Direct Space-Based Observation of Decadal Changes in NOx Emissions and Lifetime: Implications for Oxidative Capacity

The NO_x chemical lifetime in the atmosphere is nonlinearly related to its concentration and is strongly coupled to the available OH. Direct observation of changes in the NO_x emissions and lifetime over the past decades therefore provides information about the oxidative capacity of the atmosphere. Doing so required the development of a new satellite NO₂ retrieval (BEHR v3.0B) with sufficiently detailed a priori data including daily variations. Using the new BEHR retrieval, we observe shifts from NO_x-suppressed to NO_x-limited chemistry in several US cities, implying that OH concentrations will decrease in step with future NO_x emissions reductions. We examine how this change can explain the observed slowdown in US NO_x reductions after circa 2010 and show the impact that not explicitly accounting for changes in lifetime has on emissions constraints. We compare the observed changes in NO_x lifetime with those computed in a state-of-science model and discuss the implications for our current understanding of the kinetics governing the NO_x and HO_x cycles.