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Latitudinal and Seasonal Distribution of Particulate MSA over the Atlantic using a Validated Quantification Method with HR-ToF-AMS Shan Huang, Laurent Poulain, Dominik van Pinxteren, Manuela van Pinxteren, Zhijun Wu, Hartmut Herrmann, and Alfred Wiedensohler Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03186 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Environmental Science & Technology

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Latitudinal and Seasonal Distribution of Particulate

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MSA over the Atlantic using a Validated

3

Quantification Method with HR-ToF-AMS

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Shan Huang†‡, Laurent Poulain†*, Dominik van Pinxteren†, Manuela van Pinxteren†, Zhijun

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Wu§, Hartmut Herrmann†, Alfred Wiedensohler†

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†

Leibniz Institute for Tropospheric Research, Leipzig, Sachsen, 04318, Germany

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‡

Institute for Environmental and Climate Research, Jinan University, Guangzhou, Guangdong,

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511443, China

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§

11

China

College of Environmental Sciences and Engineering, Peking University, Beijing, 100871,

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*Corresponding author: Laurent Poulain, Leibniz Institute for Tropospheric Research,

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Permoserstr. 15, 04318 Leipzig, Germany. Email: [email protected]; Tel: +49 341 2717 7316;

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Fax: +49 341 2717 997316.

16 1

ACS Paragon Plus Environment


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Abstract

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Methanesulfonic acid (MSA) has been widely used as a proxy for marine biogenic sources, but it

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is still a challenge to provide an accurate MSA mass concentration with high time resolution.

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This study offers an improved MSA quantification method using high resolution time-of-flight

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aerosol mass spectrometer (HR-ToF-AMS). Particularly, the method was validated based on an

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excellent agreement with parallel offline measurements (slope = 0.88, R2 = 0.89). This

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comparison is much better than those using previously reported methods, resulting in

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underestimations of 31 to 54% of MSA concentration. With this new method, MSA mass

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concentrations were obtained during 4 North/South Atlantic cruises in spring and autumn of

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2011 and 2012. The seasonal and spatial variation of the particulate MSA mass concentration as

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well as the MSA to non-sea-salt sulfate ratio (MSA:nssSO4) over the North/South Atlantic

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Ocean were determined for the first time. Seasonal variation of the MSA mass concentration was

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observed, with higher values in spring (0.03 µg m-3) than in autumn (0.01 µg m-3). The

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investigation of MSA:nssSO4 suggests a ubiquitous and significant influence of anthropogenic

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sources on aerosols in the marine boundary layer .

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1. Introduction

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Methanesulfonic acid (MSA) has been widely found in submicrometer marine aerosol particles,

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and could be even a dominant secondary organic compound.1 Because it is formed exclusively

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by the oxidation of dimethylsulfide (DMS), which is released by phytoplankton, MSA can be a

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tracer for marine secondary organic aerosol (SOA).2-5 Also, MSA can enhance the formation of 2


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molecular clusters of sulfuric acid and amines6 and contribute significantly to particle growth.7-9

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For better understanding SOA formation and transport, accurate quantification methods of MSA

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mass concentration with a high time resolution are required.

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In the past, MSA was almost only detected by offline methods offering a time resolution from

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several hours to days. Improvements in online techniques over the last decade have allowed the

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opportunity to access a high time resolution (several minutes); these techniques include chemical

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ionization mass spectrometry (CIMS),7 particle-into-liquid sampler - ion chromatography (PILS-

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IC),10 quadrupole aerosol mass spectrometer (Q-AMS),11-13 and high resolution time-of-flight

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aerosol mass spectrometer (HR-ToF-AMS).14-17 Among these techniques, the aerosol mass

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spectrometer (AMS) is considered as one of the most widespread online instruments, measuring

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non-refractory (NR) components of particles in the submicrometer size range. MSA is not an

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individual compound typically provided by the AMS, as its signals are interpreted as organics,

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sulfate, and nitrate. The previously published quantification methods of MSA using AMS are

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similar but not identical. Additionally, a method validation by direct comparison between AMS

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and parallel measurements using a chromatographic MSA quantification by offline analysis has

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rarely been reported.11, 12 Therefore, the accuracy of the resulting MSA mass concentration in

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field measurement conditions based on the different approaches is still questionable.

