Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Although China&rsquo;s air quality has substantially improved in recent years due to the vigorous emission reduction, the Beijing-Tianjin-Hebei (BTH) region, especially its central and southern plains at the eastern foot of the Taihang Mountains, has been the most polluted area in China with persistent and severe haze in winter. Combining meteorology-chemistry coupled model simulations and multiple observations, this study explored the causes of several heavy haze events in this area in January 2017, focusing on local circulations related to mountain terrain. The study results showed that on weather scale, the configuration of the upper, middle, and lower atmosphere provided favorable weather and water vapor transport conditions for the development of haze pollution. Under the weak weather-scale systems, local circulation played a dominant role in the regional distribution and extreme values of PM_2.5. Influenced by the Taihang and Yanshan Mountains, vertical circulations and wind convergence zone were formed between the plain and mountain slopes. The vertical distribution of pollutants strongly depended on the intensity and location of the circulation. Strong and low circulation was more unfavorable to the vertical diffusion and horizontal transport of near-surface pollutants. More importantly, we found that aerosol-radiation interaction (ARI) significantly amplified the impacts of local vertical circulations on heavy haze by two mechanisms. First, ARI strengthened the vertical circulations at the lower levels, with the zonal wind speeds increasing by 0.2&ndash;0.8 m s^-1. Meanwhile, ARI could cause a substantial downward shift of the vertical circulations (~100 m). Second, ARI weakened the horizontal transport of pollutants by reducing the westerly winds below 300 m and enhancing the wind convergence below 1000 m. Under these two mechanisms, pollutants could only recirculate in a limited space. This superposition of typical local circulation and ARI eventually contributed to the accumulation of pollutants and the consequent deterioration of haze pollution in the region.