Mixing state and hygroscopicity of dust and haze particles before leaving Asian continent

Pollutants during haze and Asian dust storms are transported out of the Asian continent, affecting the regional climate and the hydrological and biogeochemical cycles. Understanding the physicochemical properties of aerosol particles is essential to quantify their impacts. In order to determine physicochemical properties of aerosols before leaving the Asian continent, we carried out a field campaign from 14 April to 2 May 2011 at a background site in the path of Asian dust and haze outflows. We measured concentrations of gaseous pollutants (SO₂, NO₂, NO, O₃, and CO), black carbon (BC), and particle number in situ and collected airborne particles for microscopic analysis. Pollutant concentrations (BC, 4 μgm³; CO, 808 ppb; SO₂, 24ppb; NO₂, 37 ppb) were highest during haze periods, except for PM_2.5 mass, which was highest (162 μgm³) during a dust storm. Seventy-one percent of haze particles were coated with organic films and 32% were internal mixtures of sulfates and refractory particles (e.g., soot, metal/fly ash, and mineral). Seventy-nine percent of haze particles have deliquescence relative humidity at 68–70%. During a dust storm, soot particles were observed among dust particles. Most dust particles were hydrophobic, and no Ca(NO₃)₂ was observed in dust particles collected during the dust storms, but up to 32% of dust particles were found to be coated with Ca(NO₃)₂ after the main dust storm moved out of the sampling area. These results indicated that both natural and anthropogenic aerosol particles in Asian outflow can undergo significant physicochemical processes before leaving the Asian continent.