Background nitrogen dioxide (NO2) over the United States and its implications for satellite observations and trends: effects of nitrate photolysis, aircraft, and open fires
Abstract. Tropospheric nitrogen dioxide (NO2) measured from satellites has been widely used to track anthropogenic NOx emissions, but its retrieval and interpretation can be complicated by the free tropospheric NO2 background to which satellite measurements are particularly sensitive. Tropospheric NO2 columns from the OMI satellite instrument averaged over the contiguous US (CONUS) show no trend after 2009, despite sustained decreases in anthropogenic NOx emissions, implying an important and rising contribution from the free tropospheric background that has not been captured in models. Here we use the GEOS-Chem chemical transport model applied to the simulation of OMI NO2 to better understand the sources and trends of background NO2 over CONUS. Previous model underestimate of the background is largely corrected by the consideration of aerosol nitrate photolysis, which increases the model NO2 column by 13 % on an annual basis (25 % in spring), and also increases the air mass factor (AMF) to convert the tropospheric slant columns inferred from the OMI spectra into vertical NO2 columns by 7 % on an annual basis (11 % in spring). The increase in the AMF decreases the retrieved NO2 columns in the satellite observations, contributing to the improved agreement with the model. Accounting for the 2009–2017 increase in aircraft NOx emissions drives only a 1.4 % mean increase in NO2 column over CONUS and a 2 % increase in the AMF, but the combination of decreasing surface NOx 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 NO2 trends. Fire smoke identification with the NOAA Hazard Mapping System (HMS) indicates that wildfires contribute 1–8 % of OMI NO2 columns over the western US in June–September and that this contribution has been increasing since 2009, contributing to the flattening of OMI NO2 trends. Future analyses of NO2 trends from satellite data to infer trends in surface NOx emissions must critically consider the effects of a rising free tropospheric background due to increasing emissions from aircraft, fires, and possibly lightning.
Ruijun Dang et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2022-1198', Anonymous Referee #1, 13 Dec 2022
- RC2: 'Comment on egusphere-2022-1198', Anonymous Referee #2, 20 Dec 2022
- AC1: 'Author response to reviewer comments', Ruijun Dang, 12 Feb 2023
Ruijun Dang et al.
Ruijun Dang et al.
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Review ‘Background nitrogen dioxide (NO2) over the United States and its implications for satellite observations and trends: effects of nitrate photolysis, aircraft, and open fires’ by Dang et al.
The study by Dang et al follows up on the ongoing conversation on the importance of free tropospheric NO2 for satellite retrievals and their interpretation. The paper puts forward interesting hypotheses and sensitivity tests for tropospheric NO2 that can be seen as guidelines for future research. The prediction of how satellite UV/Vis air mass factors may change in response to increasing aircraft NOx emissions is exciting, and provides a clear message to the retrieval community to stay on their toes in accounting for possible changes in atmospheric composition.
The paper is very well written and clear in reporting its model-based findings and providing recommendations. The one difficulty I have with judging the relevance of the findings is that many of the statements rely on the confidence the authors have in the accuracy of their baseline simulations of FT NO2. The paper would be much stronger if the authors would evaluate their simulated FT NO2 profiles with aircraft NO2 profiles that have been measured over the contiguous US over the last two decades via campaigns, or with simulations from other chemistry transport models (e.g. GMI). Such a comparison could help to substantiate the claim that including an NO2 source from nitrate photolysis in the GEOS-Chem simulation can explain ‘a large and increasing contribution from background NO2 in the free troposphere’.
Another question is why the effect of enhanced NO2 from nitrate photolysis is present so ubiquitously throughout the free troposphere. I would think that the nitrate aerosol vertical distribution has -on average- lowest mass density in the upper troposphere and highest mass density in the lower troposphere (above the boundary layer), and therefore constitutes a stronger source of NO2 in the mid-troposphere than in the upper troposphere. But the NO2 increase from nitrate photolysis shows up throughout the entire free troposphere. Could this be related to stronger radiation levels in the UT? Do the authors also see a spatial gradient in the NO2 addition with stronger NO2 additions in the warm seasons, and close to the coasts?
L169-171: please indicate for which wavelengths the photolysis of particulate nitrate occurs.
L296: I propose to (also) cite the study by Castellanos et al.  here, who showed that AMF calculations that explicitly account for aerosol absorption and scattering are on average 10% and up to a factor 2 higher than AMF calculations with implicit aerosol corrections.
Castellanos, P., Boersma, K. F., Torres, O., and de Haan, J. F.: OMI tropospheric NO2 air mass factors over South America: effects of biomass burning aerosols, Atmos. Meas. Tech., 8, 3831–3849, https://doi.org/10.5194/amt-8-3831-2015, 2015.