Observational evidence of cloud processes contributing to daytime elevated nitrate in an urban atmosphere

Nitrate has become the most important hydrophilic chemical component in PM_2.5 during serious air pollution periods in urban areas of the Pearl River Delta (PRD) region of south China, but there is a lack of fully understanding of its formation mechanisms and controlling factors, especially during daytime nitrate episodes. To fill the knowledge gap, water-soluble inorganic ions in PM_2.5 and trace gases including SO₂, HNO₃, NH₃, NOx and O₃, particle size distribution, vertical profile of aerosol backscatter density, and ground-level and vertical profiles of key meteorological variables were simultaneously measured at high time resolution in urban Guangzhou of the PRD. The remarkably enhanced nitrate mass concentrations observed at the surface-level during daytime were identified to be caused by strong boundary-layer turbulent mixing of residual aerosols produced from evaporation of low-level shallow stratocumulus clouds. This finding may have important implications on the sources of secondary inorganic aerosols in this and other similar cloudy regions.