Temperature-Dependent Nighttime Stagnation Episodes Driving Decadal Air Pollutant Exceedances in Los Angeles

Nitrogen dioxide (NO2) is a toxic air pollutant that originates primarily from anthropogenic combustion processes. It also plays a role in the generation of ozone (O3) and particulate matter (PM). Because NO2 is linked to adverse health outcomes and increased mortality, understanding its drivers and limiting exposure to high concentrations remain key objectives. Focusing on the Los Angeles metro area, previous studies have identified a relationship between NOx (NOx ≡ NO + NO2) concentrations and temperature but did not convincingly explain its drivers. We expand on past work, demonstrating how the relationship between enhanced NO2 concentrations and temperature is driven by nighttime stagnation episodes that are distinct from daytime trends. Warm nighttime conditions, marked by low planetary boundary layer heights and decreased ventilation, lead to the accumulation of pollutants, especially in the Los Angeles basin, bounded by mountainous topography on three sides. High nighttime concentrations therefore have an outsized influence on driving overall daily means. Here, we analyze 20 years of observations to show how relative increases in NO2 with temperature are consistent over time and are therefore independent of the dramatic decadal decreases in emissions. Moreover, by showing that concentrations of carbon monoxide (CO) mirror the same trends, we suggest that a common meteorological factor is influencing the ensemble rather than the temperature-dependent chemistry. We then verify this by examining trends in how atmospheric boundary layer heights and wind speeds vary with the temperature. We conclude by showing how these temperature-dependent stagnation episodes are driving exceedances in daily average NO2 concentration guidelines. Though in absolute terms exceedances are less prevalent today than in past years, their incidence is now more strongly correlated to temperature, suggesting that the role of temperature as a control for dangerously elevated NOx concentrations is growing.