Rapid nitrate formation during a dust-to-haze episode in a semi-arid city near the dust source regions

Abstract

Mineral dust can accelerate secondary aerosol formation under humid conditions, it is unclear whether it can promote secondary aerosol formation under dry conditions. Two-years comprehensive observations was conducted at a semi-arid site, near the dust source regions to illustrate this issue. Three types of episodes, including dust, anthropogenic-dominated and mixed episodes (mixed with dust and anthropogenic aerosols) were selected. Compared with the anthropogenic episodes under humid conditions, rapid nitrate formation was still observed in the mixed episodes under dry conditions, suggesting active metallic oxides in dust, like titanium dioxide, could promote the photochemical reactions of nitrogen dioxide (NO2). The detailed evolution processes were further illustrated by a typical dust-to-mixed episode. After the arrival of the dust, the titanium sharply increased tenfold, a rapid nitrate formation was observed, together with a fast increase in the two most important photochemical pollutants ozone and peroxyacetyl nitrate. The increased secondary organic carbons further illustrated the suspended dust particles accelerated the atmospheric oxidative capacity, therefore enhancing secondary aerosol formation and eventually leading to haze pollution. These results are different from those in humid regions and therefore expands the scientific understanding of the impacts of dust aerosols on haze pollution under dry conditions.