ǁ Department of Meteorology, Pennsylvania State University, University Park, PA, USA 16802

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ABSTRACT

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Organic acids have primary and secondary sources in the atmosphere, impact ecosystem

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health, and are useful metrics for identifying gaps in organic oxidation chemistry through model-

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measurement comparisons. We photooxidized (OH oxidation) primary emissions from diesel and

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biodiesel fuel types under two engine loads in an Oxidative Flow Reactor. Formic, butyric, and

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propanoic acids, but not methacrylic acid, have primary and secondary sources. Emission factors

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for these gas-phase acids varied from 0.3 – 8.4 mg kg-1 fuel. Secondary chemistry enhanced 1

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these emissions by 1.1 (load) to 4.4 (idle) x after two OH-equivalent days. The relative

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enhancement in secondary organic acids in idle versus loaded conditions was due to increased

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precursor emissions, not faster reaction rates. Increased hydrocarbon emissions in idle conditions

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due to less complete combustion (associated with less oxidized gas-phase molecules) correlated

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to higher primary organic acid emissions. The lack of correlation between organic aerosol and

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organic acid concentrations downstream of the flow reactor indicates that the secondary products

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formed on different oxidation timescales and that despite being photochemical products, organic

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acids are poor tracers for secondary organic aerosol formation from diesel exhaust. Ignoring

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secondary chemistry from diesel exhaust would lead to underestimates of both organic aerosol

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and gas-phase organic acids.

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INTRODUCTION

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Primary particle emissions from on- and off-road diesel sources are a significant fraction of the

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total aerosol budget, particularly in urban regions1, impacting human health and climate.2,

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Diesel engines account for a substantial fraction (up to 75%) of the total fine particle matter

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(PM2.5) from mobile sources in the US.4 However, primary emissions are not the only way in

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which engine emissions impact air quality. Primary emissions from engine exhaust can

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contribute to secondary organic aerosol (SOA) by the oxidation of gas-phase species (via

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functionalization reactions) to produce lower-volatility products that partition to form SOA.5, 6

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Primary emissions may also be oxidized to higher-volatility products (via fragmentation

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reactions). These products may be more (e.g. SOA, HNCO) or less toxic than their parent

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compounds.

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directly relevant to air quality and health,11, 12 with few examining the impacts of photochemical

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aging.1, 13, 30

7-10

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Online studies of gas-phase engine emissions typically focus on a few species

2


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Oxidation of hydrocarbons by OH radicals produces an array of products with numerous

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functional groups, including carboxylic acids (-C(O)OH). These functional groups act as weak

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acids, which influence not only the pH of precipitation,16,17 but potentially contribute to acid

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deposition in remote regions,14 and could influence ecosystem health.15, 16 Some larger organic

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acids have adequately low volatility to contribute to SOA formation, while the lower molecular

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weight organic acids can act as a sink for OH radicals in the aqueous-phase and influence

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chemistry in cloud droplets, contributing to SOA formation from multiphase chemistry.17-20 The

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budget of small organic acids provides insight into our understanding of oxidation chemistry.

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Specifically, multiple studies of the formic acid budget have shown missing sources of this

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organic acid,14, 21, 22 while measurements have suggested sink reactions may also be more rapid

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than previously considered.23

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Engine exhaust is a known primary source of organic acids to the atmosphere.24-26

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Secondary production of organic acids emitted from engine exhaust, however, are rarely

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reported. One study in Los Angeles indicated that secondary organic acid sources could be

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substantial in urban environments.24, 26 Further, recent work has indicated a large photochemical

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secondary source of formic acid missing from current models.22 While sources of formic acid are

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primarily biogenic,14,

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contribute to the formic acid budget and account for model-measurement discrepancies.

