Aerosol optical and radiative characteristics under contrasting pollution conditions in a typical urban valley in Northwestern China

Aerosol optical and radiative properties play a key role in climate change. Precisely pinpointing the optical and radiative properties of various types of aerosols during diverse pollution events in urban settings remains a challenge. In this study, aerosol optical and radiative properties under contrasting pollution conditions were investigated, including dusty, haze, fireworks, and clean days, in a typical urban valley in Northwestern China, based on observations from the Sun-Sky photometers and simulations from libRadtran. The results show that on dusty days aerosols have high absorption and low backscattering, while on haze days they are characterized by fine, absorptive organic aerosol particles with pronounced forward scattering in the ultraviolet and visible spectra. On fireworks days during Chinese New Year (CNY), fine-mode aerosols from fireworks dominate, with the highest scattering and the lowest absorption under the four pollution conditions, and the particle peak radius growth responds rapidly to changes in relative humidity. Aerosols generally cause Earth's surface cooling and atmospheric warming across various pollution conditions. Notably, dusty days, clean days, and haze days all exhibit a lower positive aerosol radiative forcing (ARF) in the ultraviolet (UV) spectrum at the top of the atmosphere (TOA) due to lower single-scattering albedo (SSA). In contrast, CNY uniquely displays negative UV_ARF at TOA, attributed to high-SSA non-absorptive particles. Aerosols on dusty days have higher positive radiative forcing than on haze days, emphasizing the absorptive impact of mixed dust aerosols. These findings provide valuable insights into the behavior of aerosols under various pollution conditions in a typical urban valley, contributing to a better understanding of the environmental effects of various aerosols in arid urban regions.