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.