Asian dust storm observed at a rural mountain site in southern China: chemical evolution and heterogeneous photochemistry

摘要

Heterogeneous processes on dust particles are important for understandingthe chemistry and radiative balance of the atmosphere. This paperinvestigates an intense Asian dust storm episode observed at Mount Heng(1269 m a.s.l.) in southern China on 24–26 April 2009. A set of aerosol andtrace gas data collected during the study was analyzed to investigate theirchemical evolution and heterogeneous photochemistry as the dust traveled tosouthern China. Results show that the mineral dust arriving at Mt. Hengexperienced significant modifications during transport, with largeenrichments in secondary species (sulfate, nitrate, and ammonium) comparedwith the dust composition collected at an upwind mountain top site (MountHua). A photochemical age "clock" (?Log₁₀(NO_x/NO_y)) wasemployed to quantify the atmospheric processing time. The result indicatesan obvious increase in the abundance of secondary water-soluble ions in dustparticles with the air mass atmospheric processing time. Based on theobservations, a 4-stage evolution process is proposed forcarbonate-containing Asian dust, starting from fresh dust to particlescoated with hydrophilic and acidic materials. Daytime-enhanced nitriteformation on the dust particles was also observed, which indicates therecent laboratory result of the TiO₂ photocatalysis of NO₂ as apotential source of nitrite and nitrous acid.