Fog composition at Baengnyeong Island in the eastern Yellow Sea: detecting markers of aqueous atmospheric oxidations

Abstract

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.

The acidity of atmospheric particles and clouds

“…A small fraction of nonvolatile aerosol components is sometimes present in the fine mode and tends to reduce acidity. For example, K + associated with biomass burning has been shown to cause higher pH compared to cases with very low K + levels (Bougiatioti et al, 2016).…”

“…Differences as large 4 pH units have been reported between fine and coarse particles Young et al, 2013). Bulk PM 1 and PM 2.5 acidity is more similar than fine-vs. coarse-mode acidity (pH F within 1-2 units, e.g., Bougiatioti et al, 2016;Guo et al, 2017b), but submicrometer (diameter < 1 µm) particles still show higher acidity than bulk PM 2.5 . The reason for this is the strong enrichment of aerosol with NVCs from dust and sea salt at the larger sizes (even in the fine mode) and role of sulfate in new-particle formation and surface-area-driven condensation at the small sizes ( Fig.…”

“…Although not strong sources of protons or cations, these alkaline and acidic organics may still be considered together with other water-soluble organic compounds (WSOCs) in the particulate phase in terms of their ability to influence the aerosol water content. The uptake of water due to organic components is often used to correct the solvent volume and pH F derived based on the inorganic aerosol composition (e.g., Guo et al, 2015;Bougiatioti et al, 2016). This implies that aerosol pH is reversibly influenced by the amount of water (driven by RH and composition) associated with the aerosol particles, which has been shown to drive some of the diurnal variability of pH .…”

“…Direct measurements may either provide an ensemble or bulk average pH value (Li and Jang, 2012;Jang et al, 2008;Ganor et al, 1993;, single-particle values (Craig et al, 2017;Rindelaub et al, 2016b), or intraparticle pH values (Wei et al, 2018). Considerable effort has also been spent to develop pH estimates using a combination of thermodynamic modeling with measurements of aerosol-and gas-phase composition (e.g., Guo et al, 2018aGuo et al, , 2017aGuo et al, , 2015Bougiatioti et al, 2016;Song et al, 2018;and others).…”

“…However, NH 3 measurements are challenging and often not colocated with aerosol composition measurements. Bougiatioti et al (2016) estimated that neglecting gas-phase NH 3 levels of about 0.1 to 0.7 µg m −3 in the thermodynamic equilibrium calculation of fine-aerosol pH could lead to an underestimation in the pH F of around 0.5 units, while Guo et al (2015) found neglecting gas-phase NH 3 leads to an underestimation of pH F by 1 unit. Although this magnitude of underestimation is not universally applicable when NH 3 is missing from the thermodynamic calculations, it may be a reasonable bound for most of the atmosphere.…”