POMINO: An improved satellite NO2 product and impacts on emission inversio

Abstract:

Satellite retrievals of vertical column densities (VCDs) of tropospheric nitrogen dioxide (NO₂) normally do not explicitly account for aerosol optical effects and surface reflectance anisotropy that vary with space and time. Here, we conduct an improved retrieval of NO₂VCDs over China, POMINO, from the Ozone Monitoring Instrument (OMI) and test the importance of aerosol and surface reflectance treatments. POMINO uses a parallelized LIDORT-driven AMFv6 package to derive tropospheric air mass factors, taking slant column densities from DOMINO v2. Prerequisite cloud optical properties are retrieved with a consistent procedure. Aerosol optical properties are taken from GEOS-Chem simulations with subsequent MODIS-based AOD constraints. MODIS bi-directional reflectance distribution function (BRDF) data are used for surface reflectance over lands. Level-2 data for 2012 are aggregated into monthly means on a 0.25° long. x 0.25° lat. grid.

POMINO-retrieved annual mean NO₂ VCDs vary from 15–25 x 10¹⁵ cm^-2 over the polluted North China Plain (NCP) to below 10¹⁵ cm^-2 over much of west China. The subsequently-constrained Chinese annual anthropogenic emissions are 9.05 TgN yr^-1, an increase from 2006 (Lin, 2012) by about 19%. Replacing the MODIS BRDF data with the OMLER v1 monthly climatological albedo data affects NO₂ VCDs by up to 40% for certain locations and seasons. The effect on constrained NO_x emissions is small. Assuming no aerosols in the retrieval process (as the traditional “implicit” treatment) enhances annual mean NO₂ VCDs by 15–40% over much of east China. Seasonally, NO₂ VCDs are reduced by 10–20% over parts of the NCP in spring and over north China in winter, despite the general enhancements in summer and fall. The effect on subsequently-constrained annual emissions is -5–30% with large seasonal and spatial dependence. The implicit aerosol treatment also tends to exclude days with high pollution, an important sampling bias. We conclude that the POMINO approach will be useful for quantifying NO_x pollution over China and other world regions with high aerosols and/or complex ground conditions.

Ref: Lin, J.-T. et al.: Influence of aerosols and surface reflectance on satellite NO2 retrieval: Seasonal and spatial characteristics and implications for NOx emission constraints, ACPD, 15, 1-62, 2015