The Spatiotemporal Response of Summertime Tropospheric Ozone to Changes in Local Precursor Emissions in the Lower Fraser Valley, British Columbia

This paper examines the spatiotemporal impact of changing local precursor emissions on a subset of air quality monitors in the coastal Lower Fraser Valley, British Columbia, Canada, over a 25-year period (1990–2014) marked by substantial emissions reductions. The analysis examines summertime (JJA) trends in ambient reactivity of volatile organic compounds (VOCs), , ozone, and odd oxygen (; defined as ) concentrations. To account for the potentially confounding influence of rising background ozone levels on the analysis, summer days are classified by whether appreciable local ozone production is likely or not, with a separate analysis undertaken for each set of conditions. The influence of short- and long-term meteorological variability, hebdomadal (weekly) emissions variability, and within-network spatial variability on long-term trends is accounted for by using Generalized Additive Models (GAM). The analysis suggests that long-term trends in local ambient VOC reactivity and concentrations have not occurred in a uniform fashion nor have they changed in a synchronized way. Declines in VOC reactivity appear to have occurred sooner and at a more uniform rate, but concentrations appear to have remained constant for the first 10 years before rapidly declining. Similarly, we find ozone reductions have not occurred uniformly in time nor has the network response been spatially uniform, with the smallest reductions occurring just downwind of the region's downtown core and the largest reductions about 40 km further downwind. Much of the non-uniformity in response can be attributed to changing odd-oxygen partitioning rather than changing ozone production. During our analysis period, ozone concentrations during days not conducive to ozone photochemistry appear to have increased at a rate consistent with previous background trend analyses over western North America. We use the difference between ozone concentrations on fair-weather summer days and days with little photochemistry to build a response surface for “net-produced” ozone as a function of meteorologically controlled ambient VOC reactivity and concentrations. We find that much of the predicted behaviour in local ambient ozone can be understood in light of VOC-limited conditions and differing rates of local VOC and emissions reductions. It appears that these precursor reductions have largely resulted from evolving light-duty emission control technologies and local fleet turnover.

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