Abstract. Samples of fog water were collected at Baengnyeong Island (BYI) in the Yellow Sea during the summer of 2014. The most abundant chemical species in the fog water were NH4+ (mean of 2220 µM), NO3− (1260 µM), SO4−2 (730 µM), and Na+ (551 µM), with substantial contributions from other species consistent with marine and biomass burning influence on some dates. The pH of the samples ranged between 3.48 and 5.00, with a mean of 3.94, intermediate within pH values of fog/cloud water reported previously in Southeast Asia. Back trajectories (72 h) showed that high relative humidity ( > 80 %) was encountered upwind of the sampling site by all but one of the sampled air masses, and that the fog composition at BYI can be impacted by several different source regions, including the Sea of Japan, southeastern China, northeastern China, and the East China Sea. Sulfur in the collected fog was highly oxidized: low S(IV) concentrations were measured (mean of 2.36 µM) in contrast to SO4−2 and in contrast to fog/cloud S(IV) concentrations from pollutant source regions; organosulfate species were also observed and were most likely formed through aging of mainly biogenic volatile organic compounds. Low-molecular-mass organic acids were major contributors to total organic carbon (TOC; 36–69 %), comprising a fraction of TOC at the upper end of that seen in fogs and clouds in other polluted environments. Large contributions were observed from not only acetic and formic acids but also oxalic, succinic, maleic, and other organic acids that can be produced in aqueous atmospheric organic processing (AAOP) reactions. These samples of East Asian fog water containing highly oxidized components represent fog downwind of pollutant sources and can provide new insight into the fate of regional emissions. In particular, these samples demonstrate the result of extensive photochemical aging during multiday transport, including oxidation within wet aerosols and fogs.
Abstract. Comprehensive techniques to describe the organic composition of atmospheric aerosol are needed to elucidate pollution sources, gain insights into atmospheric chemistry, and evaluate changes in air quality. Fourier transform infrared absorption (FT-IR) spectrometry can be used to characterize atmospheric organic matter (OM) and its composition via functional groups of aerosol filter samples in air monitoring networks and research campaigns. We have built FT-IR spectrometry functional group calibration models that improve upon previous work, as demonstrated by the comparison of current model results with those of previous models and other OM analysis methods. Laboratory standards that simulated the breadth of the absorbing functional groups in atmospheric OM were made: particles of relevant chemicals were first generated, collected, and analyzed. Challenges of collecting atmospherically relevant particles and spectra were addressed by including interferences of particle water and other inorganic aerosol constituents and exploring the spectral effects of intermolecular interactions. Calibration models of functional groups were then constructed using partial least-squares (PLS) regression and the collected laboratory standard data. These models were used to quantify concentrations of five organic functional groups and OM in 8 years of ambient aerosol samples from the southeastern aerosol research and characterization (SEARCH) network. The results agreed with values estimated using other methods, including thermal optical reflectance (TOR) organic carbon (OC; R2=0.74) and OM calculated as a difference between total aerosol mass and inorganic species concentrations (R2=0.82). Comparisons with previous calibration models of the same type demonstrate that this new, more complete suite of chemicals has improved our ability to estimate oxygenated functional group and overall OM concentrations. Calculated characteristic and elemental ratios including OM∕OC, O∕C, and H∕C agree with those from previous work in the southeastern US, substantiating the aerosol composition described by FT-IR calibration. The median OM∕OC ratio over all sites and years was 2.1±0.2. Further results discussing temporal and spatial trends of functional group composition within the SEARCH network will be published in a forthcoming article.
The current understanding of aqueous secondary organic aerosol (aqSOA) formation is based largely on laboratory investigations of very simple surrogate cloud water solutions that aid mechanistic understanding of aqueous oxidation but may not accurately reflect the influence of the complex ambient matrix present in authentic cloud waters on organic chemistry. In this study, unaltered ambient cloud water and 'biogenically influenced' ambient cloud water (with added pinonic acid) were photo-oxidized, atomized, and dried to simulate the formation of aqSOA in clouds, then analyzed using an Aerodyne Aerosol Mass Spectrometer. Two major chemical regimes were identified: in the first, particle organic mass is gained, then lost; sustained increases in highly oxidized fragments indicate overall organic acid formation, while increases in nominally volatile fragments suggest that evaporation may contribute to the observed mass decrease. In the second regime, the oxidation level of cloud water organic matter decreases as mass decreases, suggesting that oxidized functional groups are fragmented and lost to evaporation. Overall, the rate of aqSOA production in unaltered cloud water decreases as oxygenation increases, until organic mass loss beginning at consistent values of f44 > 0.23 ± 0.05 and O:C > 0.61 ± 0.05. We hypothesize that there may be a parameterizable 'maximum oxidation level' for cloud water above which functional group fragmentation is dominant. These experiments are among the first to quantify organic mass production in ambient cloud water and employ the most atmospherically relevant oxidant concentrations to date.
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