21, 22

poorly characterized anthropogenic sources of formic acid may

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Butyric (H8C4O2), propanoic (H6C3O2) and methacrylic (H6C4O2) acids are even more

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poorly understood than formic acid. Butyric and propanoic acids are emitted by motor vehicle

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exhaust and have been measured in urban air masses.7,

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suggest photochemical production of butyric and propanoic acids in urban air masses.29

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Methacrylic acid is known to be produced from the oxidation of isoprene30 and methacrolein,31, 32

26-28

Correlations with nitric acid also

3



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though other potential VOC precursors remain unstudied. Measurements in an urban plume

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suggest a possible urban photochemical source.29

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Because emissions vary substantially between different engine conditions and fuel types

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7, 33, 34

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Further, fuel types, engines, and gas and particle losses differ among experiments 34. In order to

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build larger datasets that better describe primary emissions and secondary production of the

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current vehicle fleet, more studies are needed to capture emission trends from a single engine

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while varying parameters such as engine load and fuel type. In particular, we note a lack of

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comparisons between diesel and biodiesel fuel. In this study, we track the photochemical aging

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of primary particle- and gas-phase emissions from an off-road diesel engine. We report the

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primary emission factors and secondary production of four small organic acids (formic acid,

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butyric acid, propanoic acid, and methacrylic acid) as a function of photochemical age, fuel type,

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and engine load condition. We express both primary emissions and secondary production on a

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per fuel burned basis to directly compare the importance of primary emissions against

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atmospheric chemistry as a function of photochemical age. We assess the photochemical

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secondary production of these organic acids, and examine how different engine conditions

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impact the total organic aerosol concentrations as a function of oxidation.

, large data sets that represent the entire in-use on- and off-road vehicle fleet are lacking.

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EXPERIMENTAL

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Experiments took place at Colorado State University’s Engines and Energy Conversion

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Laboratory in June 2015.35 A four-cylinder, turbocharged, intercooled, heavy-duty 4.5 L, 175 hp

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John Deere 4045 H Powertech Plus diesel engine generated primary gas- and particle-phase

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emissions. This engine meets the Tier III/Stage IIIA emissions standards for non-road engines 4


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and is representative of engines found in skid-steer loaders and tractors. This study does not

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include newer Tier IV emissions technologies, such as Diesel Particle Filters, Diesel Oxidation

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Catalysts, and Selective Catalytic Reduction. The engine ran on an engine dynamometer with

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diesel and biodiesel fuels to generate exhaust under both steady-state idle and 100% engine load

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operating conditions (which translates to 50% engine load operating conditions at altitude).

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Sourced locally, the diesel fuel was commercial, nonroad, ultralow-sulfur diesel. Biodiesel fuel

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(B100) was sourced from Emergent Green Energy (Minneola, KS), and produced from soy. The

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engine exhaust was diluted in a 300 L stainless steel dilution sampler for 10 minutes with HEPA

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and activated charcoal filtered room air to achieve dilution ratios of 45-110. Dilution ratios were

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chosen based on CO2 measurements, following Lipsky and Robinson 36. Diluted engine exhaust

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was sampled from the center of the dilution sampler in order to minimize losses to the dilution

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tank walls. Engine exhaust was subsequently diluted further for the gas and particle

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measurements by factors of 26-31 and 3-6, respectively.

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Oxidative Flow Reactor

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Diluted raw exhaust was aged in an Oxidative Flow Reactor (OFR, flow rate 7-8 sLpm) at a

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range of atmospheric oxidation timescales. Described in detail elsewhere,8,

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13.3 L cylindrical continuous-flow aluminum chamber with input flows such that the residence

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time was approximately 100 seconds. One UV lamp (254 nm and 185 nm light emission)

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generates OH radicals; OH radical concentration is controlled by the UV light intensity via the

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voltage applied to the lamp. To account for the high external OH reactivity (defined as the sum

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of the products of concentrations of externally introduced OH-consuming species and their

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respective reaction rates with OH) of the engine exhaust,38-40 OH concentrations in the OFR were 5


