Novel insights on causes of disproportionate trends between particulate NO ₃ ⁻ and NO _<i>x</i> emissions in Canadian urban atmospheres

Abstract. Particulate nitrate (NO3-) is a key target for air pollution control; however, its response to NOx emission reductions remains uncertain, particularly in cold climates. This study assesses long-term trends in fine- and coarse-mode NO3- (f-NO3- and c-NO3-) from 1990 to 2019 across seven Canadian cities, using data from by the National Air Pollution Surveillance (NAPS) network. The analysis reveals disproportionate trends between NO3- and NOx emissions nationwide. In Edmonton, annual mean f-NO3- decreased by approximately 60 % between 2007 and 2019, while provincial NOx emissions declined by only 10 %–20 %. Similar patterns were observed in five of the six other cities during the most recent decade. These disproportionate trends are attributed to reductions in primary f-NO3- emissions, localized dispersion processes, and wind anomalies modulated by Arctic Oscillation. In contrast, all cities exhibited a temporary increase in f-NO3- during 1998–2007, coinciding with early NOx control measures and consistent with an unintended enhancement of primary f-NO3- emissions formed within stationary combustion plumes. c-NO3- was largely insensitive to NOx reduction in most cities (except for Edmonton), with variability governed primarily by neutralization reactions with alkaline aerosols rather than by the availability of gaseous HNO3. These findings provide insight into the weak or absent response of f-NO3- to NOx emission reductions observed globally, particularly in cold-climate regions.