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Inconsistencies have been found among the MSA mass concentrations provided by different

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quantification methods working on the same AMS measurement dataset.18 Both a systematic

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calibrated relative ionisation efficiency (RIE) of MSA and a more precise reference (MSA mass

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concentration in submicrometer particles from those measurements parallel to AMS) were

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absent.

3



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To fill this gap, the present study improves the MSA quantification method using HR-ToF-AMS

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and evaluates three previously published methods.14,

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concentrations derived from a unique dataset containing HR-ToF-AMS measurements on board

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the German research vessel Polarstern during 4 cruises over the Atlantic Ocean are investigated.

15,

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Additionally, MSA mass

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2. Experiment

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Standard calibrations. To obtain mass spectral patterns of MSA, standard calibrations using the

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HR-ToF-AMS (Aerodyne Research, Inc., USA; referred to as AMS in the following text unless

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otherwise noted) were performed in the laboratory (i.e., between the cruises) and on site during

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the Polarstern campaigns. In both cases, MSA aerosol particles were nebulized by a nebulizer

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(TSI, Model 3076) from an aqueous solution of pure MSA (Sigma-Aldrich, purity >= 99.5%)

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prior to entering the AMS. Generated aerosol particles crossed a silica gel dryer to remove a

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large amount of water and keep the relative humidity (RH) of measured particles lower than

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40%. Both polydispersed MSA particles and monodispersed particles with specific size were

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selected by a differential mobility analyser (DMA, Hauke-type) and were measured by the AMS.

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The principle and instruction of the AMS was already detailed in previous publications.19, 20

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Inside the AMS, NR submicrometer particles were flash vaporized at approximately 600°C and

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then ionized to become charged fragments, which were analysed by the mass spectrometer of the

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AMS.

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To determine the RIE of MSA (RIEMSA), laboratory calibrations were carried out jointly using

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the AMS and a condensation particle counter (CPC, Model 3010, TSI Inc., USA). Different sizes 4


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(200, 225, 250, 300, and 350 nm) of monodispersed MSA aerosol particles were measured. The

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mass concentrations were kept low (< 2 µg m-3) to avoid multiple charging of particles. The

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dilution of the MSA during the calibration was made by mixing generated MSA particles with

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particle-free air. With a known particle diameter (Dp), the MSA density (ρ = 1.42 g cm-3) and the

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particle number concentration (PNC) measured by the CPC, PNC-derived MSA mass

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concentration ([MSA]CPC, in µg m-3) could be calculated using eq 1:

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[] =

(1)

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Ambient measurements. The intensive aerosol particle measurements on Polarstern were

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performed during 4 scientific cruises (CR) between Bremerhaven (BH, Germany, 53°33′N,

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8°35′E) and Cape Town (CPT, Republic of South Africa, 33°55′S, 18°25′E) or Punta Arenas

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(PA, Republic of Chile, 53°10′S, 70°56′W). These expeditions included: (1) two spring seasons

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(in the Northern Hemisphere, NH) cruises: CR1 (ANT-XXVII/4, CPT - BH, 20.04 - 20.05.2011)

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and CR3 (ANT-XXVIII/5, PA - BH, 10.04 - 15.05.2012), and (2) two autumn seasons (in the

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NH) cruises: CR2 (ANT-XXVIII/1, BH - CPT, 28.10 - 01.12.2011) and CR4 (ANT-XXIX/1, BH

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- CPT, 27.10 - 27.11.2012). The ship track of each cruise is shown in Figure 2. During the

