Measurement report: Effects of photochemical aging on the formation and evolution of summertime secondary aerosol in Beijing

Atmospheric submicrometer aerosols have a great effect on air quality and human health, while their formation and evolution processes are still not fully understood. Herein, the crucial role of atmospheric oxidation capacity, as characterized by OH exposure dose in the formation and evolution of secondary submicrometer aerosols, was systematically investigated based on a highly time-resolved chemical characterization of PM1 in a southern suburb of Beijing in summertime from 25 July to 21 August 2019. The averaged concentration of PM1 was 19.3 +/- 11.3 mu g m(-3), and nearly half (48.3 %) of the mass was organic aerosols (OAs) during the observation period. The equivalent photochemical age (t(a)) estimated from the ratios of toluene to benzene was applied to characterize the OH exposure dose of the air mass, in which an observation period with the similar sources and minimal influence of fresh emission was adopted. The relationships of non-refractory PM1 species, OA factors (i.e., one hydrocarbon-like and three oxygenated organic aerosol factors) and elemental compositions (e.g., H/C, O/C, N/C, S/C, OM/OC, and OSc) to t(a) were analyzed in detail. It was found that higher PM1 concentration accompanied longer t(a), with an average increase rate of 0.8 mu g m(-3) h(-1). Meanwhile, the formation of sulfate and more oxidized oxygenated OA were most sensitive to the increase in t(a), and their contributions to PM1 were enhanced from 22 % to 28 % and from 29 % to 48 %, respectively, as t(a) increased. In addition, OSc and the ratios of O/C and OM/OC increased with the increase in t(a). These results indicated that photochemical aging is a key factor leading to the evolution of OA and the increase in PM1 in summertime.