Long-term trends in British Columbia lower mainland air quality: Criteria air pollutants and VOC.

The lower mainland of British Columbia is a geographic region that comprises the districts of Metro Vancouver and the Lower Fraser Valley. It is situated in a complex topographical and coastal location in southwestern British Columbia. Metro Vancouver is Canada's third largest population center. Accessing the Canadian National Air Pollution Surveillance Program (NAPS) database we calculated air pollutant statistics using the Canadian Ambient Air Quality Standards (CAAQS) averaging times, numerical forms, and numerical levels for the years 2001to 2020. Man Kendall and Sen statistical methods were used to test for the presence of trends and the slope of those trends in fine particulate matter (PM_2.5), ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and volatile organic compound (VOC) ambient air concentrations. We did not determine a significant trend in 98^th percentile of the daily 24-hr average PM_2.5 concentrations. We did determine significant negative trends in the annual average of the daily 24-hr average PM_2.5 concentrations at 6 of the 9 locations. Episodic, multi-day duration elevated PM_2.5 concentrations related to forest fires were a significant influence on PM_2.5 ambient concentrations. Annual 4^th highest daily maximum 8-hr average O₃ concentrations showed no trend at 14 of 18 locations, declined at 3 locations, and increased at one location. We determined statistically significant declines in peak and average NO₂ and SO₂ concentrations, and in time-integrated annual VOC concentrations.Implications: This non-parametric, statistical analysis determines 20-year trends in British Columbia lower mainland ambient air quality for PM_2.5, O₃, NO₂, SO₂ and VOC, assesses air quality against Canadian Ambient Air Quality Standards, and highlights the importance of event-based wildfire-sourced PM_2.5.