First estimates of global free-tropospheric NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; abundances derived using a cloud-slicing technique applied to satellite observations from the Aura Ozone Monitoring Instrument (OMI)

S Choi,J Joiner,Y Choi,B N Duncan,N Krotkov,A Vasilkov,E Bucsela

doi:10.5194/acp-14-10565-2014

Abstract

Abstract. We derive free-tropospheric NO2 volume mixing ratios (VMRs) by applying a cloud-slicing technique to data from the Ozone Monitoring Instrument (OMI) on the Aura satellite. In the cloud-slicing approach, the slope of the above-cloud NO2 column versus the cloud scene pressure is proportional to the NO2 VMR. In this work, we use a sample of nearby OMI pixel data from a single orbit for the linear fit. The OMI data include cloud scene pressures from the rotational-Raman algorithm and above-cloud NO2 vertical column density (VCD) (defined as the NO2 column from the cloud scene pressure to the top of the atmosphere) from a differential optical absorption spectroscopy (DOAS) algorithm. We compare OMI-derived NO2 VMRs with in situ aircraft profiles measured during the NASA Intercontinental Chemical Transport Experiment Phase B (INTEX-B) campaign in 2006. The agreement is generally within the estimated uncertainties when appropriate data screening is applied. We then derive a global seasonal climatology of free-tropospheric NO2 VMR in cloudy conditions. Enhanced NO2 in the free troposphere commonly appears near polluted urban locations where NO2 produced in the boundary layer may be transported vertically out of the boundary layer and then horizontally away from the source. Signatures of lightning NO2 are also shown throughout low and middle latitude regions in summer months. A profile analysis of our cloud-slicing data indicates signatures of lightning-generated NO2 in the upper troposphere. Comparison of the climatology with simulations from the global modeling initiative (GMI) for cloudy conditions (cloud optical depth > 10) shows similarities in the spatial patterns of continental pollution outflow. However, there are also some differences in the seasonal variation of free-tropospheric NO2 VMRs near highly populated regions and in areas affected by lightning-generated NOx.

Highlights

Tropospheric nitrogen dioxide (NO2) is mainly produced by fossil fuel combustion, biomass burning, and soil emission near the Earth’s surface and by lightning and aircraft emissions in middle and upper troposphere
We show a case of two adjacent Ozone Monitoring Instrument (OMI) measurements in Fig. 1 for simplicity, we typically use an OMI pixel collection that consists of a number of nearby measurements collected over one OMI orbit; this minimizes the effects of random errors from both the above-cloud OMI NO2 vertical column density (VCD) and pscene
We evaluate OMI NO2 volume mixing ratios (VMRs) derived from cloud slicing using aircraft in situ NO2 measurements made during INTEX-B

Summary

Introduction

Tropospheric nitrogen dioxide (NO2) is mainly produced by fossil fuel combustion, biomass burning, and soil emission near the Earth’s surface and by lightning and aircraft emissions in middle and upper troposphere. NO2 is an important tropospheric constituent, because it is both a pollutant and climate agent It has adverse effects on human health (Brook et al, 2007) and is one of six criteria pollutants designated by the US Environmental Protection Agency (EPA). It contributes to the formation of ozone, another EPA criteria pollutant. NO2 has both direct and indirect radiative effects. Because NO2 is an ozone precursor and affects tropospheric concentrations of methane, it has indirect shortwave and long-wave radiative effects (e.g., Fuglestvedt et al, 2008; Wild et al, 2001; Shindell et al, 2009)

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