Abstract

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.

Highlights

  • With the rapid economic development and large increases in fossil energy consumption, haze pollution has become one of the most serious challenges in China, especially in the Beijing–Tianjin–Hebei (BTH) region

  • Compared to the measured vertical profiles of black carbon (BC) in Beijing, the default vertical profiles of BC from the WRF-Chem model can capture the decreases in BC mass concentration with altitude on 12 and 16–19 December when local emissions dominated, but they cannot reproduce the observed maximum mass concentration of BC around 850 m altitude on 11 December when regional transport of pollutants dominated

  • Averaged over the two severe pollution events, the model overestimated BC mass concentration by 87.4 % at the surface but underestimated BC by 33.1 % at 1000 m altitude compared with the observations in Beijing

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Summary

Introduction

With the rapid economic development and large increases in fossil energy consumption, haze pollution has become one of the most serious challenges in China, especially in the Beijing–Tianjin–Hebei (BTH) region In 2014 and 2015, the numbers of extremely serious PM2.5 (particulate matter with an aerodynamic equivalent diameter of 2.5 μm or less) pollution days (with daily mean PM2.5 > 150 μg m−3) in Beijing reached 45 and 54, respectively (He et al, 2017). The realtime hourly average concentration of PM2.5 in Beijing even reached 1000 μg m−3 during the severe haze events in January 2013, far exceeding the Chinese Ambient Air Quality Grade I Standards (35 μg m−3 for daily mean PM2.5) (Liu et al, 2017). With the implementation of the toughest-ever clean air policy since 2013, the observed annual mean PM2.5 concentrations averaged over 74 cities in China fell from 61.8 μg m−3 in 2013 to 42.0 μg m−3 in 2017 Understanding the mechanisms responsible for the occurrence of severe haze is important for air quality management planning

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