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

Abstract

Abstract. 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 (κOA) 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 κOA were observed between Aitken mode particles and accumulation mode particles. For Aitken mode particles (30–100 nm), the κOA values showed daily minima (0.02–0.07) during daytime, while the accumulation mode exhibited a daytime peak (∼ 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 (SVOA) 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 κOA value (0.1) could lead to an overestimation of the CCN number concentration (NCCN) up to 12 %–19 % at 0.1 %–0.7 % supersaturation (SS), which was more obvious at higher SS during daytime. Better prediction of NCCN could be achieved by using size-resolved diurnal κOA, which indicates that the size dependence and diurnal variations in κOA can strongly affect the NCCN 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.

Evidence of Surface-Tension Lowering of Atmospheric Aerosols by Organics from Field Observations in an Urban Atmosphere: Relation to Particle Size and Chemical Composition

“…The κ for organics (κ org ) is assumed to be 0.12. Only the size-resolved κ chem when the particle size >60 nm was calculated because HR-ToF-AMS measurements under 60 nm are considered unreliable . Since the organic mass fraction varies little with particle size for particles larger than 60 nm in this field campaign (Figure S4), the size dependence of the composition should have a small impact on κ chem .…”

Enhanced daytime secondary aerosol formation driven by gas-particle partitioning in downwind urban plumes

“…aerosol mass spectrometer and condensation particle counter) was widely used in the estimation of OA volatility (Philippin et al, 2004;Lee et al, 2010). Cai et al (2022) found that the OA volatility was higher at a particle size range of 30 to 200 nm during daytime, suggesting that the SOA formation through gas-particle partitioning could generally occur at all particle sizes. However, this method failed to provide the volatility information of different molecules of OA.…”

Impacts of Aerosol Chemical Composition on Cloud Condensation Nuclei (CCN) Activity during Wintertime in Beijing, China

“…The CCN activity closure analysis, based on the above two methods, is a key means of investigating the impacts of aerosol properties on activation processes. Although CCN measurements were conducted under different environments in the North China Plain, only a few studies established a linkage between CCN activity and chemical composition with a high time resolution [27,[29][30][31]. Additionally, due to the Clean Air Action of the Chinese government initiated in 2013, stringent emission controls have been imposed to decrease fine particulate matter pollution (PM 2.5 , particles smaller than 2.5 µm in diameter) [32].…”