35, 37

the OFR is a


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calculated from our previous estimates of the external OH reactivity for these experiments35

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following Peng et al.’s model (see Link et al. for more details).8,38 OH exposure ranged from 0 -

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9.2 x 107 molecules-h cm-3, equivalent to 0 - 2 OH days of photochemical aging assuming an

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average daily OH concentration of 1.5 x 106 molecules cm-3.41 The engine exhaust provided high

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amounts of external OH reactivity that did not adhere to traditional methods of calibrating the

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OFR, and thus the OH exposures should be taken as estimated ranges and not as absolute

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numbers. Raw exhaust was exposed to each OH exposure step for 20 minutes to allow sufficient

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time for the particle and gas-phase concentrations to stabilize and account for transport through

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the tubing. The equilibration time in the OFR is longer than the OFR residence time at organic

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aerosol concentrations less than 100 µg m-3. At higher organic aerosol concentrations we assume

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the OFR is in equilibrium.35

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According to results reported in Peng et al. (2016), SOA yields could be decreased more than

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20% by photolysis at 185 and 254 nm if photon flux and quantum yields are high. Given the

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photon fluxes employed in our system, we do not expect a significant impact of photolysis on

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SOA yields or oxidation intermediates (reactions with OH dominate over photolysis).

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Destruction by photolysis could impact a few aromatic VOCs (i.e., benzene, naphthalene), yet

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the lower UV lamp settings likely limits photolysis interference on the oxidation compounds of

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interest. Given that our study did have high external OH reactivity, dilution of the engine exhaust

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likely aided in reducing the importance of non-OH oxidants, as recommended in Peng et al

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(2016).

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Gas- and particle-phase measurements

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Primary and secondary gas-phase species were measured with high-resolution time-of-flight

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(HR-TOF) chemical ionization mass spectrometry with an acetate reagent ion (hereafter referred

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to as acetate-CIMS).42, 43 The acetate-CIMS consists of a reduced pressure ion-molecule reaction

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region coupled to an atmospheric pressure interface HR-TOF.23, 44 The instrument was operated

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in negative ion mode with a mass range of 3-500 m/Q; data analysis used Tofware and followed

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previous procedures.23 Reported concentrations were based on calibrations for the sensitivities of

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four organic acids: butyric acid (0.030 ± 0.002), formic acid (0.25 ± 0.03), methacrylic acid

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(0.050 ± 0.004), and propanoic acid (0.060 ± 0.005), in units of normalized counts per second

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per ppb (ncps ppb-1). Non-refractory particles were measured with an Aerodyne high-resolution

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time-of-flight aerosol mass spectrometer (AMS).45 Data analysis used the software package

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PIKA with recently updated elemental analysis parameterizations.46

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Emission Factor Determination

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Fuel-based emission factors (EFs, mg organic acid/kg fuel) were calculated by8:

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=

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Concentrations of the organic acid species are in units of mg cm-3 and concentrations of CO

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and CO2 are in units of g cm-3. Ci refers to the carbon mass fraction of the fuel (850 gC/kg diesel

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fuel and 770 gC/kg biodiesel fuel); MWCO2 and MWCO refer to the molecular weights (g/mol) of

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CO2 and CO, respectively. AWC is the atomic weight of carbon (g/mol). Concentrations from

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all photochemical ages were used to determine the extent of primary and secondary production at

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a given photochemical age from the combustion of a known amount of fuel. We report the

[ ] [ ] (

[] )

(1)

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secondary production in order to provide a quantitative comparison between primary emissions

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and secondary chemistry.

