Atmospheric nucleation and its effect on cloud condensation nuclei abundance at the US DOE Southern Great Plains field site

Abstract:

Atmospheric nucleation from condensable vapors in the free troposphere has been suggested to contribute to CCN and magnitude of this contribution is highly uncertain. CCN budget is affects by the size and chemical compositions of atmospheric aerosols. Therefore, we used a yearlong in-situ measurements of aerosol size spectra and CCN concentration (N_CCN) at the US DOE Southern Great Plains field site to understand the aerosol size and their impacts on CCN budget and aerosol CCN activity. A closure study between predicted and measured CCN is also conducted to investigate the influence of aerosol chemical composition on its growth to CCN. Based on formation and availability of various aerosol particles we distinguished our study in two cases. First, the new particle formation (NPF) events were favored by nucleation mode particles in the atmosphere and second Non-NPF events where Aitken mode particles are dominated in the atmosphere. We found that enhancement in the N_CCN is ~20% higher in the non-NPF events in comparison to NPF events due to lager condensation/coagulation growth of Aitken mode particles. Moreover, Aerosol particles differ depending on their origins. When the air masses came from the Atlantic Ocean, particle number concentration accounted for about ~3-5% of N_CCN at 0.4% supersaturation and about 1-4% of total number particle concentration. When air masses came from the local continental regions, we observed enhanced N_CCN is about 1.5 times higher than that during marine periods. Additionally, we analyzed the chemical composition of aerosol particles by calculating hygroscopicity parameter (κ). However, κ for these two different periods shows no significant difference. There is a slight increase in κ with increasing particle size, indicating the addition of soluble, likely inorganic, material during cloud processing. This indicated that particle size was the most important factor influencing the ability of aerosols to activate, whereas the effect of chemical composition was secondary, especially when supersaturation was high.