Impact of aerosols on the OMI troposphericNO2 retrievals over industrializedregions: how accurate is the aerosol correction of cloud-free scenes via a simple cloud model?

[摘要] The Ozone Monitoring Instrument (OMI)has provided daily globalmeasurements of tropospheric NO₂ for more than a decade. Numerousstudies have drawn attention to the complexities related to measurements oftropospheric NO₂ in the presence of aerosols. Fine particles affectthe OMI spectral measurements and the length of the average light pathfollowed by the photons. However, they are not explicitly taken into accountin the current operational OMI tropospheric NO₂ retrieval chain(DOMINO – Derivation of OMI tropospheric NO₂) product. Instead,the operational OMI O₂ − O₂ cloud retrieval algorithm is applied bothto cloudy and to cloud-free scenes (i.e. clear sky) dominated by the presenceof aerosols. This paper describes in detail the complex interplay between thespectral effects of aerosols in the satellite observation and the associatedresponse of the OMI O₂ − O₂ cloud retrieval algorithm. Then, itevaluates the impact on the accuracy of the tropospheric NO₂retrievals through the computed Air Mass Factor (AMF) with a focus oncloud-free scenes. For that purpose, collocated OMI NO₂ and MODIS(Moderate Resolution Imaging Spectroradiometer) Aqua aerosol products areanalysed over the strongly industrialized East China area. In addition,aerosol effects on the tropospheric NO₂ AMF and the retrieval of OMIcloud parameters are simulated. Both the observation-based and thesimulation-based approach demonstrate that the retrieved cloud fractionincreases with increasing Aerosol Optical Thickness (AOT), but the magnitudeof this increase depends on the aerosol properties and surface albedo. Thisincrease is induced by the additional scattering effects of aerosols whichenhance the scene brightness. The decreasing effective cloud pressure withincreasing AOT primarily represents the shielding effects of theO₂ − O₂ column located below the aerosol layers. The study cases showthat the aerosol correction based on the implemented OMI cloud model resultsin biases between −20 and −40 % for the DOMINO troposphericNO₂ product in cases of high aerosol pollution (AOT  ≥ 0.6) atelevated altitude. These biases result from a combination of the cloud modelerror, used in the presence of aerosols, and the limitations of the currentOMI cloud Look-Up-Table (LUT). A new LUT with a higher sampling must bedesigned to remove the complex behaviour between these biases and AOT. Incontrast, when aerosols are relatively close to the surface or mixed withNO₂, aerosol correction based on the cloud model results in anoverestimation of the DOMINO tropospheric NO₂ column, between 10 and20 %. These numbers are in line with comparison studies betweenground-based and OMI tropospheric NO₂ measurements in the presence ofhigh aerosol pollution and particles located at higher altitudes. Thishighlights the need to implement an improved aerosol correction in thecomputation of tropospheric NO₂ AMFs.

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