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RESULTS & DISCUSSION

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Primary and secondary sources of organic acids

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Primary emission factors were much larger for formic acid than butyric, propanoic, or

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methacrylic acid (Figure 1 and Table 1). While primary emission factors were similar for

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biodiesel and diesel fuel types, they varied significantly with engine condition with idle

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conditions producing 2 – 4 x greater emission factors than loaded conditions. While few studies

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have investigated primary emissions of organic acids in urban environments, propanoic acid

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emissions (0.3-2.3 mg kg-1) determined from this study are lower than a previous measurement

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utilizing a light-duty engine: Wentzell et al. reported a primary emission factor of propanoic acid

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for light-duty diesel vehicle exhaust ranging from 3-60 mg propanoic acid per kg fuel, depending

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on the driving mode7, compared to our measurements of 0.3-2.3 mg kg-1 for an off-road diesel

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engine. For comparison, during our experiments, total hydrocarbon emission factors were 20-34

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g kg-1 and 1.3-5 g kg-1 for idle and load engine conditions, respectively.35 The 4 – 26 x greater

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hydrocarbon emissions for idle than loaded conditions can account for the relative difference in

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organic acid emissions (i.e. higher at idle than at load). However, organic acids form a smaller

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fraction of total hydrocarbon emissions for idle (0.08±0.05%) than for load (0.27±0.15%)

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conditions, suggesting that the organic acid primary emissions cannot be merely scaled as a

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fraction of hydrocarbon emissions. The difference in chemical composition of emitted

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hydrocarbons – the more complete combustion of loaded conditions producing more oxidized

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gas-phase species than idle – may contribute to slightly more efficient formation of organic acids

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in the exhaust.

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Secondary production factors for formic, butyric, and propanoic acids increase 1.1 to 4.4-fold

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with < 2.5 equivalent days of photochemical atmospheric aging (Table 1, Figure 1). This

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enhancement was largest for idling engine conditions. Summer studies in urban environments

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have suggested that secondary, photochemical sources of small organic acids are important.22, 26

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However, a wintertime study in London found no evidence for a significant secondary source of

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formic acid,47 suggesting that oxidant loadings also impact the extent of secondary formation,

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consistent with Figure 1. Concentrations of methacrylic acid did not change significantly as a

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function of oxidative age, indicating a negligible photochemical diesel source of this acid. We

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emphasize the correlation between oxidant loadings and secondary formation to provide

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comparisons against the few studies that report secondary sources22,

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extrapolation to model output.

26, 47

and for future

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To quantify the organic acid precursor source and oxidation kinetics, we fit the photochemical

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production of each organic acid as a function of OH exposure time (Table 2, Figure 1). Note that

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because diesel and biodiesel secondary production factors were so similar and to increase the

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sample size, we did not separate the fits by fuel type, and instead fit the data separately only by

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engine load assuming pseudo first-order kinetics:48

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= + 1 − ! "#[$]% &

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The resulting fit from equation 2 provides the magnitude of precursor emissions (y0 - A, mg kg-

(2)

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1

) and the rate constant of the precursor with OH (k, cm3 molec-1 s-1). This fit equation assumes

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the initial OH reaction is the rate-limiting step. Methacrylic acid is excluded from this analysis

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because photochemical production was small, uncertainties in emissions factors were high and 9



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the resulting fits were poor (r2 < 0.7 between the observed and fit emission ratios). The oxidation

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rate constants for each organic acid are similar for both engine conditions and fuel types — the

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differences lie in the magnitude of precursor emissions. The fits suggest that organic acid

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precursor emission rates are 3 – 4x larger for idle engine conditions than load engine conditions.

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This scaling of organic acid precursor emissions is slightly narrower and at the lower end of the

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range in total organic gas emission enhancements for idle versus load engine conditions. The

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oxidation kinetics for production of the four acids range from (5.9 – 18) x 10-12 cm3 molec-1 s-1,

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and are consistent with reported rate constants for the reactions of OH with anthropogenic

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hydrocarbons, such as n-hexane and n-decane (5.20 x 10-12 and 1.1 x 10-11 cm3 molec-1 s-1,

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respectively).49 Thus, the differences in the initial oxidation levels under loaded versus idle

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conditions due to completeness of combustion do not substantially impact the rate of secondary

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organic acid production.

