Controls on surface ozone pollution in the small urban centre of Halifax, Nova Scotia

Abstract:

As the largest urban centre in Atlantic Canada (population 430,000, and growing), Halifax experiences levels of surface ozone (O3) well above WHO guidelines and provincial desirable standards but the chemistry and meteorology behind these higher ozone values are not well characterized. This study examines long-term (2000-2018) observational ozone and precursor gas data, as well as associated local emissions inventories, in order to define trends and explain changing ambient levels of ozone in Halifax. For example, provincial emissions have been consistently decreasing but ozone concentrations are beginning to rise in recent years and the cause of this rise is investigated.

Although it is known that transboundary pollution is present on some days, the significance of this transported pollution to annual trends has not been previously quantified for Halifax. We introduce and apply a spatial correlation algorithm as a novel method to diagnose transported pollution events that cause high ozone in Halifax. Meteorological conditions on elevated ozone days where no transboundary pollution is suspected are also analysed for signatures of ozone production from local precursors.

Our results clarify the sensitivity of surface ozone levels in Halifax to changing levels of precursor emissions in upstream areas like the Northeast USA, which have seen an increase in recent years following decades-long reductions (Jiang et al., 2018). The consequences these factors will have on surface ozone in Halifax are uncertain but investigated in this work. This research has significance for policy-makers working to manage risks from air pollution in growing cities subject to strong upstream pollution sources under a changing climate.