Tropospheric NO2 Patterns in Eastern Canada Using the First Year of TEMPO Observations

Abstract

The Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument enables an unprecedented assessment of diurnal and community-scale variations in tropospheric nitrogen dioxide (NO2) across North America. This study presents the first exploratory analysis of NO2 patterns in eastern Canada, including Ontario, Quebec, and the Atlantic provinces, using TEMPO observations. We analyzed TEMPO data (V03) gridded at 0.02° × 0.02° from September 2023 to August 2024 and compared it with the Tropospheric Monitoring Instrument (TROPOMI) and surface-level measurements from Canadian national regulatory monitors. With the hourly resolution of TEMPO, we observed diurnal trends and hotspots that were not recognized by once-per-day TROPOMI measurements and pinpointed undermonitored areas. NO2 in eastern Canada's eight major metropolitan areas, ports, and industrial cities similarly peaked in early morning and declined in later hours. Still, TEMPO detected variations in their hours of peaks and spikes, seasonal, and weekday-weekend distributions. In Atlantic Canada, correlations between TEMPO and TROPOMI column densities, as well as column-surface alignments, were lower (Spearman's ρ = 0.41–0.53) compared to the Quebec City-Windsor Corridor (Spearman's ρ = 0.81–0.90), primarily due to a wider dynamic range of pollution in the latter region. The two regions' TEMPO-TROPOMI mean absolute differences were 19.3% and 17.1%, respectively. Temporal variations (e.g., a later weekday morning peak in Ontario cities) and TEMPO's identification of additional undermonitored hotspots provide insights into air quality control planning. Our findings motivate future research using multiyear TEMPO data to investigate atmospheric NO2 sources, transport, exposure, and associated population health impacts in Canada.