The size-resolved cloud condensation nuclei (CCN) activity and its prediction based on aerosol hygroscopicity and composition in the Pearl Delta River (PRD) region during wintertime 2014

Abstract

Abstract. A hygroscopic tandem differential mobility analyzer (HTDMA), a scanning mobility cloud condensation nuclei (CCN) analyzer (SMCA), and an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) were used to, respectively, measure the hygroscopicity, condensation nuclei activation, and chemical composition of aerosol particles at the Panyu site in the Pearl River Delta region during wintertime 2014. The distribution of the size-resolved CCN at four supersaturations (SSs of 0.1 %, 0.2 %, 0.4 %, and 0.7 %) and the aerosol particle size distribution were obtained by the SMCA. The hygroscopicity parameter κ (κCCN, κHTDMA, and κAMS) was, respectively, calculated based upon the SMCA, HTDMA, and AMS measurements. The results showed that the κHTDMA value was slightly smaller than the κCCN one at all diameters and for particles larger than 100 nm, and the κAMS value was significantly smaller than the others (κCCN and κHTDMA), which could be attributed to the underestimated hygroscopicity of the organics (κorg). The activation ratio (AR) calculated from the growth factor – probability density function (Gf-PDF) without surface tension correction was found to be lower than that from the CCN measurements, due most likely to the uncorrected surface tension (σs∕a) that did not consider the surfactant effects of the organic compounds. We demonstrated that better agreement between the calculated and measured ARs could be obtained by adjusting σs∕a. Various schemes were proposed to predict the CCN number concentration (NCCN) based on the HTDMA and AMS measurements. In general, the predicted NCCN agreed reasonably well with the corresponding measured ones using different schemes. For the HTDMA measurements, the NCCN value predicted from the real-time AR measurements was slightly smaller (∼6.8 %) than that from the activation diameter (D50) method due to the assumed internal mixing in the D50 prediction. The NCCN values predicted from bulk chemical composition of PM1 were higher (∼11.5 %) than those from size-resolved composition measured by the AMS because a significant fraction of PM1 was composed of inorganic matter. The NCCN values calculated from AMS measurement were underpredicted at 0.1 % and 0.2 % supersaturations, which could be due to underestimation of κorg and overestimation of σs∕a. For SS values of 0.4 % and 0.7 %, slight overpredicted NCCN values were found because of the internal mixing assumption. Our results highlight the need for accurately evaluating the effects of organics on both the hygroscopic parameter κ and the surface tension σ in order to accurately predict CCN activity.

Ice nucleation by aerosols from anthropogenic pollution

“…The two cases were chosen to represent two typical convection systems with moderate and strong convective strength, respectively. For each cloud system, we perform a baseline experiment with four numerical simulations in which the number concentrations of aerosols (here confined to be hygroscopic particles with diameter > 0.1 μm that could serve as CCN under favorable supersaturation) are 100, 600, 1600, and 3200 cm −3 , covering clean to polluted conditions 38 – 40 . In each simulation, the average INP fraction is estimated to be about 1/20000, based on heterogeneously nucleated ice number concentration predicted dynamically and the total aerosol concentration initialized in our simulations.…”

Tropospheric aerosol hygroscopicity in China

“…The average κ values, reported by previous studies, were generally found to be in the range of 0.30-0.35; however, CCN activities could be significantly reduced if measurement sites were affected by fresh urban pollution or biomass burning (Rose et al, 2010;Gunthe et al, 2011;Zhang et al, 2014;Wu et al, 2017) due to enhanced contribution of soot and organics. We note that a few recent studies (Atwood et al, 2017;Zhang et al, 2017;Cai et al, 2020) also reported higher aerosol hygroscopicity, as shown in Table S5. For example, the average κ values observed at the Xinzhou site (Zhang et al, 2017) appeared to be larger than those reported at other continental sites probably because aerosols arriving at this site were heavily aged; in addition, two studies which investigated aerosol CCN activities in the marine boundary layer reported larger κ values (Atwood et al, 2017;Cai et al, 2020) compared to those at continental sites.…”

“…Under marine background conditions, the average κ a values were determined to be 0.65±0.11 and 0.46±0.17 for the accumulation and Aitken modes (Atwood et al, 2017). Compared to marine background conditions, CCN activities were reduced after extensive precipitation, with average κ a values determined to be 0.54±0.14 and 0.34±0.11 for the accumulation and Aitken modes, whereas during periods impacted by biomass burning, κ a values were reduced to 0.40±0.03 for the accumulation mode but increased instead to 0.56 ± 0.25 for the Aitken mode (Atwood et al, 2017 (Cai et al, 2020), and no obvious dependence of κ a on particle size (50-100 nm) was observed. The campaign-averaged κ was determined to be ∼ 0.40 (Cai et al, 2020), larger than these measured in the PRD region but smaller than those measured over remote marine regions.…”

Secondary aerosol formation alters CCN activity in the North China Plain

“…The tandem differential mobility analyzer (TDMA) inversion algorithm (Gysel et al, 2009) was applied to calculate the Probability Density Function of GF (GF-PDF). More details about this system can be referred to Cai et al (2018) and Hong et al (2018).…”