Abstract

This study investigated impacts of regional transport and meteorology on ground-level ozone (O3) in the smog season (April–September) during 1996–2015 in Windsor, Ontario, Canada. Data from five upwind stations in the US, which are within 310 km (i.e., Allen Park and Lansing in Michigan, Erie, National Trail School, and Delaware in Ohio), were included to assess the regional characteristics of O3. The five US stations showed high degrees of similarity with O3 concentrations in Windsor, with overall strong correlations (r = 0.567–0.876 for hourly O3 and r = 0.587–0.92 for 8 h max O3 concentrations) and a low degree of divergence, indicating that O3 pollution in the study area shares regional characteristics. Meteorological conditions played important roles in O3 levels in Windsor. High O3 concentrations were associated with southerly and southwesterly air mass from which polluted and hot air mass was transported and that enhanced local photochemical O3 production. In contrast, northerly flows brought in clean, cool, and dry air mass, and led to low O3 concentrations. Strong correlations were found between numbers of days with 8 h max O3 concentrations greater than 70 ppb and numbers of days with daily max temperature greater than 30 °C, as well as between daily max temperatures and 8 h max O3 concentrations. Nearly half (45%) of the high O3 days (≥90th percentile) occurred in dry tropical weather during 1996–2015, and the 90th percentile 8 h max O3 was associated with dry tropical weather. Occurrences of both southerly flow hours and dry tropical weather type in the smog season increased during the study period. If there were more hot and dry days in the next few decades due to climate change, the effect of emission control on reducing peak O3 values would be diminished. Therefore, continued regional and international efforts are essential to control precursors’ emissions and to mitigate O3 pollution in Windsor.

Highlights

  • Ground-level ozone (O3 ) is a secondary air pollutant, produced by photochemical reactions between volatile organic compounds (VOC) and nitrogen oxides (NOX ) [1]

  • In urban areas where NOX levels are normally high, O3 levels increase with VOC levels but decrease due to the nitric oxide (NO) scavenging effect

  • O3 concentrations in the smog season exhibited a high degree of similarity between Windsor and the five United States (USA) sites within 310 km, suggesting that the O3 pollution in Windsor and at the five sites in the US showed regional characteristics, likely due to their similar emission sources of O3 precursors and the shared weather conditions

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Summary

Introduction

Ground-level ozone (O3 ) is a secondary air pollutant, produced by photochemical reactions between volatile organic compounds (VOC) and nitrogen oxides (NOX ) [1]. O3 production is non-linearly related to levels VOCs and NOX , and mainly depends on the ratio of the two [2,3]. In urban areas where NOX levels are normally high, O3 levels increase with VOC levels but decrease due to the nitric oxide (NO) scavenging effect. At low NOX levels, O3 production is limited by NOX levels but with little impact by VOCs. The degree of O3 scavenging by NO is strongly affected by the ratio of NO to NO2. The decreasing ratios of NO to NO2 have been observed in recent

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