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Proc Natl Acad Sci U S A. 2020 Feb 10. pii: 201919343. doi: 10.1073/pnas.1919343117. [Epub ahead of print]

An unexpected catalyst dominates formation and radiative forcing of regional haze.

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College of Global Change and Earth System Science, Beijing Normal University, 100875 Beijing, China.
Department of Atmospheric Sciences, Texas A&M University, College Station, TX 77843.
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125.
State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, 100080 Beijing, China.
State Environmental Protection Key Lab of Environmental Risk Assessment and Control on Chemical Processes, East China University of Science and Technology, 200237 Shanghai, China.
Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing University of Information Science and Technology, 210044 Nanjing, China.
Department of Chemistry, Texas A&M University, College Station, TX 77843.
School of Atmospheric Physics, Nanjing University of Information Science & Technology, 210044 Nanjing, China.
State Key Laboratory of Severe Weather, Chinese Meteorological Administration, Chinese Academy of Meteorological Sciences, 100081 Beijing, China.
Key Laboratory of Atmospheric Chemistry, Chinese Meteorological Administration, Chinese Academy of Meteorological Sciences, 100081 Beijing, China.
Department of Chemistry, College of Natural Sciences, The University of Texas at Austin, Austin, TX 78712.
Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, School of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, 510006 Guangzhou, China.
Key Laboratory of Geographic Information Science of the Ministry of Education, School of Geographic Sciences, East China Normal University, 200241 Shanghai, China.
State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, 100871 Beijing, China.
Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093
Department of Atmospheric Sciences, Texas A&M University, College Station, TX 77843;


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.


air pollution; black carbon; climate; haze; multiphase chemistry

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Conflict of interest statement

The authors declare no competing interest.

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