Divergent changes in aerosol optical hygroscopicity and new particle formation during a heatwave of summer 2022

Abstract. As a crucial climate-forcing driver, the aerosol optical enhancement factor (f(RH)) is significantly modulated by chemical compositions and the evolution of particle number size distribution (PNSD), e.g., during new particle formation (NPF). However, mechanisms regulating aerosol optical hygroscopicity during different NPF days, particularly those under heatwaves due to global warming, remain poorly understood. In the hot summer of 2022 in urban Chongqing of southwest China, simultaneous measurements of aerosol optical and hygroscopic properties, PNSD, and bulk chemical compositions were conducted. Two distinct types of NPFs were identified: NPFs with relatively polluted periods (NPFpolluted) and clean cases during heatwave-dominated periods (NPFclean, HW). Compared to the NPFpolluted events, NPFclean, HW occurred approximately 1 h earlier, and the subsequent growth was prolonged, accompanied by a smaller aerosol effective radius (Reff) and lower formation/growth rate during heatwaves. This agreed with the concurrently increased aerosol hemispheric backscattering fraction and scattering Ångström exponent. A generally higher f(RH) was observed on NPF days than in non-event cases, partly attributable to distinct changes in PNSD patterns during NPF days. Moreover, heatwave-induced stronger photooxidation may intensify the formation of more hygroscopic secondary components and prolong the atmospheric aging/subsequent growth of both pre-existing and newly formed particles, largely contributing to the enhanced f(RH), especially during NPFclean, HW days. The higher f(RH) and lowered Reff could synergistically elevate the aerosol direct radiative forcing, specifically under persistent heatwave conditions. Further in-depth exploration of molecular-level characterizations and the aerosol radiative impacts of both direct and indirect interactions under heatwaves in a warming climate is recommended.