GOME-2A retrievals of tropospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in different spectral ranges – influence of penetration depth

Lisa K Behrens,Andreas Richter,John P Burrows,Enno Peters,Andreas Hilboll,Henk Eskes

doi:10.5194/amt-11-2769-2018

Abstract

Abstract. In this study, we present a novel nitrogen dioxide (NO2) differential optical absorption spectroscopy (DOAS) retrieval in the ultraviolet (UV) spectral range for observations from the Global Ozone Monitoring Instrument 2 on board EUMETSAT's MetOp-A (GOME-2A) satellite. We compare the results to those from an established NO2 retrieval in the visible (vis) spectral range from the same instrument and investigate how differences between the two are linked to the NO2 vertical profile shape in the troposphere.As expected, radiative transfer calculations for satellite geometries show that the sensitivity close to the ground is higher in the vis than in the UV spectral range. Consequently, NO2 slant column densities (SCDs) in the vis are usually higher than in the UV if the NO2 is close to the surface. Therefore, these differences in NO2 SCDs between the two spectral ranges contain information on the vertical distribution of NO2 in the troposphere. We combine these results with radiative transfer calculations and simulated NO2 fields from the TM5-MP chemistry transport model to evaluate the simulated NO2 vertical distribution.We investigate regions representative of both anthropogenic and biomass burning NO2 pollution. Anthropogenic air pollution is mostly located in the boundary layer close to the surface, which is reflected by large differences between UV and vis SCDs of ∼ 60 %. Biomass burning NO2 in contrast is often uplifted into elevated layers above the boundary layer. This is best seen in tropical Africa south of the Equator, where the biomass burning NO2 is well observed in the UV, and the SCD difference between the two spectral ranges is only ∼ 36 %. In tropical Africa north of the Equator, however, the biomass burning NO2 is located closer to the ground, reducing its visibility in the UV.While not enabling a full retrieval of the vertical NO2 profile shape in the troposphere, our results can help to constrain the vertical profile of NO2 in the lower troposphere and, when analysed together with simulated NO2 fields, can help to better interpret the model output.

Highlights

Nitrogen dioxide (NO2) is an important indicator for natural phenomena and anthropogenic air pollution, as it is produced in the troposphere by, e.g., biomass burning or combustion of fossil fuels (Lee et al, 1997)
The slant column densities (SCDs) depend on the measurement geometry, surface spectral reflectance (SSR), and wavelength, and the air mass factors (AMFs) depend on the a priori vertical NO2 profile
The SCD differences can be partly explained by the different measurement geometries and SSR, which change during the year, but other factors like injection height and relative vertical distribution have to contribute as well

Summary

Introduction

Nitrogen dioxide (NO2) is an important indicator for natural phenomena and anthropogenic air pollution, as it is produced in the troposphere by, e.g., biomass burning or combustion of fossil fuels (Lee et al, 1997). Biomass burning is important in equatorial regions like central Africa, whereas anthropogenic emissions are mostly important in the industrialized mid-latitudes like China or Europe. Solar radiation scattered in the Earth’s atmosphere can be measured, and the amount of trace gases inverted mathematically from the depth of molecular absorption bands.

Methods

Results

Conclusion