Abstract
Abstract. The chemical composition of fine particulate matter (PM2.5), the size distribution and number concentration of aerosol particles (NCN), and the number concentration of cloud condensation nuclei (NCCN) were measured at the northern tip of Taiwan during an intensive observation experiment from April 2017 to March 2018. The parameters of aerosol hygroscopicity (i.e., activation ratio, activation diameter and kappa of CCN) were retrieved from the measurements. Significant variations were found in the hygroscopicity of aerosols (kappa – κ – of 0.18–0.56, for water vapor supersaturation – SS – of 0.12 %–0.80 %), which were subject to various pollution sources, including aged air pollutants originating in eastern and northern China and transported by the Asian continental outflows and fresh particles emitted from local sources and distributed by land–sea breeze circulations as well as produced by processes of new particle formation (NPF). Cluster analysis was applied to the back trajectories of air masses to investigate their respective source regions. The results showed that aerosols associated with Asian continental outflows were characterized by lower NCN and NCCN values and by higher kappa values of CCN, whereas higher NCN and NCCN values with lower kappa values of CCN were observed in the aerosols associated with local air masses. Besides, it was revealed that the kappa value of CCN exhibited a decrease during the early stage of an event of new particle formation, which turned to an increasing trend over the later period. The distinct features in the hygroscopicity of aerosols were found to be consistent with the characteristics in the chemical composition of PM2.5. This study has depicted a clear seasonal characteristic of hygroscopicity and CCN activity under the influence of a complex mixture of pollutants from different regional and/or local pollution sources. Nevertheless, the mixing state and chemical composition of the aerosols critically influence the aerosol hygroscopicity, and further investigations are necessary to elucidate the atmospheric processing involved in the CCN activation in coastal areas.
Highlights
Aerosols suspended in the atmosphere allow for the condensation of water vapor under certain supersaturation conditions and subsequently evolve into cloud droplets
This study presents the observation of aerosol hygroscopicity parameters, including κ, the activation diameter (Dcut) and the activation ratio (AR = number concentration of cloud condensation nuclei (NCCN)/NCN) of cloud condensation nuclei (CCN) at a coastal research station (CAFÉ) in northern Taiwan during a 1-year campaign from April 2017 to March 2018
High levels of NCN and NCCN were consistently observed in spring and summer, whereas kappa values were elevated in autumn and minimal in summer
Summary
Aerosols suspended in the atmosphere allow for the condensation of water vapor under certain supersaturation conditions and subsequently evolve into cloud droplets. The activation of cloud condensation nuclei (CCN) depends on the size and chemical composition of aerosol particles, as well as on the meteorological conditions Cheung et al.: Hygroscopic properties and cloud condensation nuclei activity of atmospheric aerosols feld and Pandis, 1998). Atmospheric aerosols are a mixture of different chemical species rather than a single compound and exist in various size ranges and mixing states. While the hygroscopicity and CCN activity of a single component can be characterized in laboratories, the properties of their mixture in ambient air are difficult to estimate, owing to the complexity in the physiochemical characteristics of aerosols. In situ measurements of physicochemical properties of aerosols and CCN in critical geographical areas in the climate system could provide a means of constraining the representation of relevant schemes in global climate models (Khairoutdinov and Randall, 2001; Morales Betancourt and Nenes, 2014; Seinfeld et al, 2016)
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