The concentration of fine particulate matter (PM2.5) across China has decreased by 30–50% over the period 2013–2018 due to stringent emission controls. However, the nitrate component of PM2.5 has not responded effectively to decreasing emissions of nitrogen oxides and has actually increased during winter haze pollution events in the North China Plain. Here, we show that the GEOS-Chem atmospheric chemistry model successfully simulates the nitrate concentrations and trends. We find that winter mean nitrate would have increased over 2013–2018 were it not for favourable meteorology. The principal cause of this nitrate increase is weaker deposition. The fraction of total inorganic nitrate as particulate nitrate instead of gaseous nitric acid over the North China Plain in winter increased from 90% in 2013 to 98% in 2017, as emissions of nitrogen oxides and sulfur dioxide decreased while ammonia emissions remained high. This small increase in the particulate fraction greatly slows down deposition of total inorganic nitrate and hence drives the particulate nitrate increase. Our results suggest that decreasing ammonia emissions would decrease particulate nitrate by driving faster deposition of total inorganic nitrate. Decreasing nitrogen oxide emissions is less effective because it drives faster oxidation of nitrogen oxides and slower deposition of total inorganic nitrate.
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Surface PM2.5 observations across China from the China Ministry of Ecology and Environment (MEE) national network can be downloaded from quotsoft.net/air. The anthropogenic emission inventory is from www.meicmodel.org. MERRA-2 reanalysis data are from https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/data_access/. Information about the observed PM2.5 species concentrations used in this work are summarized in the Supplementary Table. PM2.5 species observation data are deposited at https://doi.org/10.7910/DVN/VHFTLQ. The National Nitrogen Deposition Monitoring Network (NNDMN) version 1.0 database is from ref. 35. Source data are provided with this paper.
The GEOS-Chem model code version 12.3.1 is open source (https://doi.org/10.5281/zenodo.2633278). Code for calculations and data processing is available from the corresponding author upon request.
Action Plan on Prevention and Control of Air Pollution (in Chinese) (Chinese State Council, 2013); http://www.gov.cn/zwgk/2013-09/12/content_2486773.htm
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This work was funded by the Harvard–NUIST Joint Laboratory for Air Quality and Climate, the Samsung PM2.5 Strategic Research Program and Samsung Advanced Institute of Technology. H.L. is supported by the National Key Research and Development Program of China (grant no. 2019YFA0606804). G.L. and F.Y. acknowledge funding support from NASA under grant no. NNX17AG35G. Y.S. acknowledges support from the Beijing Municipal Natural Science Foundation (8202049).
The authors declare no competing interests.
Peer review information Nature Geoscience thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended Data Fig. 2 Spatial and seasonal patterns of the mass concentrations of PM2.5 and its major components (OA, BC, sulfate, nitrate, and ammonium) over China in 2013.
a-f, Spatial distributions of observed annual mean concentrations (circles) are compared to the GEOS-Chem model (background). g-i, Scatter plots of observed and modeled monthly mean sulfate, nitrate, and ammonium concentrations for winter (December-January-February; filled circles) and summer (June-July-August; open circles). Also shown in panels g-i are the 1:1 lines, the correlation coefficients (r) between model and observations, and the corresponding reduced-major-axis regressions and slopes. PM2.5 observations are from the China Ministry of Ecology and the Environment (MEE) national air quality monitoring network. OA and BC observations in Beijing, Handan, and Shanghai are from refs. 70,71,72. Sulfate, nitrate, and ammonium observations are from the Campaign on Atmospheric Aerosol Research network of China (CARE-China)36,48.
Extended Data Fig. 3 Same as Extended Data Fig. 1 but for the year 2015 including January, February, July, and December.
GEOS-Chem results (blue dotted lines) are compared to observations (black solid lines). Observations are from the Station for Observing Regional Processes of the Earth System (SORPES; 118.97° E, 32.1° N) in Nanjing, and are detected by the Monitor for AeRosols and GAses in Ambient air (MARGA; Metrohm, Switzerland)3,73. The abnormally low nitrate in summer 2013 is mainly due to meteorological influence (Supplementary Fig. 3).
Extended Data Fig. 5 Linear regression trends of temperature and RH from 2013 to 2017 for annual mean, summer, and winter conditions.
Temperature and RH are from the MERRA-2 reanalysis data from the NASA Goddard Earth Sciences (GES) Data and Information Services Center (https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/data_access/). The dashed rectangles define the North China Plain region (113.75°-118.75° E, 35°-41° N).
Extended Data Fig. 6 Thermodynamic regime for ammonium nitrate particulate formation in the North China Plain in winter.
The figure shows the molar ratio R = [NH3T] /(2 × [SO42-] + [NO3T]) as a function of sulfate-nitrate-ammonium (SNA) PM2.5 concentrations in daily mean GEOS-Chem results for the North China Plain in winters 2013-2017. Formation of nitrate PM2.5 is nitrate-limited if R > 1 (ammonia in excess) and ammonia-limited if R < 1 (nitrate in excess). The black dashed line indicates R = 1. This figure can be compared to Fig. 4a from ref. 44 which showed the same plot for observations in Beijing in December 2015 and December 2016. Bisulfate (HSO4-) in acid particles would modify the acid-base balance but we find from ISORROPIA II calculations that it accounts for less than 5% of total sulfate in the model, consistent with wintertime Beijing observations44.
Extended Data Fig. 7 2013-2017 trends of PM2.5 nitrate, the particulate fraction of total nitrate ([NO3-]/[NO3T] molar ratio), and NO3T lifetime against deposition simulated by GEOS-Chem without implementation of the new wet deposition scheme in ref. 34.
Results are from GEOS-Chem driven by 2013 and 2017 MEIC emissions with 2017 meteorology applied to the two years.
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Zhai, S., Jacob, D.J., Wang, X. et al. Control of particulate nitrate air pollution in China. Nat. Geosci. 14, 389–395 (2021). https://doi.org/10.1038/s41561-021-00726-z