Novel insights on causes of disproportionate trends between particulate NO₃⁻ and NO_x emissions in Canadian urban atmospheres

Abstract. Particulate nitrate (NO₃⁻) is a key target for controlling air pollution, yet its response to NO_x abatement remains uncertain in cold climates. This study assesses trends of fine- and coarse-mode NO₃⁻ (f-NO₃⁻ and c-NO₃⁻) during 1990–2019 in seven Canadian cities, making use of the long-term data collected by the National Air Pollution Surveillance (NAPS) network, and revealed disproportionate trends between NO₃⁻ and NO_x emissions across Canada. In Edmonton, annual mean f-NO₃⁻ decreased by ~60 % from 2007–2019 while provincial NO_x emissions declined by only 10–20 %; comparable patterns were also observed in five out of the six other cities in the most recent decade. Such disproportionate trends were diagnosed to be caused by reduced primary f-NO₃⁻ emissions, localized dispersion, and Arctic Oscillation–modulated wind anomalies. Conversely, all cities exhibited a transient f-NO₃⁻ increase during 1998–2007, coincident with early NO_x controls and consistent with unintended enhancement of primary emissions of f-NO₃⁻ formed within stationary-combustion plumes. c-NO₃⁻ was largely insensitive to NO_x reduction in most cities (except Edmonton), with its trends governed by neutralization reactions with alkaline aerosols rather than HNO₃ availability. These findings can help interpret the weak or absent f-NO₃⁻ response to NO_x reductions worldwide, especially in cold-climate regions.