Optical characteristics and formation mechanism of haze events in Qingdao, a coastal city in China.

During the field campaign conducted in Qingdao, Shandong Province, from December 2019 to January 2020, six significant haze episodes occurred. To investigate the optical characteristics and formation mechanisms of haze in this region, we utilized an optical particle counter to measure the size distribution of aerosol particles. The results showed that the average Angström exponent value during the campaign was 1.45, indicating that fine particulate matter dominated. Additionally, the aerosol scattering coefficient, absorption coefficient, and single scattering albedo were measured using a cavity-attenuated phase shift instrument. It was observed that both scattering and extinction coefficients significantly increased during the haze episodes, while SSA notably decreased at the onset of haze, which may be related to newly emitted aerosol particles. Air quality monitors detected a marked increase in sulfur compounds and nitrogen oxides during the haze episodes, with nitrogen oxides being higher than sulfur compounds, indicating that local pollution was primarily driven by vehicular emissions. Using the HYSPLIT4 model, we analyzed atmospheric transport trajectories and found that regional transport played a critical role in haze events. Analysis of micropulse lidar data revealed that the boundary layer height during the haze episodes was significantly lower than during non-haze periods. The lower boundary layer height restricted pollutant dispersion, exacerbating pollution levels. High relative humidity also contributed to the hygroscopic growth of aerosols, altering atmospheric thermal balance and playing a crucial role in the formation of haze events.