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Polarstern campaigns, the AMS was deployed in an air-conditioned container located on the first

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deck of the ship (just above the bridge). It ran alternatively between V-mode (MS + PToF modes;

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MS for “mass spectrum”, and PToF for “particle time of flight”) and W-mode (only MS-mode)

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at a time resolution of 2 min. A constant collection efficiency (CE) of 0.7 was determined by the

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inter-comparison of total or speciation mass concentrations between AMS and parallel

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measurements (detailed in SI-1). This CE was applied in all Polarstern measurements to calculate

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bulk and speciation mass concentrations. Additionally, the composition dependent collection 5



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efficiency (CDCE)21 is discussed in SI-1. All AMS data were analysed with the toolbox Squirrel

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v1.56D for the unit mass resolution (UMR) data analysis and the Pika v1.15D for the high

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resolution (HR) data analysis based in Igor Pro (WaveMetrics Inc., version 6.22A).

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To reduce the signal noise, 20min-average (corresponding to approximately 0.05° of latitude)

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values of the MSA mass concentrations will be used in the following analysis. Particle-free

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measurements provide the detection limit of MSA, which could be calculated as three times the

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standard deviation of the mass concentration.20 For the 20min averaged data, the detection limits

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are equal to original detection limits (2min data) divided by √10 (the square root of the ratio

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between averaged time resolution and default time resolution). The detection limits of MSA

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(20min) during the four cruises are: 0.002 µg m-3 (CR1), 0.003 µg m-3 (CR2), 0.002 µg m-3

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(CR3), and 0.006 µg m-3 (CR4). They are within the range of 0.0009 µg m-3 reported by Zorn et

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al.14 using HR-ToF-AMS to 0.025 µg m-3 reported by Langley et al.12 using Q-AMS.

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For offline measurements, a high volume Digitel filter sampler (DHA-80, Digitel Elektronik AG,

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Switzerland) was sitting on the roof of the container to collect daily filter samples of particulate

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matter with diameter smaller than 1 µm (PM1) from midnight to midnight (coordinated universal

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time). To obtain MSA mass concentrations, these filter samples were later analysed using

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capillary electrophoresis with a UV-detector (CE-DAD, Spectra Phoresis 1000, Thermo

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Separation Products, Waltham, MA, USA) as described in previous study.22 Due to

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contamination by either ship exhausts (soot) or seawater in stormy weather, only 45 filter

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samples (i.e., 45 days’ worth of data out of 86 sampling days) can represent Atlantic aerosols.

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6


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3. Results and discussions

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Quantification of MSA. According to the standard calibrations using polydisperse MSA aerosol

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particles, the mass spectral pattern of pure MSA is obtained and shown together with those

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provided in previous studies (SI-2, Figure S2).11, 17 Compared to the Q-AMS which can only

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provide the UMR mass spectra, the HR-ToF-AMS can easily identify ions missed (e.g., CH2+,

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m/z 14) or wrongly recognized (CH2S+, m/z 46, as “nitrate”) in the UMR mass spectra. The

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intensity variation of each MSA fragment (error bars in Figure S2) is on average 23.7 ± 11.0 %

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(standard deviation / the mean intensity), indicating that the MSA mass spectral pattern based on

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several standard calibrations covering two years is relatively stable. Exceptions involve

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ammonium related ions such as NH2+ (m/z 16) and NH3+ (m/z 17) with intensity variations of

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approximately 40%. NH2+ and NH3+ do not correspond to MSA, but could be attributed to

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ambient NH3 contamination or even the water effect (the huge water peak may influence the

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quantification of the adjacent NH+ peak). Moreover, the signal at m/z 18 (H2O+) could mainly be

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related to residual water from particle generation. Thus, these signals are removed from the MSA

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spectral pattern used later for quantification.