209 210

SOA production

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The change in precursor emissions as a function of engine load impacts secondary production

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of not only organic acids, but also of organic aerosol. Loaded conditions emit less unburned fuel

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due to more complete combustion, contributing to different primary compositions and thus

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different extents of gas-phase organic oxygenation with oxidation.7,

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intermediate volatility organic compounds from raw primary exhaust have been shown to be

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higher at idle conditions.33, 50, 52 The fits of photochemical production of organic acids described

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above suggest the idling conditions produce not only more primary organic acids, but also more

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precursors for secondary formation of organic acids, than loaded conditions; however, this raises

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the question of the extent to which hydrocarbon precursor emissions impact secondary organic 10


33, 50, 51

Both volatile and

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aerosol formation, and the extent to which organic acids can be used as tracers for this chemistry.

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While the differences in photochemical aging in each experiment varied due to the high but

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variable OH reactivities present, Figure 2a demonstrates that the enhancement in organic aerosol

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due to secondary chemistry was consistently larger for idle conditions than for loaded conditions

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– a similar trend to the organic acids described above. Also similar to patterns in the organic

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acids, primary emissions and secondary production of organic aerosol were indistiguishable for

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biodiesel and diesel fuels.

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The carbon in organic aerosol is 0.06-1.4% of that in CO + CO2 for idle conditions, and 0.03-

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0.34% for loaded conditions (both fuel types). The carbon in C1-C4 organic acids is only 0.08-

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0.2% of the carbon in CO+CO2 for idle conditions and 0.07-0.13% for loaded conditions. Thus,

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as expected, both organic aerosol and short-chain organic acids are small carbon reservoirs.

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However, both forms of organic carbon can have large implications to human health on short

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timescales. Particles impact respiratory and cardiovascular health; for example, fine particulate

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concentrations of 30 µg m-3 can potentially increase mortality rates by up to 30%.53 The organic

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acids and other emitted reactive organic carbon in the gas-phase spurs chain initiation of ozone

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production, particularly in urban environments, which are typically NOx-saturated. The relatively

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large secondary formation of SOA and organic acids relative to primary emissions observed in

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these experiments emphasizes the importance of considering secondary chemistry of vehicle

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exhaust to avoid underestimating human health impacts from particulate matter and ozone from

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transport-related sources.

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To gain further insight in the relative importance of secondary production of organic acids

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versus aerosol from diesel exhaust, we examine the bulk properties of the detected gas and

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particle phases. The AMS provides a quantitative measurement of the elemental composition of 11



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bulk non-refractory aerosol, while the acetate CIMS provides only a qualitative metric. To obtain

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elemental composition of organic compounds in the gas-phase, we use the full suite of organic

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species detected by acetate-CIMS, which we define as any ions containing C, H and O atoms

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(hereafter referred to as CHO species). The elemental composition of each ion is weighted by ion

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signal to calculate elemental ratios of C, H and O. This gas-phase elemental composition is

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qualitative due to uncertainties around the variance in instrument sensitivity and the assumption

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that each molecule is detected with uniform sensitivity by acetate-CIMS.54 Trends in

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composition (i.e. increases or decreases in oxygenation) are robust, as interferences from ion-

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molecule clustering or variance in sensitivity should not systematically affect the relative

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direction of changes in total gas-phase elemental composition. Both particles and gases exhibit

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increased O:C and decreased H:C ratios with increased OH exposure (see color bar).

