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 PM_2.5 aerosols (σ_{sp_dry}) and analyzed the chemical composition of PM_2.5. We also monitored the size distribution of particles in the range of ~10-700 nm to observe the particle growth (PG_size). Results showed that the concentrations of secondary inorganic aerosols (SIAs) and the mean size of PM_2.5 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 PM_2.5. Thus, the increase in σ_{sp_dry} was particularly dramatic and highly sensitive to the ambient RH in severe haze stages. When the ratio of SIAs to PM_2.5 (M_SIAs) exceeded 0.35 during the polluted environment, the water content, PG_size, and σ_{sp_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 M_SIAs in terms of describing the correlation of σ_{sp_dry} variation with pollution severity. The estimation of the respective contributions of individual components to σ_{sp_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 PG_size under humid conditions are the key factors contributing to the increased σ_{sp_dry} via enhanced mass scattering efficiency and increased PM_2.5 in the severe haze observed in this study.