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As the most prominent MSA fragment containing a sulfur atom14 and a negligible ion

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contribution in ammonium sulfate, sulfuric acid, and organo-sulfates,12, 17 the fragment CH3SO2+

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is selected as the reference ion for MSA. With the stable mass spectral pattern of pure MSA,

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other MSA fragments can be calculated in relation to the CH3SO2+ intensity obtained from the

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HR mass spectra analysis. Accordingly, all MSA fragments can be extracted from the ambient

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aerosol particle signals. Although MSA mass concentration can be calculated solely using HR 7



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data, the fragmentation table23 embedded in the UMR data analysis tool Squirrel allows users to

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obtain both MSA mass concentration and more accurate mass concentrations of sulfate, organics

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and nitrate which exclude MSA signals. For this purpose, the fragmentation table was modified

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from the default one as follows: (1) establishing a specific MSA column containing MSA

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fragments at 32 different mass-to-charge ratios based on their individual ratio to CH3SO2+ and

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(2) excluding MSA signals from the default organics, sulfate and nitrate. Finally, the sum of all

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cells in the MSA column gives the total MSA intensities, which can be converted to mass

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concentration.

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Since the UMR fragmentation table was used, the cell at m/z 79 of the MSA column originally

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represents all fragments at this m/z. There are 5 different ions detected at m/z 79 adjacent to

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CH3SO2+ (SI-2, Figure S3) by the HR mass spectrometer, including Br+ (m/z 78.918), CH3O4+

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(m/z 79.003), C5H3O+ (m/z 79.018), C5H5N+ (m/z 79.042) and C6H7+ (m/z 79.055). All of these

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ions were possible to be found in ambient measurements, so all of them were chosen to fit the

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mass spectrum. Hence, a factor representing the fractional contribution of the key ion CH3SO2+

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to the total m/z 79 is needed for extracting the CH3SO2+ signal. This factor f(CH3SO2+) can be

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calculated with HR data as shown in eq 2: =

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!"# ∑ %&'(

(2)

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where ICH3SO2 is the signal intensity of CH3SO2+ and ∑Imz79 is the total signal intensity of all ions

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at m/z 79. The f(CH3SO2+) is not a constant value, but varies as a function of time, which can

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enhance the accuracy of MSA quantification. The modified fragmentation table with the factor at

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m/z 79 is presented in Table S2. 8


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According to the comparison between MSA mass concentrations from AMS and those estimated

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by PNC from CPC (detailed in SI-3, Figure S4), an RIE of 1.2 can be determined for MSA for

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the current instrumental conditions. The resulting value of RIE is comparable to the one (1.15)

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used by Langley et al.12 and Phinney et al.11 and the average (1.3) of RIEs for organics and

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sulfate estimated by Zorn et al.14 and Huang et al.17 Although the RH was always lower than 40%

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in calibrations, a certain amount of residual water cannot be excluded. Hence, a unit CE was

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used because MSA particles should be liquid droplets with virtually no loss due to bouncing on

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the AMS vaporizer. Also, the organic components in aerosol particles may be independent of the

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water content, making the residual water only affect the particle diameter. Given that the

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uncertainty of the size accuracy of DMA was small (±3.5%),24 the water content should have

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negligible influence on the PNC estimation, and a limited effect on RIE determination.

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Method validation. The present quantification method was directly applied to the AMS dataset

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during the Polarstern measurements. The calculated MSA mass concentration is compared to that

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obtained from parallel filter measurements. As shown in Figure 1a, excellent agreement is found

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(slope = 0.88, R2 = 0.89), which validates the present method for ambient measurements. The

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intercept of 0.002 is considered to be negligible as compared with the median of MSA mass

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concentrations derived from offline measurements (0.023 µg m-3). Considering that MSA mass

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concentrations are not normally distributed (the standard deviation is as high as the average

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value), the median value is used in this study to present the variation of MSA. Using offline

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measurements as the reference, the AMS slightly underestimated the MSA mass concentration

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by 12%, which is within the AMS measurement uncertainty of ±30%.25-27 It is worth noting that

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this discrepancy includes not only AMS uncertainties but also those of the individual upper size

9



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cutting (PM1 for offline and near-PM1 for the AMS) as well as uncertainties associated with the

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offline techniques (e.g., adsorption and desorption of reactive species and water, analytical

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uncertainties, etc.).

