Formation, radiative forcing, and climatic effects of severe regional haze
Authors: Lin Yun , Wang Yuan , Pan Bowen , Hu Jiaxi , Guo Song , Levy Zamora Misti , Tian Pengfei , Su Qiong , Ji Yuemeng , Zhao Jiayun , Gomez-Hernandez Mario , Hu Min , Zhang Renyi 
Published: April 2022

Abstract. Severe regional haze events, which are characterized by
exceedingly high levels of fine particulate matter (PM), occur frequently in
many developing countries (such as China and India), with profound
implications for human health, weather, and climate. The occurrence of the
haze extremes involves a complex interplay between primary emissions,
secondary formation, and conducive meteorological conditions, and the
relative contributions of the various processes remain unclear. Here we
investigated severe regional haze episodes in 2013 over the Northern China
Plain (NCP), by evaluating the PM production and the interactions between
elevated PM and the planetary boundary layer (PBL). Analysis of the
ground-based measurements and satellite observations of PM properties shows
nearly synchronized temporal PM variations among the three megacities
(Beijing, Baoding, and Shijiazhuang) in this region and a coincidence of the
aerosol optical depth (AOD) hotspots with the three megacities during the
polluted period. During the clean-to-hazy transition, the measured
oxygenated organic aerosol concentration ([OOA]) well correlates with the
odd-oxygen concentration ([Ox]=[O3]+[NO2]), and the
mean [OOA] / [Ox] ratio in Beijing is much larger than those in other
megacities (such as Mexico City and Houston), indicating highly efficient
photochemical activity. Simulations using the Weather Research and
Forecasting (WRF) model coupled with an explicit aerosol radiative module
reveal that strong aerosol–PBL interaction during the polluted period
results in a suppressed and stabilized PBL and elevated humidity, triggering
a positive feedback to amplify the haze severity at the ground level. Model
sensitivity study illustrates the importance of black carbon (BC) in the
haze–PBL interaction and the aerosol regional climatic effect, contributing
to more than 30 % of the PBL collapse and about half of the positive
radiative forcing on the top of the atmosphere. Overall, severe regional
haze exhibits strong negative radiative forcing (cooling) of −63 to −88 W m−2 at the surface and strong positive radiative forcing (warming) of
57 to 82 W m−2 in the atmosphere, with a slightly negative net
radiative forcing of about −6 W m−2 on the top of the atmosphere. Our
work establishes a synthetic view for the dominant regional features during
severe haze events, unraveling rapid in situ PM production and inefficient
transport, both of which are amplified by atmospheric stagnation. On the
other hand, regional transport sufficiently disperses gaseous aerosol
precursors (e.g., sulfur dioxide, nitrogen oxides, volatile organic
compounds, and ammonia) during the clean period, which subsequently result
in rapid in situ PM production via photochemistry during the transition period and
via multiphase chemistry during the polluted period. Our findings highlight
the co-benefits for reduction in BC emissions, which not only improve local
and regional air quality by minimizing air stagnation but also mitigate the
global warming by alleviating the positive direct radiative forcing.