Identifying decadal trends in deweathered concentrations of criteria air pollutants in Canadian urban atmospheres with machine learning approaches
This study investigates long-term trends of criteria air pollutants, including NO 2, CO, SO 2, O 3 and PM 2.5, and (NO 2+O 3) measured in ten Canadian cities during the last two to three decades and associated driving forces in terms of emission reductions, perturbations from varying weather conditions and large-scale wildfires, and changes in O 3 sources and sinks. Two machine-learning methods, including random forest algorithm and boosted regression trees, were used to extract deweathered mixing ratios (or mass concentrations) of the pollutants. The Mann-Kendall analysis of the deweathered and original annual average concentrations of the pollutants showed that, on the time scale of 20 years or longer, the perturbation from varying weather conditions exerted a very minor influence on the decadal trends of original annual averages (within ±2 %) in ~70 % of the cases, and a moderate influence up to 16 % of the original trends in the other 30 % cases. NO 2, CO and SO 2 showed decreasing trends in the last two to three decades in all the cities except CO in Montreal. O 3 showed increasing trends in all the cities, except Halifax, mainly due to weakened titration reaction between O 3 and NO. (NO 2+O 3), however, showed decreasing trends in all the cities, except Victoria, because the increase in O 3 is much less than the decrease in NO 2. In three of the five eastern Canadian cities, emission reductions dominated the decreasing trends in PM 2.5, but no significant trends in PM 2.5 were observed in the other two cites. In five western Canadian cities, increasing or no significant trends in PM 2.5 were observed, likely due to unpredictable large-scale wildfires overwhelming or balancing the impacts of emission reductions on PM 2.5. In addition, despite improving air quality during the last two decades in most cities, air quality health index of above 10 (representing very high-risk condition) still occasionally occurred after 2010 in western Canadian cities because of the increased large-scale wildfires.
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