Evidence of haze-driven secondary production of supermicrometer aerosol nitrate and sulfate in size distribution data in South Korea

Abstract

This study reports measurements of size-resolved aerosol composition at a site in Incheon along with other aerosol characteristics for contrast between Incheon (coastal) and Seoul (inland), South Korea, during a transboundary pollution event during the early part of an intensive sampling period between 4 and 11 March 2019. Anthropogenic emissions were dominant in the boundary layer over the study region between 4 and 6 March, with much smaller contributions from dust, smoke, and sea salt. The meteorology of this period (shallow boundary layer, enhanced humidity, and low temperature) promoted local heterogeneous formation of secondary inorganic and organic species, including high nitrate (NO3-) relative to sulfate (SO42-). Seoul exhibited higher PM2.5 levels than Incheon, likely due to local emissions. The following findings point to secondary aerosol formation and growth sensitivity to water vapor during this pollution event: (i) significant concentrations of individual inorganic and organic acids in the supermicrometer range relative to their full size range (∼40 %) at higher humidity; (ii) high correlation (r=0.95) between oxalate and SO42-, a marker of secondary aqueous production of oxalate; (iii) increased sulfur and nitrogen oxidation ratios as a function of humidity; and (iv) matching composition apportionment (for soluble ions) between the PM1 and PM2.5−1 size fractions. The last finding confirms that PM1 aerosol composition measurements fully capture PM2.5 composition apportionment (for soluble ions) during haze events and may therefore be reliably applied in modeling studies of such events over the full PM2.5 size range. However, the differences evident in the periods following the haze event imply that under other atmospheric conditions PM1 composition measurements will not fully reflect the apportionment of PM2.5 aerosols. The study period was marked by relatively low temperatures that made NO3- the most abundant species detected, pointing to the sensitivity of PM2.5 levels and composition as a function of season during such transboundary events. For instance, other such events in previous studies exhibited more comparable levels between SO42- and NO3- coincident with higher temperatures than the current study. This dataset can contribute to future evaluation of model PM2.5 composition to better support regulatory efforts to control PM2.5 precursors.