Surface ozone in the North American pollution outflow region of Nova Scotia: Long-term analysis of surface concentrations, precursor emissions and long-range transport influence

Abstract

Nova Scotia is in the air pollution outflow region of North America and experiences elevated surface air pollution episodes regularly. We present the first analysis of long-term (2000–2018) hourly ozone (O3) and ozone precursor gas trends at three Nova Scotia provincial monitoring locations (‘urban’ Halifax, ‘suburban’ Halifax at Lake Major and ‘regional background’ at Aylesford Mountain), together with an examination of provincial ozone precursor emission trends. The precursors considered are nitrogen oxides (NOx) and volatile organic compounds (VOC). Over the common data period for the three stations (2006–2017) the annual 99th percentile hourly ozone concentrations decreased significantly in Halifax (−0.77 ppb/year; p = 0.02) and at Aylesford (−0.65 ppb/year; p = 0.03), in agreement with regional trends. No statistically significant changes were found in high ozone at suburban Lake Major (−0.02 ppb/year; p = 0.94), but the annual 25th percentile, median, and 75th percentile hourly ozone concentrations increased significantly (p < 0.01) only at this location (1.09 ppb/year, 1.23 ppb/year, 1.03 ppb/year, respectively). This is consistent with the strong NOx-O3 titration effect in downtown Halifax decreasing over time. After correcting for the titration effect over the full study period, the annual median Halifax total ozone is found to be decreasing (−0.45 ppb/year; p = 0.02).Over the full study period, ambient concentrations of precursors decreased significantly (p < 0.01) in Halifax by −2.34 ppb/year for NOx and −3.41 μg/m3/year for VOCs, as did the ozone formation potential (OFP) of VOCs (−9.66 μg/m3/year; p < 0.01). The underlying emissions of precursors also decreased significantly (p < 0.01) in and near Halifax (provincial NOx: 3982 ppb/year; provincial VOCs: 1680 tonnes/year), also in agreement with regional trends. However, a linear regression during 2012–2018 showed a decelerating reduction of provincial NOx emissions. Provincial total VOC emissions also decreased approximately ten times slower during 2012–2018, albeit this was not statistically significant. Finally, we present an algorithm to identify events of long-range transported pollution in Halifax, based on spatial correlations in ozone across surface monitoring stations in Nova Scotia. Between 2000 and 2018, transported upstream pollution was found to be connected to between 45% and 63% of 1444 days when surface ozone was elevated in Halifax. The frequency with which transported pollution events occur increased by 15% across this time period.