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Interestingly, the O:C of both primary organic aerosol and organic acidic gas emissions is higher

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for idle than loaded conditions in these experiments, despite the fact that combustion under

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loaded conditions should be more complete and thus more oxidized. While the different

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photochemical ages in each experiment make the comparison less obvious, experiments do have

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higher O:C (and lower H:C) values in both the gas- and particle-phase for idle (closed marker)

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engine conditions versus loaded (open marker) engine conditions at a given photochemical age

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(Figure 2b). For example, the particle phase O:C (H:C) shifts from 0.05 (2.1) at 0 days of

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equivalent OH aging to 0.45 (1.67) for 2.2 days of equivalent OH aging; the gas-phase acid O:C

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(H:C) shifted from 1.1 (1.9) to 1.9 (1.1) during the same experiment. This greater degree of

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oxygenation correlates with the total organic aerosol concentration: under idle conditions, the

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engine emissions generate more SOA than under loaded conditions (Figure 2a). Interestingly, the

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bulk properties of both engine loads and both fuel types overlap and follow the same trajectory in 12


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elemental composition, suggesting that the chemistry of the combustion engine follows a similar

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chemical trajectory as the chemistry in the OFR in both the gas and particle phase. The O:C

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ratios observed in the gas-phase are greater than in the particle phase but fall within a much

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narrower dynamic range; this could be due to instrument artifacts from the acetate-CIMS and the

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narrow window of gas-phase molecules observed (i.e. the bias towards carboxylic acids, which

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are more oxygenated than other functional groups), or could represent a real trend in the gas-

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phase products of combustion to highly oxidized and relatively volatile products. Further

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investigation with more quantitative metrics is warranted.

274 275 276

ATMOSPHERIC IMPLICATIONS

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Formic acid closely followed nitric acid (HNO3) during 2013 summertime field measurements

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in Brent, AL as part of the Southern Oxidant and Aerosol Study, with high correlations (r2 =

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0.78).22, 23 This correlation was used as evidence that formic acid was driven by OH chemistry in

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that biogenic-dominated system, and that formic acid had a comparable lifetime to HNO3.23

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Here, we see similarly strong correlations (r2 = 0.7 – 0.9) between HNO3 and formic, butyric and

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propanoic acids (Figure 3ab, Figure S5). The correlation between HNO3 and methacrylic acid is

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weak (r2 = 0.4), confirming the lack of a significant photochemical source of that acid from

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diesel exhaust. OH-driven production of formic, butyric and propanoic acids thus likely occurs in

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urban environments influenced by diesel emissions. However, the OFR provides a measurement

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of photochemical production with little influence of dry deposition or other loss factors expected

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in the ambient environment..

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While these small organic acids are well correlated with HNO3, they are less well correlated

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with organic aerosol concentrations (r2 = 0.1-0.6; Figure 3c,d; Figure S5). Although the binned

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data looks visually similar to Figures 3a and b, points at low aerosol concentrations skew the

291

correlation. This suggests that SOA production occurs on a different timescale from the

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photochemical formation of formic, butyric, and propanoic acids. The rate of gas-phase

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oxidation of engine emissions is fast (5.9 – 18 x 10-12 cm3 molec-1 s-1, Table 2), while the time to

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partition to the particle phase and establish equilibrium takes longer.55 While the utility of OFRs

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to study SOA formation is under debate

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SOA formation may occur (albeit to a lesser extent) in the ambient atmosphere. This highlights

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the limitation of using only equilibrium considerations, rather than kinetics, to accurately model

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SOA growth in an OFR.

38, 39, 56

, this lagtime between gas-phase oxidation and

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Fuel type (diesel vs biodiesel) did not impact the organic aerosol mass concentrations or gas-

300

phase organic acid chemistry. However, different engine loads produced different amounts of

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both primary and secondary organic acids and aerosol. The primary emissions of both gas-phase

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acids and particle-phase organics correlate with differences in total hydrocarbon emissions,

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although loaded conditions produce relatively more organic acid per hydrocarbon than idle

304

conditions. This would be consistent with different combustion cycles for the idle versus load

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engine conditions producing not only different amounts of precursor emissions, but also slightly

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less oxidized hydrocarbons. However, we note that the bulk O:C ratios of both the CIMS and

307

AMS measurements contradict this idea, as they are higher for idle than loaded conditions. The

