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Discussion papers | Copyright
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Research article 07 Mar 2018

Research article | 07 Mar 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Improved aerosol correction for OMI tropospheric NO2 retrieval over East Asia: constraint from CALIOP aerosol vertical profile

Mengyao Liu1,2, Jintai Lin1, K. Folkert Boersma2,3, Gaia Pinardi4, Yang Wang5, Julien Chimot6, Thomas Wagner5, Pinhua Xie7,8,9, Henk Eskes2, Michel Van Roozendael4, François Hendrick4, Pucai Wang10, and Yingying Yan1 Mengyao Liu et al.
  • 1Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
  • 2Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
  • 3Meteorology and Air Quality department, Wageningen University, Wageningen, the Netherlands
  • 4Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
  • 5Max Planck Institute for Chemistry, Mainz, Germany
  • 6Department of Geoscience and Remote Sensing (GRS), Civil Engineering and Geosciences, TU Delft, the Netherlands
  • 7Anhui Institute of Optics and Fine Mechanics, Key laboratory of Environmental Optics and Technology, Chinese Academy of Sciences, Hefei, China
  • 8CAS Center for Excellence in Urban Atmosphe ric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, China
  • 9School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei, China
  • 10IAP/CAS, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

Abstract. Satellite retrieval of vertical column densities (VCDs) of tropospheric nitrogen dioxide (NO2) is critical for NOx pollution and impact evaluation. For regions with high aerosol loadings, the retrieval accuracy is greatly affected by whether aerosol optical effects are treated implicitly (as additional effective clouds) or explicitly, among other factors. Our previous POMINO algorithm explicitly accounts for aerosol effects to improve the retrieval especially in polluted situations over China, by using aerosol information from GEOS-Chem simulations with further monthly constraint by MODIS/Aqua AOD data. This study updates the retrieval algorithm to POMINO v1.1, by constructing a monthly climatological dataset of aerosol extinction profiles, based on Level-2 CALIOP/CALIPSO data over 2007–2015, to better constrain the modeled aerosol profiles.

We find that GEOS-Chem captures the month-to-month variation of CALIOP aerosol layer height but with a systematic underestimate by about 300–600m (season and location dependent), due to a too strong vertical gradient of extinction above 1km. Correcting the model aerosol extinction profiles results in small changes in retrieved cloud fraction, increases in cloud top pressure (within 2–6% in most cases), and increases in tropospheric NO2 VCD by 4–16% over China on a monthly basis in 2012. The improved NO2 VCDs (in POMINO v1.1) are more consistent with independent ground-based MAX-DOAS observations (R2 = 0.80, NMB = −3.4%) than POMINO (R2 = 0.80, NMB = −9.6%) and DOMINO v2 (R2 = 0.68, NMB = −2.1%) are. Especially on haze days, R2 reaches 0.76 for POMINO v1.1, much higher than that for POMINO (0.68) and DOMINO v2 (0.38). Furthermore, the increase in cloud pressure likely reveals a more realistic vertical relationship between cloud and aerosol layers, with aerosols situated above the clouds in certain months instead of always below the clouds. Our POMINO v1.1 algorithm will be applied to the recently launched TropOMI sensor.

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Publications Copernicus
Short summary
China has become the world's largest emitting country of NOx, which mainly come from vehicle exhausts, power plants, etc. However, there is no official ground-based measurements before 2013, so satellite have been widely used to monitor and analyze NOx pollution here. Aerosol is the key factor to influence the accuracy of the satellite NOx product. Our study provide a more accurate way to account aerosol's influence comparing to current wildly used products.
China has become the world's largest emitting country of NOx, which mainly come from vehicle...