Preprints
https://doi.org/10.5194/egusphere-2023-2968
https://doi.org/10.5194/egusphere-2023-2968
20 Dec 2023
 | 20 Dec 2023

Identifying decadal trends in deweathered concentrations of criteria air pollutants in Canadian urban atmospheres with machine learning approaches

Xiaohong Yao and Leiming Zhang

Abstract. This study investigates long-term trends of criteria air pollutants, including NO2, CO, SO2, O3 and PM2.5, and (NO2+O3) 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 O3 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. NO2, CO and SO2 showed decreasing trends in the last two to three decades in all the cities except CO in Montreal. O3 showed increasing trends in all the cities, except Halifax, mainly due to weakened titration reaction between O3 and NO. (NO2+O3), however, showed decreasing trends in all the cities, except Victoria, because the increase in O3 is much less than the decrease in NO2. In three of the five eastern Canadian cities, emission reductions dominated the decreasing trends in PM2.5, but no significant trends in PM2.5 were observed in the other two cites. In five western Canadian cities, increasing or no significant trends in PM2.5 were observed, likely due to unpredictable large-scale wildfires overwhelming or balancing the impacts of emission reductions on PM2.5. 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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Xiaohong Yao and Leiming Zhang

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • AC1: 'Supplementary Information to egusphere-2023-2968', Leiming Zhang, 21 Dec 2023
  • RC1: 'Comment on egusphere-2023-2968', Anonymous Referee #1, 09 Jan 2024
  • RC2: 'Comment on egusphere-2023-2968', Anonymous Referee #2, 31 Jan 2024

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • AC1: 'Supplementary Information to egusphere-2023-2968', Leiming Zhang, 21 Dec 2023
  • RC1: 'Comment on egusphere-2023-2968', Anonymous Referee #1, 09 Jan 2024
  • RC2: 'Comment on egusphere-2023-2968', Anonymous Referee #2, 31 Jan 2024
Xiaohong Yao and Leiming Zhang
Xiaohong Yao and Leiming Zhang

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Short summary
This study investigates long-term trends of criteria air pollutants, including NO2, CO, SO2, O3 and PM2.5, and (NO2+O3) 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 O3 sources and sinks.