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On the other hand, all of the previous methods could not properly estimate MSA mass

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concentration when applying them individually on the Polarstern campaigns as shown in Figure

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1b (the Phinney-Langley method), 1c (the Ge method) and 1d (the HKUST method). The

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Phinney-Langley method refers to the MSA quantification method with Q-AMS firstly published

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by Phinney et al.11 and later detailed by Langley et al.12 The Ge method estimates MSA mass

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concentration by scaling up three characteristic ions (see SI-4) suggested by Ge et al.15 The

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HKUST method, described in SI-4, is published by Huang et al.17 from Hong Kong University of

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Science and Technology (HKUST) based on a HR-ToF-AMS. In general, all three methods

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underestimate the MSA mass concentration compared to that from filter measurements, only

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taking 31% to 54% of the latter. The relatively best representativeness (slope = 0.54) but worst

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correlation (R2 = 0.78) is found in the case using the Phinney-Langley method with its RIE of

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1.15 (Figure 1b). The intercept is not negligible (0.007 µg m-3), accounting for 30% of the

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median of MSA mass concentrations derived from filter measurements. This could be attributed

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to the low mass resolution (i.e., UMR) provided by Q-AMS. In the Phinney-Langley method, the

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intensity of the key ion CH3SO2+ (m/z 78.985) is equal to total intensity of all fragments at m/z

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79 minus the intensity of non-MSA ions (e.g., C6H7+, m/z 79.055), which is assumed to be the

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same as their relative ions such as C7H9+ (m/z 93.070). This may lead to an incorrect intensity of

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the key ions if those relative ions are from different origins. It will consequently enlarge the

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uncertainties of MSA estimation. Moreover, the Q-AMS may improperly interpret some

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fragments of MSA, e.g., CH2+ and CH2S+ as mentioned before, resulting in an underestimation of 10


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the MSA mass concentration. Nevertheless, both the Ge method and the HKUST method provide

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MSA mass loadings using the HR approach that correlated well with that from the offline

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approach (R2 = 0.90 and 0.89, both with a negligible intercept: 0.001). It confirms that the

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selected key ion(s) are well representative for MSA.

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In the Ge method, three characteristic fragments are considered and scaled up to calculate the

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total MSA mass concentration (SI-4, eq S1).15 Organic-equivalent mass concentrations of the

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three key ions take 14.7% (i.e., the scaling factor) of the total MSA mass concentration

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according to the HR mass spectrum of pure MSA.15 The MSA mass concentration by applying

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this scaling factor only represents one-third (33%) of that from offline measurements (Figure 1c),

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suggesting that this factor is not properly used, at least for the Polarstern measurements. A

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similar scaling factor derived from the standard MSA spectral pattern in the Polarstern study was

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0.068 ± 0.007. However, when applying this factor to the equation used by Ge et al.,15 the

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updated MSA mass concentration is still 33% lower than that from offline measurements (slope

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= 0.67, intercept = 0.002, R2 = 0.90).

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The HKUST method considered the RIE of each species and the CE (details are provided in SI-4,

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eq S2 and eq S3). However, it also cannot appropriately estimate the MSA mass concentration

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during the Polarstern cruises compared with results from the offline method, giving only 31% of

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what the filter measured (Figure 1d). In this method, 9.7% is a key value (eq S2) showing the

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contribution of CH3SO2+ intensity to total MSA fragments in the standard mass spectrum, and

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the RIEMSA of 1.3 is the average of RIEs for organics and sulfate.17 However, in the present

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study, these two factors were 4.2 ± 0.5% and 1.2, respectively, according to the standard

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calibrations. Applying them to the HKUST method can markedly reduce the gap between MSA 11



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mass concentrations from AMS and from filter measurements (slope = 0.88, intercept = 0.001,

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R2 = 0.91). Thus, this could be used as a simpler estimation method without modifying the

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fragmentation table but cannot give more accurate mass concentrations of organics, sulfate, and

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nitrate excluding MSA signals.

