Modeling and Chemical Analysis Used as Tools to Understand Decade-Long Trends of Ozone Air Pollution in the Lower Fraser Valley, British Columbia, Canada

Abstract

The Lower Fraser Valley (LFV) B.C., a largely deindustrialized region with emissions dominated by the transportation sector, has experienced large reduction in precursor emissions over the last 20 years. While these reductions have resulted in concomitant changes in summertime ambient ozone concentrations, the changes have not been uniform across the region, with trends in long term behavior differing noticeably between the eastern and western portions of the valley. In this work, we draw upon previous modeling, observational and experimental studies to infer how the sensitivity of ground-level ozone to precursor emissions has changed over the last two decades. This work is notable for several reasons. First we establish not only VOC- and NO_x-ozone sensitivities during summertime ozone episodes, but across the full range of summertime meteorological conditions. Next, we examine how these sensitivities have changed over the last two decades. We also examine how these sensitivities vary spatially across the LFV and finally we use the above findings to explain observed summertime ozone trends in terms of changing ozone sensitivities and changing emission rates.

Question and Answer

QUESTIONER: S.Aksoyoglu

QUESTION: You mentioned that background O₃ increased. Did you check if there is any increase in radiation of the last decades due to decreasing particles, leading to increased production of O₃?

ANSWER: We have looked at the effects of summertime temperatures on O₃, as a proxy for insolation (shown in trend plots on the presentation slides). It appears to be responsible for much of the scatter in the maximum ozone values, however, has little effect below 95th percentile. Unfortunately, the Metro Vancouver air quality network is not equipped to collect long-term incoming solar radiation data.

QUESTIONER: Lucas Hennem

QUESTION: I have concerns over defining the lowest percentile of ozone as the “background”, since without local anthropogenic emissions it is unlikely that that ozone concentrations would go to the very low values observed in urban centers.

ANSWER: Using solely statistical trend analysis to define “background” is certainly a very limiting approach. The values are inevitably tied to local emission sources. However, as we see from summertime diurnal ozone profiles, there is an approximately equal increase in nighttime levels of O₃ present for both urban West LFV and rural East LFV over the more recent years as compared to pre-AQMP years, indicating a large-scale change, rather than a local response.

QUESTIONER: Amir Hakami

QUESTION: How did you evaluate your VOC or NO_x-limited areas, and would it be possible to compare those with model-based DDM chemical regimes?

ANSWER: We used Sillman indicators to establish the connection between changing ozone sensitivity and the long-lived indicator [O₃]/[NO_y]. The model indicates that when the 8-h average afternoon [O₃]/[NO_y] concentration is below 7.0, the airmass is VOC-sensitive, otherwise it is NO_x-sensitive. It would certainly be interesting to compare our results with those obtained using a DDM-based method.