Effects of continental emissions on Cloud Condensation Nuclei (CCN) activity in northern South China Sea during summertime 2018

Abstract

<p><strong>Abstract.</strong> Aerosol particles in marine atmosphere have been shown to significantly affect cloud formation, atmospheric optical properties, and climate change. However, high temporal and spatial resolved atmospheric measurements over sea are currently sparse, limiting our understanding of aerosol properties in marine atmosphere. In this study, a ship-based cruise campaign was conducted over northern South China Sea (SCS) region (19°37′ N to 22°43′ N, 113°44′ E to 118°12′ E) during summertime 2018. Chemical compositions of the non-refractory PM1 (NR-PM1), particle number size distribution (PNSD) and size-resolved cloud condensation nuclei (CCN) activity (at supersaturation ss = 0.18 %, 0.34 %, and 0.59 %) were measured by a time-of-flight aerosol chemical speciation monitor (ToF-ACSM), and the combination of a cloud condensation nuclei counter (CCNc) and a scanning mobility particle sizer (SMPS), respectively. Overall, aerosol particles exhibited a unimodal distribution (centering at 60∼80 nm) and dominated by sulfate (~46 %) in the NR-PM1, similar to the characteristic of previously-reported background marine aerosols. Two polluted episodes were respectively observed at the beginning (P1, 6th–8th August) and at the end (P2, 25th–26th August) of the campaign and both were characterized by high particle number concentrations (NCN) which were shown to originate from local emissions or pollutants from long range transport. Two relatively clean periods (C1, 9th–10th and C2, 19th–21st August) prior to and after tropical storm Bebinca (11th–15th August) were also classified due to substantial removal of pollutants by strong winds and rainfalls accompanying with the storm. A value of about 0.4 for aerosol hygroscopicity parameter κ measured in this study falls in a range of values (i.e., 0.2–1.0) reported previously for urban atmosphere and for remote marine atmosphere. </p> <p> The concentrations of trace gases (i.e., O<sub>3</sub>, CO, NO<sub>X</sub>) and particles (NCN and NCCN at ss = 0.34 %) were elevated at the end of the campaign and decreased with the offshore distance, suggesting important impacts of anthropogenic emissions from the inland Pearl River Delta (PRD) region on the northern SCS. A good correlation between NOX concentration and NCN implies similar sources (e.g., heavy ship, traffic, and biomass burning) for NOx and particles. The results showed that the NCCN/NCN,tot and the κ values obtained from the CCNc measurement (ss = 0.34 %) had no clear correlation either with the offshore distance or with the concentrations of the particles. Back trajectory analysis showed that the air pollutants originated from local emissions and from inland China continent via long range transport during P1 and P2, respectively. In addition, the air was affected by air masses from southwest and from Indo-China Peninsula during the clean C1 and C2 periods respectively. Chemical composition measurements showed an increase of organic mass fraction and no obviously different κ values were obtained from CCN measurements during C2 and P2, implying that the air masses carried pollutants from local sources during long range transport from Indo-China Peninsula and from the inland China continent respectively during the above two periods. Our study highlights dynamical variations of particle properties and the impact of long range transport from the China continent and Indo-China Peninsula on the northern SCS region during summertime.</p>

Tropospheric aerosol hygroscopicity measurements in China

“…113º44'E to 118º12'E) in August 2018 (Cai et al, 2020), and no obvious dependence of κa 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. This is because the air in north South China Sea was affected by both continental air masses (low hygroscopicity) and marine background (high hygroscopicity).…”

“…Bimodal hygroscopicity distributions were observed for 40-200 nm particles, and the average κ increased with particle size, from 0.213 at 40 nm to 0.312 at 200 nm. The κ values derived from size-resolved chemical compositions measured using AMS were significantly lower than those derived from GF measurements (Cai et al, 2017;Cai et al, 2018), probably because using a constant κ value (0.1) may underestimate hygroscopicity of aerosol organics. Aerosol composition and hygroscopic growth at 90% RH were investigated at this site in September-October (Hong et al, 2018), using an ACSM and a H-TDMA.…”

“…Aerosol hygroscopic growth (at 90% RH) were further measured at this site in November-December 2014 (Cai et al, 2017;Cai et al, 2018). Bimodal hygroscopicity distributions were observed for 40-200 nm particles, and the average κ increased with particle size, from 0.213 at 40 nm to 0.312 at 200 nm.…”

“…Dazhengang (23º00'N, 113º21'E, 150 m above the sea level), is surrounded by residential areas without major pollution sources nearby and can be considered as a suburban site in Guangzhou (Tan et al, 2013). Several aerosol hygroscopic growth measurements at 90% RH have been carried out at this site since 2011 (Tan et al, 2013;Jiang et al, 2016;Cai et al, 2017;Tan et al, 2017;Cai et al, 2018;Hong et al, 2018;Liu et al, 2018).…”

“…Size-resolved CCN activities were investigated at the Panyu site in November-December 2014 (Cai et al, 2018), and the average κa were found to increase from 0.21 at 58 nm to 0.30 at 156 nm, because mass fractions of organics, measured using AMS, decreased with particle size. The average κ derived from H-TDMA measurements agreed well with those derived from CCN measurements; however, they were larger than those calculated from size-resolved chemical compositions, and the difference between measured and calculated κ increased with particle size (Cai et al, 2018). This discrepancy was probably because assuming a constant κ (0.1) may underestimate the hygroscopicity of aerosol organics.…”