Elucidating the sulfate formation during haze episodes in wintertime Beijing: insights from sulfur and triple oxygen isotope.

Frequently occurred severe haze events in heavily populated China in recent years have beenattributed to extensive industrial and residential emissions, together with enhancedsecondary aerosol formation under stagnant meteorological condition, especially inwinter. Sulfate, as the major inorganic component of fine particles (PM2.5), has beenfound to form rapidly under high relative humidity conditions. Currently, a largegap exists in model-estimated sulfate concentration and field observation, withthe latter often being magnitudes higher. While model simulation is improving,multiple oxygen and sulfur isotopic compositions of atmospheric sulfate offer anindependent, often specific constraint on atmospheric secondary sulfate formationpathways. Here we report inorganic components, together with sulfur (δ34S) and triple-oxygenisotopes (δ18O, Δ17O) of sulfate in PM2.5 samples collected every 12 hours in the city ofBeijing, northern China in winter 2015. During the sampling period, PM2.5 concentrationwent up to as high as 178.9 ± 154.9 μg m−3. Sulfate had an average concentration of 20.9 ±20.9 μg m−3, with a mean δ34S values of 5.5±2.4‰ ranging from 1.3‰ to 9.4‰Ṫhe δ34Sis higher during heavily polluted days (average at 6.8‰) than that during relatively cleandays (average at 2.0‰). Meanwhile, the average sulfate Δ17O (Δ17O = δ18O -0.5305×δ17O) during heavy haze episodes was 0.62 ± 0.33‰ suggesting that aqueousoxidation pathway of S(IV)–H2O2 played a significant role on the enhanced sulfateformation during the evolution of haze episodes associated with fog processing. [ABSTRACT FROM AUTHOR]