Inconsistent decadal variations between surface and free tropospheric nitrogen oxides over United States

Abstract. Decreases in surface emissions of nitrogen oxides (NO_x = NO + NO₂) in North America have led to substantial improvements in air-quality over the last several decades. Here we show that satellite observations of tropospheric nitrogen dioxide (NO₂) columns over the contiguous United States (US) do not decrease after about 2009, while surface NO₂ concentrations continue to decline through to the present. This divergence, if it continues, could have a substantial impact on surface air quality due to mixing of free-tropospheric air into the boundary layer. Our results show only limited contributions from local effects such as fossil fuel emissions, lightning, or instrument artifacts, but we do find a possible relationship of NO₂ changes to decadal climate variability. Our analysis demonstrates that the intensity of transpacific transport is stronger in El Niño years and weaker in La Niña years, and consequently, that decadal-scale climate variability impacts the contribution of Asian emissions on North American atmospheric composition. Because of the short lifetime, it is usually believed that the direct contribution of long-range transport to tropospheric NO_x distribution is limited. If our hypothesis about transported Asian emissions is correct, then this observed divergence between satellite and surface NO_x could indicate mechanisms that allow for either NO_x or its reservoir species to have a larger than expected effect on North American tropospheric composition. These results therefore suggest more aircraft and satellite studies to determine the possible missing processes in our understanding of the long-range transport of tropospheric NO_x.