Does nighttime chemistry of isoprene impact air quality in polluted environments?

Exposure to poor air quality is the top environmental risk factor of premature mortality globally. It is estimated that 91 % of the world's population lives in places where air quality exceeds World Health Organisation guideline limits and that 1 in 9 deaths globally are a result of exposure to air pollution. Volatile organic compounds (VOC) are emitted to the atmosphere from both man-made and natural sources. They play a key role in the formation of ozone, a toxic pollutant, and they can also increase the amount of particulate matter, through the formation of secondary organic aerosol (SOA). Isoprene constitutes nearly half of all global emissions of VOCs to the atmosphere, with a flux of ~600 Tg per year. The dominant source of isoprene is biogenic emissions from trees and plants, with a small emission in gasoline vehicle exhaust. In remote regions, isoprene can dominate the formation of secondary organic aerosol. However, the contribution of isoprene to secondary organic aerosol formation is still under debate, especially in regions where biogenic emissions combine with those from urban environments. In large megacities, there can be significant concentrations of isoprene as a result of urban green spaces. The interaction of isoprene with man-made pollutants, such as NOx and sulfate aerosol, can actually increase the amount of toxic pollutants that can form in urban areas.

Recent aircraft observations of polluted power plant plumes that had mixed with air containing high levels of isoprene, found that the nighttime chemistry of isoprene with nitrate radicals was a much more efficient source of secondary organic aerosol than previously thought. One possible explanation for the discrepancy between these measurements and previous laboratory results is that the atmospheric lifetime of isoprene derived peroxy radicals in these plumes could be significantly longer than has been replicated in the lab, and could lead to previously unknown reactions dominating. Understanding the interplay between isoprene and anthropogenic emissions is crucial if we are to reduce secondary pollutants such as ozone and aerosols.

This project aims to understand the impact of nighttime chemistry of isoprene on air quality in polluted urban and rural environments. The student will use a chemical model to interpret data obtained at the SAPHIR simulation chamber at the Forschungszentrum Julich to determine if current chemical mechanisms can replicate the oxidation of isoprene by nitrate radicals, the main loss route at night. The student will then design and take part in a new set of experiments at SAPHIR to more accurate simulate isoprene oxidation in polluted areas during the night. The student will also attempt to quantify the contribution of isoprene nitrate secondary organic aerosol to particulate matter in polluted atmospheres and determine the key factors that control the impact of this chemistry on air quality.