Divergent changes in aerosol optical hygroscopicity and new particle formation induced by heatwaves

Abstract. As a crucial climate-forcing driver, the aerosol optical enhancement factor (f(RH)) is significantly modulated by the evolution of particle number size distribution (PNSD), e.g., during new particle formation (NPF). The mechanisms regulating aerosol optical hygroscopicity during different NPF events and non-event days, particularly those influenced by heatwaves due to global warming, remain poorly understood. In the extremely 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 NPF were identified: the ones with relatively polluted period (P1) and clean cases during heatwave-dominated period (P2). Heatwaves triggered NPF earlier and prolonged the subsequent growth, resulting in smaller aerosol effective radius (R_eff) and lower growth rate. This agreed with the concurrently increased aerosol hemispheric backscattering fraction and scattering Ångström exponent. f(RH) was generally higher during NPF events in comparison to that for non-event cases in both periods. Heatwave-induced stronger photooxidation may intensify the formation of more hygroscopic secondary components, as well as the subsequent growth of pre-existing particles and newly formed ultrafine ones, thereby enhancing aerosol optical hygroscopicity especially during heatwave-influenced NPF events. The promoted f(RH) and lowered R_eff could synergistically elevate the aerosol direct radiative forcing, specifically under persistent heatwave conditions. Further in-depth exploration on molecular-level characterizations and aerosol radiative impacts of both direct and indirect interactions during weather extremes (e.g., heatwaves) with the warming climate are recommended.