Measurement report: Distinct size dependence and diurnal variation in organic aerosol hygroscopicity, volatility, and cloud condensation nuclei activity at a rural site in the Pearl River Delta (PRD) region, China

Organic aerosol (OA) has a significant contribution to cloud formation and hence climate change. However, high uncertainties still exist in its impact on global climate, owing to the varying physical properties affected by the complex formation and aging processes. In this study, the hygroscopicity, volatility, cloud condensation nuclei (CCN) activity, and chemical composition of particles were measured using a series of online instruments at a rural site in the Pearl River Delta (PRD) region of China in fall 2019. During the campaign, the average hygroscopicity of OA (icoA) increased from 0.058 at 30 nm to 0.09 at 200 nm, suggesting a higher oxidation state of OA at larger particle sizes, supported by a higher fraction of extremely low volatility OA (ELVOA) for larger size particles. Significantly different diurnal patterns of kappa(OA) were observed between Aitken mode particles and accumulation mode particles. For Aitken mode particles (30-100 nm), the kappa(OA) values showed daily minima (0.02-0.07) during daytime, while the accumulation mode exhibited a daytime peak (similar to 0.09). Coincidently, a daytime peak was observed for both aged biomass burning organic aerosol (aBBOA) and less oxygenated organic aerosol (LOOA) based on source apportionment, which was attributed to the aging processes and gas-particle partitioning through photochemical reactions. In addition, the fraction of semi-volatile OA (S VOA) was higher at all measured sizes during daytime than during nighttime. These results indicate that the formation of secondary OA (SOA) through gas-particle partitioning can generally occur at all diameters, while the aging processes of pre-existing particles are more dominated in the accumulation mode. Furthermore, we found that applying a fixed kappa(OA) value (0.1) could lead to an overestimation of the CCN number concentration (N-CCN) up to 12 %-19 % at 0.1 %-0.7 % supersaturation (SS), which was more obvious at higher SS during daytime. Better prediction of N-CCN could be achieved by using size-resolved diurnal kappa(OA), which indicates that the size dependence and diurnal variations in kappa(OA) can strongly affect the N-CCN at different SS values. Our results highlight the need for accurately evaluating the atmospheric evolution of OA at different size ranges and their impact on the physicochemical properties and hence climate effects.