Details

Autor(en) / Beteiligte

• Silvern, Rachel F
• Jacob, Daniel J
• Mickley, Loretta J
• Sulprizio, Melissa P
• Travis, Katherine R
• Marais, Eloise A
• Cohen, Ronald C
• Laughner, Joshua L
• Choi, Sungyeon
• Joiner, Joanna
• Lamsal, Lok N

Titel

Using satellite observations of tropospheric NO.sub.2 columns to infer long-term trends in US NO.sub.x emissions: the importance of accounting for the free tropospheric NO.sub.2 background

Ist Teil von

• Atmospheric chemistry and physics, 2019-07, Vol.19 (13), p.8863

Ort / Verlag

Copernicus GmbH

Erscheinungsjahr

2019

Quelle

EZB Electronic Journals Library

Beschreibungen/Notizen

• The National Emission Inventory (NEI) of the US Environmental Protection Agency (EPA) reports a steady decrease in US NO.sub.x emissions over the 2005-2017 period at a rate of 0.1 Tg N a.sup.-1 (53 % decrease over the period), reflecting sustained efforts to improve air quality. Tropospheric NO.sub.2 columns observed by the satellite-based Ozone Monitoring Instrument (OMI) over the US show a steady decrease until 2009 but a flattening afterward, which has been attributed to a flattening of NO.sub.x emissions, contradicting the NEI. We show here that the steady 2005-2017 decrease in NO.sub.x emissions reported by the NEI is in fact largely consistent with observed network trends of surface NO.sub.2 and ozone concentrations. The OMI NO.sub.2 trend is instead similar to that observed for nitrate wet deposition fluxes, which is weaker than that for anthropogenic NO.sub.x emissions, due to a large and increasing relative contribution of non-anthropogenic background sources of NO.sub.x (mainly lightning and soils). This is confirmed by contrasting OMI NO.sub.2 trends in urban winter, where the background is low and OMI NO.sub.2 shows a 2005-2017 decrease consistent with the NEI, and rural summer, where the background is high and OMI NO.sub.2 shows no significant 2005-2017 trend. A GEOS-Chem model simulation driven by NEI emission trends for the 2005-2017 period reproduces these different trends, except for the post-2009 flattening of OMI NO.sub.2, which we attribute to a model underestimate of free tropospheric NO.sub.2 . Better understanding is needed of the factors controlling free tropospheric NO.sub.2 in order to relate satellite observations of tropospheric NO.sub.2 columns to the underlying NO.sub.x emissions and their trends. Focusing on urban winter conditions in the satellite data minimizes the effect of this free tropospheric background.