Quantifying NOx emissions from a city and a small country with the TROPOMI and OMI satellite instruments

Folkert Boersma,Alba Lorente,Maarten Krol,Marina Zara,Henk Eskes

doi:10.5194/egusphere-egu2020-11042

Abstract

&lt;p&gt;The TROPOMI and OMI satellite sensors provide an exciting perspective on the sources, dispersion, and fate of air pollution, and in particular on nitrogen dioxide (NO&lt;sub&gt;2&lt;/sub&gt;). Yet it is still difficult to relate satellite observations of tropospheric NO&lt;sub&gt;2&lt;/sub&gt; columns to the underlying NO&lt;sub&gt;x&lt;/sub&gt; emissions and their trends. Robust interpretation of satellite data relies on a good understanding of the accuracy and representativeness of the satellite data itself, but also on the relationship between NO&lt;sub&gt;x&lt;/sub&gt; emissions and the observable NO&lt;sub&gt;2&lt;/sub&gt; amount. This relationship is influences by local chemistry, mixing and dispersion, and by the NO&lt;sub&gt;2&lt;/sub&gt; amount in the free troposphere. We address these issues via two examples:&lt;/p&gt;&lt;p&gt;(1) Direct estimation of NO&lt;sub&gt;x&lt;/sub&gt; emissions from the satellite-observed build-up of pollution over the city of Paris. After validating NO&lt;sub&gt;2&lt;/sub&gt; measurements from TROPOMI over the heart of Paris, we analyse the observed build-up of NO&lt;sub&gt;2&lt;/sub&gt; pollution over the city along with the wind. Over the city, recently emitted NO&lt;sub&gt;x&lt;/sub&gt; has been oxidized to limited degree, facilitating the use of TROPOMI data to directly determine the strength and distribution of emissions from the city. From the observed build-up of NO&lt;sub&gt;2&lt;/sub&gt; pollution, we find highest NO&lt;sub&gt;x&lt;/sub&gt; emissions on cold weekdays in February 2018, and lowest emissions on warm weekend days in spring 2018.&lt;/p&gt;&lt;p&gt;(2) Trends in NO&lt;sub&gt;2&lt;/sub&gt; over The Netherlands. We use the QA4ECV OMI NO&lt;sub&gt;2&lt;/sub&gt; record to investigate trends in tropospheric NO&lt;sub&gt;2&lt;/sub&gt; columns over Europe, and in particular over The Netherlands between 2005 and 2018. In spite of the differences in metrics and sampling techniques, the NO&lt;sub&gt;2&lt;/sub&gt; measured in the Dutch atmosphere from space and from the ground follows a trend that is consistent with predictions by emission inventories. Surface NO&lt;sub&gt;2&lt;/sub&gt; is reduced by 32% in 2018 relative to 2005, OMI NO&lt;sub&gt;2&lt;/sub&gt; by 35%, and NO&lt;sub&gt;x&lt;/sub&gt; emissions by 32%-38% depending on the inventory. Interestingly, the Dutch surface concentrations reveal an upward trend in the NO&lt;sub&gt;2&lt;/sub&gt;:NO ratio in line with O&lt;sub&gt;3&lt;/sub&gt; increases. This suggests that the NO&lt;sub&gt;2&lt;/sub&gt; makes up an increasing share of the NO&lt;sub&gt;x&lt;/sub&gt; in the lower atmosphere as NO&lt;sub&gt;x&lt;/sub&gt; emissions decline. This needs to be accounted for when interpreting NO&lt;sub&gt;2&lt;/sub&gt; trends as proxy for NO&lt;sub&gt;x&lt;/sub&gt; trends.&lt;/p&gt;

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