Satellite NO2 trends reveal pervasive impacts of wildfire and soil emissions across California landscapes

Nitrogen dioxide (NO _2 ) plays a pivotal role in the production of secondary pollutants, most importantly ozone (O _3 ) and particulate matter. Regulatory controls have greatly reduced NO _2 in cities, where most of the surface monitoring occurs, but the change in rural environments is less certain. Here, we present summertime (June–September) spatio-temporal patterns of NO _2 concentrations using satellite and ground observations across California from 2009–2020, quantifying the differences in NO _2 trends for five distinct land cover classes: urban, forests, croplands, scrublands (shrublands, savannas, and grasslands), and barren (minimally vegetated) lands. Over urban environments, NO _2 columns exhibited continued but weakening downward trends (−3.7 ± 0.3%a ^−1 ), which agree fairly well with contemporaneous trends estimated from the surface air quality network (−4.5 ± 0.5%a ^−1 ). In rural (i.e., non-urban) parts of the state, however, secular trends are insignificant (0.0–0.4 ± 0.4%a ^−1 ) or in the case of remote forests are rapidly on the rise (+4.2 ± 1.2%a ^−1 ). Sorting the NO _2 columns by air temperature and soil moisture reveals relationships that are commensurate with extant parameterizations but do indicate a stronger temperature dependence. We further find that rapidly rising temperatures and, to a lesser extent, decreasing precipitation in response to climate change are acting to increase soil NO _x emissions, explaining about one-third of the observed NO _2 rise in non-urban regions across California. Finally, we show that these trends, or their absence, can be attributed predominantly to the dramatic rise in wildfire frequency, especially since the turn of the 21st century.