The importance of accounting for the free tropospheric Nitrogen Dioxide background

Using satellite observations of tropospheric Nitrogen Dioxide (NO₂) columns to infer long-term trends in US nitrogen oxide (NO_x) emissions

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• The National Emission Inventory (NEI) of the US EPA reports a steady decrease of US NO_x emissions over the 2005-2017 period, while tropospheric NO₂ columns observed by OMI show a steady decrease until 2009 but a flattening afterward, which has been attributed to a flattening of US NO_x emissions.

• We show that the steady decrease of NO_x emissions reported by the NEI is in fact largely consistent with observed network trends of surface NO₂ and ozone concentrations, and consistent with chemical transport modeling using GEOS-Chem.

• The post-2009 flattening of the OMI NO₂ trend is instead due to the increasing relative importance of the non-anthropogenic NO₂ background, rather than a flattening of US NO_x emissions.

• Better understanding is needed of factors controlling free tropospheric NO₂ in order to relate satellite observations of tropospheric NO₂ columns to underlying NO_x emissions and their trends.

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Technical description of figure:

Relative trends since 2005 of NEI NO_x emissions, OMI tropospheric NO₂ columns, surface NO₂ concentrations, and nitrate wet deposition fluxes averaged over the contiguous US. The left panel shows observations for sites with continuous annual records for 2005-2017 (2016 for SEARCH) and the right panel shows the GEOS-Chem model values sampled at the corresponding sites. NEI NO_x emissions are the same in both panels.

Scientific significance, societal relevance, and relationships to future missions:

Following increasingly stringent emission controls targeted at improving air quality since the early 2000s, the EPA NEI reports a steady decrease in US fuel combustion emissions of nitrogen oxides (NO_x) over the 2005-2017 period. Tropospheric NO₂ columns over the US observed by OMI, presumed to be a proxy for NO_x emissions if most most of the information in the NO₂ tropospheric column originates from the boundary layer, show a leveling off after 2009, leading to the suggestion that the NEI emission trend is in error and that related air quality gains have halted.

Here we conduct a comprehensive analysis of 2005-2017 trends in US NO_x emissions by using the GEOS-Chem chemical transport model to concurrently interpret the trends observed in OMI NO₂ columns, nitrogen wet deposition fluxes, and surface observations of NO₂ and ozone. We show the flattening of the OMI NO₂ trend is in fact not inconsistent with the sustained decrease in NO_x emissions reported by the NEI and that the NEI emission trend is consistent with other atmospheric observations of NO_x and ozone trends. Our results demonstrate the importance of accounting for the free tropospheric NO₂ background when using satellite observations of NO₂ columns to infer NO_x emissions and their trends. The concern is minor in highly polluted areas where NO_x emissions are sufficiently high to dominate over the background influence. In the US, however, NO_x emissions have now decreased to the point that NO₂ columns over non-urban areas are mostly contributed by the free tropospheric background. Accounting for this poorly understood background will become increasingly important as NO_x emissions continue to decrease in the developed world, and in tropical regions that are undergoing rapid development but have a deep troposphere and intense lightning. The upcoming TEMPO mission will need to properly account for free tropospheric NO₂ in their retrieval and will be able to leverage the cloud-slicing technique developed for OMI to separate boundary layer and free tropospheric NO₂ with higher spatial and temporal resolution.

Data sources:

OMI observations of tropospheric NO₂ columns are from the NASA operational retrieval level 2 version 3.0 (https://mirador.gsfc.nasa.gov/; Krotkov et al., 2017) after removing cloudy scenes (cloud radiance fraction > 0.5), bright surfaces (surface reflectivity > 0.3), and observations affected by the so-called row anomaly (Dobber et al., 2008). NO_x emissions are from the US EPA National Emission Inventory (NEI; https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data). Surface NO₂ data are from the mainly urban US EPA Air Quality System (AQS) (https://www.epa.gov/aqs; Demerjian, 2000) sites and two rural sites in the southeast from the Southeastern Aerosol Research and Characterization Study (SEARCH) network (https://www.dropbox.com/sh/o9hxoa4wlo97zpe/AACbm6LetQowrpUgX4vUxnoDa?dl=0; Hansen et al., 2003; Edgerton et al., 2006), and nitrate wet deposition observations are from the National Acid Deposition Program (NADP; https://nadp.slh.wisc.edu/data/NTN/). Model results are from the GEOS-Chem global 3-D chemical transport model version 11-02c (http://www.geos-chem.org) using NASA MERRA-2 assimilated meteorological data (Gelaro et al., 2017) at the native 0.5°× 0.625° horizontal resolution over North America and adjacent oceans with dynamic boundary conditions from a global simulation.

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References: Silvern, R.F., Jacob, D.J., Mickley, L.J., Sulprizio, M.P., Travis, K.R., Marais, E.A., Cohen, R.C., Laughner, J.L., Choi, S., Joiner, J., and Lamsal, L.N.: Using satellite observations of tropospheric NO₂ columns to infer long-term trends in US NO_x emissions: the importance of accounting for the free tropospheric NO₂ background, Atmospheric Chemistry and Physics, 19, 8863–8878, https://doi.org/10.5194/acp-19-8863-2019, 2019 .

7.2019