An unexpected catalyst dominates formation and radiative forcing of regional haze

Abstract

Although regional haze adversely affects human health and possibly counteracts global warming from increasing levels of greenhouse gases, the formation and radiative forcing of regional haze on climate remain uncertain. By combining field measurements, laboratory experiments, and model simulations, we show a remarkable role of black carbon (BC) particles in driving the formation and trend of regional haze. Our analysis of long-term measurements in China indicates declined frequency of heavy haze events along with significantly reduced SO2, but negligibly alleviated haze severity. Also, no improving trend exists for moderate haze events. Our complementary laboratory experiments demonstrate that SO2 oxidation is efficiently catalyzed on BC particles in the presence of NO2 and NH3, even at low SO2 and intermediate relative humidity levels. Inclusion of the BC reaction accounts for about 90–100% and 30–50% of the sulfate production during moderate and heavy haze events, respectively. Calculations using a radiative transfer model and accounting for the sulfate formation on BC yield an invariant radiative forcing of nearly zero W m−2 on the top of the atmosphere throughout haze development, indicating small net climatic cooling/warming but large surface cooling, atmospheric heating, and air stagnation. This BC catalytic chemistry facilitates haze development and explains the observed trends of regional haze in China. Our results imply that reduction of SO2 alone is insufficient in mitigating haze occurrence and highlight the necessity of accurate representation of the BC chemical and radiative properties in predicting the formation and assessing the impacts of regional haze.

Aerosol–photolysis interaction reduces particulate matter during wintertime haze events

“…Based on laboratory experiments and field observations during wintertime haze events in Beijing and Xi'an, the aqueous oxidation of SO 2 by NO 2 is proposed to play a key role in sulfate formation under the conditions of high RH and NH 3 neutralization (37). Further complementary laboratory experiments have shown efficient SO 2 oxidation by NO 2 catalyzed on BC particles in the presence of NH 3 even in intermediate RH conditions (39). Additionally, the rapid aerosol nucleation and growth, caused by highly elevated concentrations of gaseous aerosol precursors or abundant organics from photooxidation of vehicular exhaust, also have large potentials to promote secondary aerosol formation during haze events (38,40,41).…”

Characterization of the aerosol chemical composition during the COVID-19 lockdown period in Suzhou in the Yangtze River Delta, China

“…Noticeably, these regulatory measures have resulted in significant reduction in primary PM emissions, but the secondary pollutant ozone (O 3 ) remains prominent ( Huang et al, 2021 ; Li et al, 2019b ; Wang et al, 2020d ; Zeng et al, 2019 ). Numerous studies have indicated that compound air pollution has resulted from the primary pollutants emitted by industry, power plants, traffic and heating processes and from the secondary pollutants generated by complex physical, chemical and biological processes ( An et al, 2019 ; Fenger, 2009 ; Guo et al, 2020 ; Li et al, 2019c ; Zhang et al, 2020 ; Zhu et al, 2011 ). These pollutants may quickly accumulate at high concentrations under static meteorological conditions and then undergo cross border transport processes, leading to complicated effects on human health and the environment ( Huang et al, 2014 ; Lou et al, 2020; Wang et al, 2014b ; Xu et al, 2013 ; Zhang et al, 2020 ).…”

“…Numerous studies have indicated that compound air pollution has resulted from the primary pollutants emitted by industry, power plants, traffic and heating processes and from the secondary pollutants generated by complex physical, chemical and biological processes ( An et al, 2019 ; Fenger, 2009 ; Guo et al, 2020 ; Li et al, 2019c ; Zhang et al, 2020 ; Zhu et al, 2011 ). These pollutants may quickly accumulate at high concentrations under static meteorological conditions and then undergo cross border transport processes, leading to complicated effects on human health and the environment ( Huang et al, 2014 ; Lou et al, 2020; Wang et al, 2014b ; Xu et al, 2013 ; Zhang et al, 2020 ). Additionally, air pollution also has complex interactions with large scale weather and climate ( Cai et al, 2017 ; Lou et al, 2019 ; Quinn and Bates, 2003 ).…”

“…The contributions of distinct emission sources should first be clarified when implementing the joint prevention and control of regional air pollution. Therefore, the key step is to distinguish the precursors and their sources that contribute the most to the pollution when studying its formation mechanisms and impact factors ( An et al, 2019 ; Cheng et al, 2016 ; Li et al, 2019a ; Zhang et al, 2020 ). For now, the impacts of emission reduction measurements on air quality have been discussed for several important activity periods (Beijing Olympics, APEC, G20 Summit) ( Cai and Xie, 2011 ; Li et al, 2017a ; Shen et al, 2017 ; Wang et al, 2016 , 2018 ; Xu et al, 2016 ; Zhang et al, 2016 ).…”

Unexpected air pollution with marked emission reductions during the COVID-19 outbreak in China

“…S4), leading to 40 to 100% increases in AOD during the city lockdown. Possible factors that explain enhanced PM 2.5 and ozone levels in the face of declining precursor gas emissions include the complex chemistry of secondary aerosols and ozone (7,13) as well as the meteorological influence (8). Changes in relative humidity (RH), near-surface wind speed and direction, planetary boundary layer (PBL) height, and precipitation between the 2020-CLD and CLIM-LNY are shown in Fig.…”