Influence of aerosols and surface reflectance on satellite NO2 retrieval: seasonal and spatial characteristics and implications for NOx emission constraints

摘要

Satellite retrievals of vertical column densities (VCDs) of troposphericnitrogen dioxide (NO) normally do not explicitly account for aerosoloptical effects and surface reflectance anisotropy that vary with space andtime. Here, we conduct an improved retrieval of NO VCDs over China,called the POMINO algorithm, based on measurements from the Ozone MonitoringInstrument (OMI), and we test the importance of a number of aerosol andsurface reflectance treatments in this algorithm. POMINO uses a parallelizedLIDORT-driven AMFv6 package to derive tropospheric air mass factors viapixel-specific radiative transfer calculations with no look-up tables, takingslant column densities from DOMINO v2. Prerequisite cloud optical propertiesare derived from a dedicated cloud retrieval process that is fully consistentwith the main NO retrieval. Aerosol optical properties are taken fromGEOS-Chem simulations constrained by MODIS aerosol optical depth (AOD)data. MODIS bi-directional reflectance distribution function (BRDF) dataare used for surface reflectance over land. For the present analysis, POMINOlevel-2 data for 2012 are aggregated into monthly means on a 0.25°long. × 0.25° lat. grid.POMINO-retrieved annual mean NO VCDs vary from15–25 × 10 cm over the polluted North China Plain(NCP) to below 10 cm over much of western China. Using POMINOto infer Chinese emissions of nitrogen oxides leads to annual anthropogenicemissions of 9.05 TgN yr, an increase from 2006 (Lin, 2012) by about19 %. Replacing the MODIS BRDF data with the OMLER v1 monthlyclimatological albedo data affects NO VCDs by up to 40 % forcertain locations and seasons. The effect on constrained NO emissionsis small. Excluding aerosol information from the retrieval process (this isthe traditional "implicit" treatment) enhances annual mean NO VCDs by15–40 % over much of eastern China. Seasonally, NO VCDs arereduced by 10–20 % over parts of the NCP in spring and over northernChina in winter, despite the general enhancements in summer and fall. Theeffect on subsequently constrained annual emissions is between −5 and+30 % with large seasonal and spatial dependence. The implicit aerosoltreatment also tends to exclude days with high pollution, since aerosols areinterpreted as effective clouds and the respective OMI pixels are oftenexcluded by cloud screening; this is a potentially important sampling bias.Therefore an explicit treatment of aerosols is important for space-basedNO retrievals and emission constraints. A comprehensive independentmeasurement network with sufficient spatial and temporal representativenessis needed to further evaluate the different satellite retrieval approaches.