Simulated impacts of vertical distributions of black carbon aerosol on meteorology and PM<sub>2.5</sub> concentrations in Beijing during severe haze events

Abstract

Abstract. Vertical profiles of black carbon (BC) play a critical role in modifying the meteorological conditions such as temperature, planetary boundary layer height (PBLH), and regional circulation, which influence surface layer concentrations of PM2.5 (particulate matter with a diameter of 2.5 µm or less; the surface layer covers from 0 to 79.5 m). However, BC vertical profiles in current models usually have large uncertainties. In this study, by using measurements of BC vertical profiles in Beijing collected by King Air 350 aircraft and the Weather Research and Forecasting with Chemistry model (WRF-Chem) coupled with an improved integrated process (IPR) analysis scheme, we investigated the direct radiative effect (DRE) of BC with different vertical profiles on meteorology and PM2.5 concentrations in Beijing during two severe haze events (11–12 and 16–19 December 2016). Compared with measurements in Beijing, the model overestimated BC concentrations by 87.4 % at the surface and underestimated BC mass by 14.9 % at altitudes of 300–900 m as averaged over the two pollution events. The BC DRE with the default vertical profiles from the model heated the air around 300 m altitude, but the warming would be stronger when BC vertical profiles were modified for each day using the observed data during the two severe haze events. Accordingly, compared to the simulation with the default vertical profiles of BC, PBLH was reduced further by 24.7 m (6.7 %) and 6.4 m (3.8 %) in Beijing in the first and second haze events, respectively, with the modified vertical profiles, and hence the surface layer PM2.5 concentrations were higher by 9.3 µg m−3 (4.1 %) and 5.5 µg m−3 (3.0 %) over central Beijing, owing to increased positive contributions of vertical mixing and chemical processes. Furthermore, we quantified by sensitivity experiments the roles of BC vertical profiles with six exponential decline functions (C(h)=C0×e-h/hs and hs​​​​​​​ = 0.35, 0.48, 0.53, 0.79, 0.82, and 0.96) parameterized on the basis of the observations. A larger hs means less BC at the surface and more BC in the upper atmosphere, resulting in less solar radiation reaching the ground and consequently a stronger cooling at the surface (+0.21 with hs of 0.35 vs. −0.13∘ with hs of 0.96). Our results indicate that it is very important to have accurate vertical profiles of BC in simulations of meteorology and PM2.5 concentrations during haze events.