Pollution severity-dependent aerosol light scattering enhanced by inorganic species formation in Beijing haze.

The dependence of aerosol optical properties on the chemical composition and size of particles in haze in Beijing was studied. We measured the scattering coefficient of dehydrated PM2.5 aerosols (σsp_dry) and analyzed the chemical composition of PM2.5. We also monitored the size distribution of particles in the range of ~10-700 nm to observe the particle growth (PGsize). Results showed that the concentrations of secondary inorganic aerosols (SIAs) and the mean size of PM2.5 particles (scattering Ångström exponent decreasing) increased with the deterioration of the air quality and increase in relative humidity (RH) which enhanced mass scattering efficiency and increased PM2.5. Thus, the increase in σsp_dry was particularly dramatic and highly sensitive to the ambient RH in severe haze stages. When the ratio of SIAs to PM2.5 (MSIAs) exceeded 0.35 during the polluted environment, the water content, PGsize, and σsp_dry showed distinct increases, indicating that the formation of SIAs enhanced water vapor condensation and particle growth. This finding revealed the existence of a critical value for MSIAs in terms of describing the correlation of σsp_dry variation with pollution severity. The estimation of the respective contributions of individual components to σsp_dry with the IMPROVE formula revealed that ammonium nitrate and ammonium sulfate were the two largest contributors. These results indicate that the rapid formation of SIAs and PGsize under humid conditions are the key factors contributing to the increased σsp_dry via enhanced mass scattering efficiency and increased PM2.5 in the severe haze observed in this study.