Abstract
Abstract. A variant of the limb–nadir matching technique for deriving tropospheric NO2 columns is presented in which the stratospheric component of the NO2 slant column density (SCD) measured by the Ozone Monitoring Instrument (OMI) is removed using non-coincident profiles from the Optical Spectrograph and InfraRed Imaging System (OSIRIS). In order to correct their mismatch in local time and the diurnal variation of stratospheric NO2, OSIRIS profiles, which were measured just after sunrise, were mapped to the local time of OMI observations using a photochemical box model. Following the profile time adjustment, OSIRIS NO2 stratospheric vertical column densities (VCDs) were calculated. For profiles that did not reach down to the tropopause, VCDs were adjusted using the photochemical model. Using air mass factors from the OMI Standard Product (SP), a new tropospheric NO2 VCD product – referred to as OMI-minus-OSIRIS (OmO) – was generated through limb–nadir matching. To accomplish this, the OMI total SCDs were scaled using correction factors derived from the next-generation SCDs that improve upon the spectral fitting used for the current operational products. One year, 2008, of OmO was generated for 60° S to 60° N and a cursory evaluation was performed. The OmO product was found to capture the main features of tropospheric NO2, including a background value of about 0.3 × 1015 molecules cm−2 over the tropical Pacific and values comparable to the OMI operational products over anthropogenic source areas. While additional study is required, these results suggest that a limb–nadir matching approach is feasible for the removal of stratospheric NO2 measured by a polar orbiter from a nadir-viewing instrument in a geostationary orbit such as Tropospheric Emissions: Monitoring of Pollution (TEMPO) or Sentinel-4.
Highlights
Nadir satellite instruments can measure daily global maps of tropospheric nitrogen dioxide (NO2), which, at the surface, is a pollutant linked to smog and acid rain
The technique of matching nadir- and limb-viewing satellite retrievals to quantify tropospheric NO2 is explored in this work using Ozone Monitoring Instrument (OMI) nadir measurements and Optical Spectrograph and InfraRed Imaging System (OSIRIS) limb measurements to create the OmO tropospheric NO2 dataset
The new OmO tropospheric NO2 dataset uses information from OSIRIS profile measurements in order to estimate the stratospheric contribution to OMI slant column density (SCD) measurements
Summary
Nadir satellite instruments can measure daily global maps of tropospheric nitrogen dioxide (NO2), which, at the surface, is a pollutant linked to smog and acid rain. Various methods have been used to separate stratospheric and tropospheric contributions from the total NO2 vertical column density (VCD) measured by nadir satellites Several of these techniques assume that the NO2 distribution over a remote, non-polluted location, like the Pacific Ocean, is dominated by the stratospheric component. In order to account for biases between the modelled stratosphere and the satellite measurements, the model data can be scaled to the satellite measurements through comparisons in the Pacific Ocean (Richter et al, 2005) or the measured total column NO2 can be assimilated in the model (e.g., Boersma et al, 2007) These techniques, as applied to operational data products for OMI, are described in more detail in Sect. A discussion of these results and future applications is given Sect. 5
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