Abstract
Abstract. The Ozone Monitoring Instrument (OMI) has provided daily global measurements of tropospheric NO2 for more than a decade. Numerous studies have drawn attention to the complexities related to measurements of tropospheric NO2 in the presence of aerosols. Fine particles affect the OMI spectral measurements and the length of the average light path followed by the photons. However, they are not explicitly taken into account in 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 both to cloudy and to cloud-free scenes (i.e. clear sky) dominated by the presence of aerosols. This paper describes in detail the complex interplay between the spectral effects of aerosols in the satellite observation and the associated response of the OMI O2 − O2 cloud retrieval algorithm. Then, it evaluates the impact on the accuracy of the tropospheric NO2 retrievals through the computed Air Mass Factor (AMF) with a focus on cloud-free scenes. For that purpose, collocated OMI NO2 and MODIS (Moderate Resolution Imaging Spectroradiometer) Aqua aerosol products are analysed over the strongly industrialized East China area. In addition, aerosol effects on the tropospheric NO2 AMF and the retrieval of OMI cloud parameters are simulated. Both the observation-based and the simulation-based approach demonstrate that the retrieved cloud fraction increases with increasing Aerosol Optical Thickness (AOT), but the magnitude of this increase depends on the aerosol properties and surface albedo. This increase is induced by the additional scattering effects of aerosols which enhance the scene brightness. The decreasing effective cloud pressure with increasing AOT primarily represents the shielding effects of the O2 − O2 column located below the aerosol layers. The study cases show that the aerosol correction based on the implemented OMI cloud model results in biases between −20 and −40 % for the DOMINO tropospheric NO2 product in cases of high aerosol pollution (AOT ≥ 0.6) at elevated altitude. These biases result from a combination of the cloud model error, used in the presence of aerosols, and the limitations of the current OMI cloud Look-Up-Table (LUT). A new LUT with a higher sampling must be designed to remove the complex behaviour between these biases and AOT. In contrast, when aerosols are relatively close to the surface or mixed with NO2, aerosol correction based on the cloud model results in an overestimation of the DOMINO tropospheric NO2 column, between 10 and 20 %. These numbers are in line with comparison studies between ground-based and OMI tropospheric NO2 measurements in the presence of high aerosol pollution and particles located at higher altitudes. This highlights the need to implement an improved aerosol correction in the computation of tropospheric NO2 AMFs.
Highlights
Nitrogen oxides (NOx = NO+NO2) play a key role in atmospheric chemistry, regulating the level of ozone and maintaining the oxidizing capacity in the troposphere
The behaviour of the Ozone Monitoring Instrument (OMI) cloud model over cloud-free scenes dominated by aerosols was studied as well as the accuracy of the cloud-model-based aerosol correction of tropospheric NO2 Air Mass Factor (AMF)
This study focused on the operational OMI DOMINO product for cloud-free scenes, its behaviour in the presence of aerosol-dominated scenes that were selected based on collocated MODIS Aqua aerosol products, and the comparison with numerical simulated study cases
Summary
Nitrogen oxides (NOx = NO+NO2) play a key role in atmospheric chemistry, regulating the level of ozone and maintaining the oxidizing capacity in the troposphere. The retrieval technique of the OMI tropospheric NO2 Vertical Column Density (VCD) (Boersma et al, 2004) is common to all the other similar satellite missions (Burrows et al, 1999; Bovensmann et al, 1999). The backscattered solar radiation is captured in daylight in the visible spectral domain by the instrument at the Top Of the Atmosphere (TOA) and processed through the Differential Optical Absorption Spectroscopy (DOAS) retrieval approach. The associated assumptions play a crucial role in the accuracy of the tropospheric NO2 VCD
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