Secondary aerosol formation in continental outflow conditions during ACE‐Asia

Abstract

Ground‐based aerosol size distribution measurements at the Gosan, Korea, sampling site during ACE‐Asia showed occasionally elevated concentrations of nucleation mode aerosol particles, with subsequent growth to the Aitken mode. Similar results from aircraft and ship‐based measurements at around the same time indicated that at least one event exhibited a broad spatial extent. One of the most pronounced events, with total number concentrations of particles as high as 105 cm−3, occurred on 12 April at the same time as a large dust event, when the total aerosol surface area of particles with diameters less than 10 μm was increase by a factor of 1.7 compared to the average value for the study. Aircraft data showed layers of enlarged SO2 concentration coinciding with an increased aerosol total number concentration, with the highest detected SO2 concentration being 12 ppbv and aerosol number concentration reaching 8 × 104 cm−3 at 800 m altitude. Aircraft data showed that newly formed particles and dust were stratified in different layers in marine environment and transported to the surface close to the island. Atmospheric thermodynamic data suggested that particles were formed above the marine boundary layer and transported by vertical mixing toward the surface as a result of a 3 K north‐to‐south increase in the sea surface temperature. This temperature increase changed atmosphere stratification from stable to neutral/slightly unstable and deepened the internal mixed layer from below 300 m to above 800 m altitude. Measured hygroscopic properties of nucleation mode aerosol particles were consistent with those of ammonium sulfate particles. Here we reported a novel case study of tropospheric aerosol formation identifying particle chemical composition of nucleation mode aerosols in real time by measuring their water uptake properties. Then we demonstrated that particles formed in a layer aloft and were transported to the ground site by growth of the mixed boundary layer while nucleation was chemically induced and did not require turbulent mixing.