Background nitrogen dioxide (NO<sub>2</sub>) over the United States and its implications for satellite observations and trends: effects of nitrate photolysis, aircraft, and open fires
<p><span id="page6272"/>Tropospheric nitrogen dioxide (NO<span class="inline-formula"><sub>2</sub>)</span> measured from satellites has been widely used to track anthropogenic NO<span class="inline-formula"><sub><i>x...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-06-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/23/6271/2023/acp-23-6271-2023.pdf |
| Summary: | <p><span id="page6272"/>Tropospheric nitrogen dioxide (NO<span class="inline-formula"><sub>2</sub>)</span> measured from
satellites has been widely used to track anthropogenic NO<span class="inline-formula"><sub><i>x</i></sub></span> emissions,
but its retrieval and interpretation can be complicated by the free tropospheric NO<span class="inline-formula"><sub>2</sub></span> background to which satellite measurements are
particularly sensitive. Tropospheric NO<span class="inline-formula"><sub>2</sub></span> vertical column densities
(VCDs) from the spaceborne Ozone Monitoring Instrument (OMI) averaged over the contiguous US
(CONUS) show no trend after 2009, despite sustained decreases in
anthropogenic NO<span class="inline-formula"><sub><i>x</i></sub></span> emissions, implying an important and rising
contribution from the free tropospheric background. Here, we use the
GEOS-Chem chemical transport model applied to the simulation of OMI NO<span class="inline-formula"><sub>2</sub></span>
to better understand the sources and trends of background NO<span class="inline-formula"><sub>2</sub></span> over
CONUS. The previous model underestimate of the background is largely corrected
by the consideration of aerosol nitrate photolysis, which increases the
model NO<span class="inline-formula"><sub>2</sub></span> VCDs by 13 % on an annual basis (25 % in spring) and
also increases the air mass factor (AMF) to convert the tropospheric slant
column densities (SCDs) inferred from the OMI spectra into VCDs by 7 % on
an annual basis (11 % in spring). The increase in the AMF decreases the
retrieved NO<span class="inline-formula"><sub>2</sub></span> VCDs in the satellite observations, contributing to the
improved agreement with the model. Accounting for the 2009–2017 increase in
aircraft NO<span class="inline-formula"><sub><i>x</i></sub></span> emissions drives only a 1.4 % mean increase in NO<span class="inline-formula"><sub>2</sub></span>
VCDs over CONUS and a 2 % increase in the AMF, but the combination of
decreasing surface NO<span class="inline-formula"><sub><i>x</i></sub></span> emissions and increasing aircraft emissions is
expected to drive a 14 % increase in the AMF over the next decade that
will be necessary to account for in the interpretation of satellite NO<span class="inline-formula"><sub>2</sub></span>
trends. Fire smoke identification with the National Oceanic and Atmospheric Administration (NOAA) Hazard Mapping System (HMS)
indicates that wildfires contribute 1 %–8 % of OMI NO<span class="inline-formula"><sub>2</sub></span> VCDs over the
western US in June–September and that this contribution has been increasing
since 2009, contributing to the flattening of OMI NO<span class="inline-formula"><sub>2</sub></span> trends. Future
analyses of NO<span class="inline-formula"><sub>2</sub></span> trends from satellite data to infer trends in surface
NO<span class="inline-formula"><sub><i>x</i></sub></span> emissions must critically consider the effects of a rising free tropospheric background due to increasing emissions from aircraft, fires,
and possibly lightning.</p> |
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| ISSN: | 1680-7316 1680-7324 |