Abstract
Emissions of nitrogen oxides (NOx = NO + NO2) in the United States have
declined significantly during the past three decades. However, satellite
observations since 2009 indicate total column NO2 is no longer declining
even as bottom-up inventories suggest continued decline in emissions.
Multiple explanations have been proposed for this discrepancy including
1) the increasing relative importance of non-urban NOx to total column
NO2, 2) differences between background and urban NOx lifetimes, and 3)
that the actual NOx emissions are declining more slower after 2009. Here
we use a deep learning model trained by NOx emissions and surface
observations of ozone to assess consistency between the reported NOx
trends between 2005-2014 and observations of surface ozone. We find that
the 2005-2014 trend from older satellite-derived emission estimates
produced at low spatial resolution best reproduce ozone in low NOx
emission (background) regions, reflecting the blending of urban and
background NOx in these low-resolution top-down analyses. The trend from
higher resolution satellite-based estimates, which are more capable of
capturing the urban emission signature, is in better agreement with
ozone in high NOx emission regions, and is consistent with the trend
based on surface observations of NO2. In contrast, the 2005-2014 trend
from the US Environmental Protection Agency (EPA) National Emission
Inventory (NEI) results in an underestimate of ozone. Our results
confirm that the satellite-derived trends reflect anthropogenic and
background influences and that the 2005-2014 trend in the NEI inventory
is overestimating recent reductions in NOx emissions.