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fit parameters from organic acid emission factors and secondary production factors as a function

309

of OH exposure time suggest that the magnitude of precursor emissions (i.e. y-intercepts) are

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greater for idle conditions than loaded conditions (Table 2). The relative magnitudes of these 14


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precursor emissions for idle versus loaded conditions (idle precursor emissions are 4-6 x larger

312

than loaded conditions) were different from the relative magnitude of total hydrocarbon

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emissions (idle precursor emissions are 4-26 x larger than loaded conditions), suggesting that the

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level of oxidation in hydrocarbon precursors (controlled by the completeness of combustion,

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higher in loaded conditions than idle) also influences the secondary production of organic acids.

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That is, the extent of oxidation and magnitude of the precursor mix does not alter the chemical

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pathways that ultimately produce carboxylic acids. This is different from the formation of

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secondary organic aerosol in this system. The organic aerosol oxidation chemistry in the OFR

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follows similar trajectories in terms of elemental composition (O:C and H:C) for the different

320

engine loads despite different precursor emissions – the more complete combustion of load

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engine conditions merely starts the OFR oxidation process at a more oxidized point along the

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trajectory than the less complete combustion of idle engine conditions. The evolution of

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elemental ratios during OFR oxidation appears similar to the combustion chemistry. Consistent

324

with this interpretation, Jathar et al.19 determined that SOA yields from different loaded

325

conditions and fuel types were similar across OA mass concentrations and precursor emission

326

magnitudes.

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Observations of small organic acids are useful metrics by which to test our understanding of

328

oxidation chemistry using model-measurement comparison. The observations described herein

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point to not only a primary source of these compounds from diesel exhaust, but also a complex

330

secondary source with hydrocarbon precursors that do not linearly follow changes in

331

hydrocarbon emissions. Recent studies suggest a large missing source of formic acid on both

332

global and regional scales.22,

333

formic acid from anthropogenic sources such as diesel engine exhaust could be a relevant source

24, 26

Our measurements imply that photochemical production of

15



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of formic acid in urban environments. For the California South Coast Air Basin region, we

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estimate 40 kg/day of formic acid emitted from non-road diesel engines and up to 1000 kg/day of

336

formic acid formed including the photochemical enhancement of formic acid emission factors

337

over 10 hours of aging. This estimate is based on the measured formic acid:CO concentration

338

ratio in this study (0.07 mmol formic acid mol CO-1) and CO emissions from non-road diesel

339

vehicles (619,000 kg/day57). Sources of formic acid, including both direct emission and in situ

340

formation, at a coastal site in Southern California were estimated at 31,600 kg/day.58 Satellite

341

measurements suggest 1 x 1011 kg of formic acid are produced globally per year from forest

342

emissions.14 Photochemical sources of formic acid from diesel exhaust may be one contribution

343

to formic acid concentrations in urban regions or regions of low biogenic emissions. However,

344

the secondary formic acid source from diesel exhaust is insufficient to account for the

345

discrepancy between modelled and measured formic acid concentrations.

346

Previous studies have suggested a secondary anthropogenic source of formic, butyric, and

347

propanoic acids,24, 26 and the experiments described herein validate this hypothesis and constrain

348

their formation rates. The production of secondary organic acids does not directly correlate with

349

the production of secondary organic aerosol, suggesting that the OH-driven formation of formic

350

acid operates at a different timescale from the production of organic aerosol, at least in this diesel

351

exhaust OFR experiment. This highlights the necessity for separate tracers for these processes,

352

despite both being driven by OH photochemistry of hydrocarbon emissions: while the

353

hydrocarbon precursors for both organic aerosol and acids correlate with the total primary

354

hydrocarbon emissions, different subsets of those emissions may drive production rates. These

355

experiments highlight the importance of including both primary and secondary anthropogenic

356

sources in budgets of these lower molecular weight organic acids, although these sources may 16


Page 17 of 25


357

change in the future with emerging engine technologies (i.e., diesel particulate filters, selective

358

catalytic reduction). Our measurements also confirm that methacrylic acid does not have a strong

359

primary or secondary anthropogenic source from diesel exhaust, and may remain an adequate

360

tracer for biogenic sources (i.e., isoprene and monoterpene oxidation). Measurements of

361

methacrylic acid in urban regions may be due to nearby biogenic sources or transport of biogenic

362

oxidation products, or non-diesel anthropogenic sources.

