Controls on the production of ground level ozone in Halifax, Nova Scotia

Abstract

Atmospheric ozone (O₃) is a secondary pollutant produced via complex chemical reactions involving precursors like nitrogen oxides (NO_x), Volatile Organic Compounds (VOC) and carbon monoxide (CO). The key to reducing ozone concentrations is to regulate the precursors. Using data from the National Air Pollutant Surveillance (NAPS) program, possible controls on ozone production are examined for Halifax and other regions across Nova Scotia, including meteorological and chemical variables.<br />Long-term trends of ozone and its precursors in Nova Scotia were analyzed to see the effect of emissions regulations across North America. Diurnal and monthly averages were also examined to show short-term patterns in Halifax. It was found that 1-hour average ozone levels are relatively steady in Halifax under decreasing NO_x emissions, due to a reduction of the strong NO_x titration effect on O₃; however, Total Ozone (TO), comprised of O₃ and NO₂, is decreasing, along with maximum 1-hr ozone. Additionally, it was found that there are no strong signatures of biogenic VOC influence on ozone levels in Halifax (as opposed to anthropogenic VOC), although more frequent measurements of VOC are needed to better study VOC-sensitivity. There is indication from the high NO_x levels that ozone production is on the whole VOC-sensitive in Halifax.<br />Lastly, long-range transport was examined during an event of maximum elevated O₃ at the background air Aylesford NAPS station in 2016. The high O₃ levels were correlated with high values in Halifax and transport to both locations was consistent with pollutant import from the the northeast U.S. Flow to Halifax vs. Aylesford is in general more complicated because there is no obvious correlation between ozone in Halifax and at the Aylesford NAPS station, which requires further study.<br />All of the above findings are policy-relevant in the regulation of ozone precursors.