Simulated impacts of vertical distributions of black carbon aerosol on meteorology and PM_2.5 concentrations in Beijing during severe haze events

Vertical profiles of black carbon (BC) play a critical role in BC-meteorology interaction which influences PM2.5 (particulate matter with a diameter of 2.5 μm or less) concentrations. In this study, we used the Weather Research and Forecasting with Chemistry model (WRF-Chem) coupled with an improved integrated process (IPR) analysis scheme to investigate 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 December 2016 and 16–19 December 2016). The vertical profiles of BC in Beijing collected by King-Air350 aircraft can be classified into two types: the first type was characterized by decreases in BC concentration with altitude, which was the case mainly controlled by local emissions; the second type had maximum BC concentration around 900 hPa, which was mainly affected by regional transport from the polluted south/southwest region. 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 altitude 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 observed data during the two severe haze events. Accordingly, compared to the simulation with the default vertical profiles of BC, planetary boundary layer heights (PBLH) were reduced further by 24.7 m (6.7 %) and 6.4 m (3.8 %) in Beijing and simulated PM2.5 concentrations were higher by 9.3 μg m−3 (4.1 %) and 5.5 μg m−3 (3.0 %) over central Beijing in the first and second haze events, respectively, with modified vertical profiles. 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 and the vertical profile dominated by regional transport. A larger hs leads to a sharper decline of BC concentrations with altitude (less BC at the surface and more BC in the upper atmosphere), resulting in a stronger cooling at the surface (+0.21 with hs of 0.35 vs. −0.13 °C with hs of 0.96) and hence larger reductions in PBLH (larger BC-induced increases in PM2.5). Relative to the simulation without BC DRE, the mean PM2.5 concentrations were increased by 5.5 μg m−3 (3.4 %) and 7.9 μg m−3 (4.9 %) with BC DRE when hs values were 0.35 and 0.96, respectively. 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.