Relative effects of open biomass burning and open crop straw burning on haze formation over central and eastern China: modeling study driven by constrained emissions
Authors: Mehmood Khalid , Wu Yujie , Wang Liqiang , Yu Shaocai , Li Pengfei , Chen Xue , Li Zhen , Zhang Yibo , Li Mengying , Liu Weiping , Wang Yuesi , Liu Zirui , Zhu Yannian , Rosenfeld Daniel , Seinfeld John H. 
Published: February 2020

Abstract. Open biomass burning (OBB) has a high potential to
trigger local and regional severe haze with elevated fine particulate
matter (PM2.5) concentrations and could thus deteriorate ambient air
quality and threaten human health. Open crop straw burning (OCSB), as a
critical part of OBB, emits abundant gaseous and particulate pollutants,
especially in fields with intensive agriculture, such as in central and eastern
China (CEC). This region includes nine provinces, i.e., Hubei, Anhui, Henan, Hunan,
Jiangxi, Shandong, Jiangsu, Shanghai, and Fujian. The first four ones are
located inland, while the others are on the eastern coast. However,
uncertainties in current OCSB and other types of OBB emissions in chemical
transport models (CTMs) lead to inaccuracies in evaluating their impacts on
haze formations. Satellite retrievals provide an alternative that can be
used to simultaneously quantify emissions of OCSB and other types of OBB,
such as the Fire INventory from NCAR version 1.5 (FINNv1.5), which,
nevertheless, generally underestimates their magnitudes due to unresolved
small fires. In this study, we selected June 2014 as our study period,
which exhibited a complete evolution process of OBB (from 1 to 19 June) over
CEC. During this period, OBB was dominated by OCSB in terms of the number of
fire hotspots and associated emissions (74 %–94 %), most of
which were located at Henan and Anhui (> 60 %) with intensive
enhancements from 5 to 14 June (> 80 %). OCSB generally
exhibits a spatiotemporal correlation with regional haze over the central part
of CEC (Henan, Anhui, Hubei, and Hunan), while other types of OBB emissions
had influences on Jiangxi, Zhejiang, and Fujian. Based on these analyses, we
establish a constraining method that integrates ground-level PM2.5
measurements with a state-of-art fully coupled regional meteorological and
chemical transport model (the two-way coupled WRF-CMAQ) in order to derive
optimal OBB emissions based on FINNv1.5. It is demonstrated that these
emissions allow the model to reproduce meteorological and chemical fields
over CEC during the study period, whereas the original FINNv1.5
underestimated OBB emissions by 2–7 times, depending on
specific spatiotemporal scales. The results show that OBB had substantial
impacts on surface PM2.5 concentrations over CEC. Most of the OBB
contributions were dominated by OCSB, especially in Henan, Anhui, Hubei, and
Hunan, while other types of OBB emissions also exerted an influence in Jiangxi,
Zhejiang, and Fujian. With the concentration-weighted trajectory (CWT)
method, potential OCSB sources leading to severe haze in Henan, Anhui,
Hubei, and Hunan were pinpointed. The results show that the OCSB emissions
in Henan and Anhui can cause haze not only locally but also regionally
through regional transport. Combining with meteorological analyses, we can
find that surface weather patterns played a cardinal role in reshaping
spatial and temporal characteristics of PM2.5 concentrations.
Stationary high-pressure systems over CEC enhanced local PM2.5
concentrations in Henan and Anhui. Then, with the evolution of
meteorological patterns, Hubei and Hunan in the low-pressure system were
impacted by areas (i.e., Henan and Anhui) enveloped in the high-pressure
system. These results suggest that policymakers should strictly undertake
interprovincial joint enforcement actions to prohibit irregular OBB,
especially OCSB over CEC. Constrained OBB emissions can, to a large extent,
supplement estimations derived from satellite retrievals as well as reduce
overestimates of bottom-up methods.