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Although the three different methods can properly reproduce the dynamics of the MSA

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concentrations, they all strongly underestimated the real concentration if directly applying them.

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The differences of MSA concentrations from the three quantification methods can be directly

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attributed to discrepancy of the MSA fragment pattern (fractional contribution). For example, the

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low intensities of the main sulfate ion SO+ and SO2+ (comparing with the key ion CH3SO2+) in

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the Phinney-Langley method (SI-2, Figure S2) significantly contributed to the underestimation of

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MSA mass concentrations when applying this method to the Polarstern dataset. The

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fragmentation pattern and RIE might be linked to instrumental dependency (e.g., vaporization

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temperature, ionization source and mass spectrometer tuning). Nevertheless, the influence of

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variable AMS vaporizer temperature on the standard MSA mass spectrum remains unclear. Zorn

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et al.14 suggested an instrument dependency of MSA quantification because the MSA spectral

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pattern varied dramatically while changing the vaporizer temperature from 160°C to 800°C. In

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contrast, Ovadnevaite et al.16 reported only a 2% variation in the signal intensity of a pure MSA

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mass spectrum over a smaller range from 550°C to 650°C; this range more closely approximates

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the AMS working conditions in the ambient measurements. Therefore, even though the

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previously reported methods have been used without calibration in some studies,13, 28 performing

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the standard calibrations before quantifying MSA using AMS is still highly recommended

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according to the method comparison and evaluation in this study.

12


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Latitudinal distribution of MSA. Based on the validated quantification method using AMS as

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previously discussed, the marine secondary product tracer MSA can be derived in high time

258

resolution during the different Polarstern measurements. As shown in Figure 2, the spatial and

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seasonal distribution of MSA mass concentration along the ship tracks during the four cruises is

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provided. Five-day air mass back trajectories at 950 hPa are also illustrated in Figure 2, with the

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background graph showing simultaneous mass concentration of chlorophyll-a (Chl-a).

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masses at 950 hPa (approximately 500 m above sea level) were selected to represent a well-

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mixing situation in the marine boundary layer (MBL) since the height of the MBL in the open

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ocean is from 400 m to around 1700 m.29 The data of the marine biomass indicator Chl-a (rolling

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32-day average) was obtained from the moderate resolution imaging spectroradiometer (MODIS)

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aboard the satellite Aqua (http://oceancolor.gsfc.nasa.gov/cgi/l3). The time resolution of Chl-a is

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lower than that of the MSA mass concentration (20min). Nevertheless, typical seasonal blooms,

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the spring blooms due to the seasonal increase in solar radiation and redistributed nutrients to

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surface waters, could often persist for a few weeks to months.30 Hence, the monthly Chl-a level

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can at least represent the spring blooms and report the highest spots of Chl-a observed during the

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selected period, consequently still providing the strongest source of DMS.

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Detailed seasonal average, median, and standard deviation values in both hemispheres are

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provided in Table S1. In general, MSA shows a relatively higher mass concentration in spring

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(0.03 µg m-3) than in autumn (0.01 µg m-3) in both hemispheres. This seasonal variation on mass

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concentrations is consistent with the previous measurements taken over the ocean.4, 16, 31-33 The

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median value of MSA concentrations in this study is similar to those values observed in the

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summer over the Atlantic Ocean in its northern (median varying from 0.02 to 0.04 µg m-3 for

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different marine air masses),16 tropical (from 0.027 to 0.063µg m-3, average: 0.042 µg m-3),34 and 13


Air


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southern regions (from

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