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

暴露于恶劣的空气质量是全球过早死亡的首要环境风险因素。据估计，世界上91%的人口生活在空气质量超过世界卫生组织指导标准的地方，全球每9例死亡中就有1例是由于暴露于空气污染。挥发性有机化合物（VOC）从人造和天然来源排放到大气中。它们在形成臭氧（一种有毒污染物）方面起着关键作用，并且它们还可以通过形成二次有机气溶胶（SOA）来增加颗粒物的数量。异戊二烯占全球挥发性有机化合物排放量的近一半，每年的排放量约为600 Tg。异戊二烯的主要来源是树木和植物的生物排放物，汽油车辆废气中也有少量排放物。在偏远地区，异戊二烯可以主导二次有机气溶胶的形成。然而，异戊二烯对二次有机气溶胶形成的贡献仍有争议，特别是在生物排放与城市环境排放联合收割机相结合的地区。在大的特大城市，由于城市绿色空间，异戊二烯可能会大量集中。异戊二烯与人造污染物（如氮氧化物和硫酸盐气溶胶）的相互作用实际上会增加城市地区可能形成的有毒污染物的数量。最近飞机对污染的发电厂羽流进行了观察，这些羽流与含有高水平异戊二烯的空气混合，发现异戊二烯与硝酸根的夜间化学反应是二次有机气溶胶的有效来源，比以前认为的要有效得多。这些测量结果与先前实验室结果之间的差异的一个可能解释是，这些羽流中异戊二烯衍生的过氧自由基的大气寿命可能比实验室中复制的要长得多，并且可能导致先前未知的反应占主导地位。了解异戊二烯和人为排放之间的相互作用是至关重要的，如果我们要减少二次污染物，如臭氧和气溶胶。本项目旨在了解异戊二烯的夜间化学对污染的城市和农村环境中的空气质量的影响。学生将使用化学模型来解释在Forschungszentrum Julich的SAPHIR模拟室中获得的数据，以确定当前的化学机制是否可以复制硝酸根对异戊二烯的氧化，这是夜间的主要损失途径。然后，学生将设计并参加SAPHIR的一系列新实验，以更准确地模拟夜间污染地区的异戊二烯氧化。学生还将尝试量化异戊二烯硝酸盐二次有机气溶胶对污染大气中颗粒物的贡献，并确定控制这种化学物质对空气质量影响的关键因素。

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.