Biogenic isoprene in the Lower Fraser Valley, British Columbia
Tropospheric ozone is formed by photochemical reactions between nitrogen oxides (NOx) and volatile organic compounds. In some regions, biogenic isoprene may be a significant contributor to the production of tropospheric ozone. The contribution of biogenic isoprene is an important aspect of regional ozone chemistry as it represents an ozone precursor that cannot be eliminated through emissions controls. The purpose of this study was to characterise the sources, emission rates, and ambient concentrations of isoprene in the Lower Fraser Valley, British Columbia, and to evaluate its contribution to the regional production of tropospheric ozone. Field studies were conducted to identify natural sources of isoprene in the region and to quantify the rates of emission. Ambient isoprene monitoring data were used to examine isoprene's relationship with temperature, determine its source, construct diurnal time-concentration profiles, and investigate the chemical and physical factors that limit its concentrations. The Integrated Empirical Rate model examined the chemical characteristics of the Lower Fraser Valley air mass during ozone events and studies evaluating isoprenes relative reactivity were reviewed. Few species of plants in the Lower Fraser Valley were found to emit isoprene in appreciable amounts. Ambient concentrations of total isoprene were correspondingly low throughout the day and in all seasons. Measurable concentrations of isoprene in winter and on cool summer days, as well as the positive correlation of isoprene with hydrocarbons known to be emitted from anthropogenic mobile sources, indicated that isoprene in the Lower Fraser Valley originated from both biogenic and anthropogenic sources. Therefore, biogenic isoprene constituted only a fraction of the small total isoprene load. Diurnal profiles of isoprene showed that the times of peak concentrations of isoprene did not coincide with peak NOx concentrations nor with the times of optimal ozone-producing meteorological conditions. Finally, in spite of isoprene's high reactivity, ' biogenic' isoprene was estimated to be responsible for only ~7.5% of the total production of ozone downwind of Vancouver's centre during the mild episode of 1993. Thus, the weight of evidence from the research performed and reviewed in this thesis indicates that 'biogenic' isoprene is not a major contributor to the production of ozone in the Lower Fraser Valley.