363

364 365

Table 1. Ranges for both primary emission factors and secondary production factors of formic,

366

butyric, propanoic, and methacrylic acids. Ranges of secondary production factors as a function

367

of photochemical age are shown within the parentheses, while the primary emission factor is

368

shown outside the parentheses.

369 370

Table 2. Fit parameters of the emission factors for formic, butyric and propanoic acid as a

371

function of OH exposure time to equation 2 for either idle or loaded conditions. Biodiesel and 17



Page 18 of 25

372

diesel conditions are grouped together due to their similar emissions pattern. The primary

373

emission factor (mg kg-1) is taken as the difference between y0 (mg kg-1) and A (mg kg-1). The

374

precursor emission factor (mg kg-1) is taken as the y-intercept y0. The rate constant k is in units

375

of cm3 molec-1 s-1. r2 values refer to the correlation between the observed emission factors and

376

the fit emission factors from equation 2.

377 378

Figure 1. Average emission factors and secondary production for formic (a), butyric (b),

379

propanoic (c), and methacrylic (d) acids from a diesel engine as a function of oxidative age. Blue

380

points refer to diesel fuel, red points refer to biodiesel fuel; circles refer to idle engine conditions

381

and triangles refer to loaded engine conditions. Error bars represent the standard deviation of the

382

mean. The dashed line indicates the fit from Equation 2; fit parameters are shown in Table 2.

383

18


Page 19 of 25


384 385

Figure 2. a) Organic aerosol mass enhancement (total organic aerosol / primary organic aerosol

386

concentration) as a function of estimated photochemical age. Colors indicate fuel type (blue and

387

red for diesel and biodiesel, respectively), and the symbols indicate engine condition (circles and

388

triangles for idle and load, respectively). Error bars represent standard deviation of the mean for

389

2 experiments for each fuel type and load condition. b) The Van Krevelen diagram of the bulk

390

organic aerosol (AMS) and gas-phase (acetate-CIMS) elemental ratios for one representative

391

experiment for each fuel type and engine load. The color indicates estimated photochemical age

392

(OH equivalent days). Black symbols refer to zero-OH oxidation.

393 394

19



395

Page 20 of 25

.

396 397 398

Figure 3. Correlations of formic acid and propanoic acid with nitric acid (a,b) and organic

399

aerosol (c,d). The color of each point represents the estimated photochemical age (OH equivalent

400

days). Each point and error bar represents an average and standard deviation of the signals for all

401

experiments under all engine conditions and fuel types (n = 7).

402

ASSOCIATED CONTENT

20


Page 21 of 25


403

Supporting Information. Figures and text describing the percent contribution of the four

404

organic acids to the measured organic aerosol concentrations, estimated mass concentrations of

405

the CHO species measured in the CIMS, and correlations of butyric and methacrylic acids with

406

nitric acid and organic aerosol are shown in the Supporting Information. This material is

407

available free of charge via the Internet at http://pubs.acs.org.

408

AUTHOR INFORMATION

409

Corresponding Author

410

*E-mail: [email protected]

411

Author Contributions

412 413

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

414 415

ACKNOWLEDGEMENT

416

This worked was funded by an Arnold and Mabel Beckman Young Investigator Award (DKF)

417

and Colorado State University (SJ). We thank Kirk Evans, Liam Lewane and Nathan Reed for

418

support. Data can be acquired per request to the corresponding author.

419 420 421 422 423 424

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