Variation of secondary coatings associated with elemental carbon by single particle analysis

The mixing state of elemental carbon (EC) with secondary species has been highlighted as a major uncertainty in assessing its climate impact. However, the extent to which secondary coatings are present on EC and the underlying processes remained poorly understood in China, where there is a high loading of EC produced from extensive usage of fossil fuels and biomass. A single particle aerosol mass spectrometer (SPAMS) was applied to detect the chemical compositions associated with EC at the Guangdong Atmospheric Supersite, China. Efforts were made to track the variation of secondary coatings on EC. It is the first report on the direct observation of secondary compositions associated with EC with high time resolution in China. The hourly average number of EC-containing particles accounted for ∼33% (21-50%) of total analyzed particles over the sampling period. EC was found to be extensively internally mixed with sulfate (97.4% in number), nitrate (89.5%), oxidized organics (69.6%), and/or ammonium (80%). The results also indicate that secondary processing on EC in condensation (0.2-0.5 μm) and droplet (0.7-1.1 μm) modes is different. Active photochemical formation of oxidized organics and ammonium sulfate during daytime, and formation of ammonium nitrate during nighttime led to a distinct diurnal circle of mixing state of EC in the condensation mode. However, the photochemical aging may have limited or negligible influence on the mixing state and growth of EC in the droplet mode. These findings improve the understanding of the evolution of physicochemical properties of EC, and may help to model its climate impact.