Characterization of submicron aerosols and effect on visibility during a severe haze-fog episode in Yangtze River Delta, China

Particle size, composition and optical properties were measured at a regional atmosphere background station in the Yangtze River Delta (YRD) to understand the formation and evolution of haze-fog episodes in Jan. 2013. The peak of particle number size distribution was in the size range of 80-100 nm during the measurements. PM₁ mass concentration contributed 84% to the total particle mass (PM₁₀). Based on visibility and ambient relative humidity, three types of weather conditions (i.e., clear, haze and fog) were classified in this study. The extinction coefficients of PM₁ and PM₁₀ under dry conditions were simulated by the Mie model. Under dry conditions, PM₁ was found to contribute approximately 91% to the light extinction coefficient of PM₁₀. However, the PM₁ with the assumption of dry state was found to contribute approximately 85% to the ambient extinction coefficient of PM₁₀ during clear conditions, 58% during haze conditions and approximately 41% during fog conditions. The variation of the dry PM₁ contribution was related to the water uptake of particles under different relative humidity conditions. A severe haze-fog event on Jan. 14-17 was discussed in more detail as a case study. Two episodes were chosen to show that nitrate and organics dominated the aerosol component during the severe haze-fog episode and were related to secondary aerosol formation and air mass origin. Nitrate played a more dominant role than sulfate in heavy haze formation in the YRD region, which was different from the North China Plain region.