Urban oxidising capacity measurements using inert and reactive tracers

根据定义，城市环境是许多人生活或工作的环境。在一天的过程中，人们会在户外呆上不同的时间，暴露在不同水平的空气污染物中。在白天，阳光可以提供所需的能量来产生羟基自由基(HO)，这是一种非常活跃的物种，在大气中像洗涤剂一样，与空气污染物发生反应。因此，我们想知道城市大气中有多少HO，以及它的水平如何在一天的过程中和每天作为一年中的时间的函数变化。使用一种名为FAGE的仪器直接测量Ho是可能的，英国的一些城市(例如伯明翰和伦敦)已经进行了这一测量。这些数据帮助我们了解了Ho在一个地方是如何变化的，但FAGE仪器太昂贵了，无法复制，让我们可以在整个城市进行测量。HO的生成和破坏非常迅速，因此对HO的测量也可以告诉我们它的产生速率与其去除速率的比率，实际上FAGE也可以告诉我们它的去除速率。因此，我们可以从这些测量中计算出产量，但这些来自一些城市的其他测量表明，根据我们目前的理解，HO的产量被低估了。一种可能性是，有我们不知道的失踪消息来源？鉴于HO和其他氧化剂对城市空气质量的重要性，试图找出这些缺失的来源是至关重要的。考虑到这一点，我们开发了一种新技术，使我们能够测量HO(和其他洗涤剂)污染物在城市环境中移动时遇到的量。通过这种方式，我们将能够建立一个关于污染物在城市中的去除率如何随一天的时间、季节、污染负荷等变化的图像。我们还将能够估计城市环境中颗粒物的生成速度，并开始了解是什么控制了它们的产生。颗粒可以直接释放到大气中，例如从汽车尾气中释放(称为初级颗粒)，但也可以在大气中产生，即所谓的次级颗粒。我们想要更多地了解次级粒子的来源，因为我们可以对初级粒子来源做出合理的估计。众所周知，颗粒物对空气质量不好，降低空气中的颗粒物浓度将是非常有益的。为了进行这些测量，我们将释放少量与氧化剂反应的有机分子，如HO，以及一些不与任何东西反应的分子。我们已经标记了活性分子，这样我们就可以将它们与已经进入城市的分子区分开来，并将测量释放点顺风向活性和惰性物质的水平。当这些分子分散时，它们的水平会因为稀释而下降，惰性物质会告诉我们稀释速度，活性分子会下降得更多，因为它们不仅会分散，而且会发生反应。通过使用这两条信息，我们可以估计它们的化学去除率，从而估计存在的Ho的量。其他污染物的测量和气象将同时进行，届时我们将能够估计这些污染物被清除的速度。包含我们目前城市空气质量知识的计算机模型将与所有测量数据进行比较，然后我们将能够测试一些关于HO和其他氧化剂缺失来源的假设。我们还将在夜间进行研究，在那里HO水平非常低，一种不同的氧化剂硝酸根(NO3)会接管。我们有一台可以测量NO3的仪器，因此我们可以将一个地点的NO3测量与污染物通过城市时经历的NO3进行比较。这样就可以进行详细的比较

By definition, the urban environment is one where many people either live or work. During the course of a day, people will be outdoors for varying lengths of time and be exposed to different levels of air pollutants. During daytime, sunlight can provide the energy needed to produce the hydroxyl radical (written as HO), which is an extremely reactive species that acts like a detergent in the atmosphere, reacting with air pollutants. Therefore, we want to know how much HO there is in the urban atmosphere and how its level changes over the course of a day and from day to day as a function of time of the year. It is possible to measure HO directly using an instrument called FAGE and this has been carried out for some cities in the UK (e.g. Birmingham and London). These data have helped us to understand how HO varies in one location but the instrument FAGE is too expensive to replicate to allow us to make measurements across a city. HO is made and destroyed very rapidly, so a measurement of HO also tells us the ratio of the rate of its production to the rate of its removal and indeed FAGE can also tell us about the rate of its removal. Therefore, we can calculate the rate of production from these measurements, but these other measurements from a number of cities suggest that the rate of production of HO is underestimated based on our current understanding. One possibility is that there are missing sources that we don't know about? Given the importance of HO and other oxidants to urban air quality it is vital to try to work out what these missing sources are.With this in mind we have developed a new technique to allow us to measure the amount of HO (and other detergents) pollutants encounter as they move through the urban environment. In this way we will be able to build up a picture of how removal rates for pollutants varies across a City as a function of time of day, season, pollution loading etc. We will also be able to estimate how rapidly particles are made in the urban environment and begin to understand what controls their production. Particles can be released into the atmosphere directly, e.g. from car exhausts, (called primary particles) but can also be made in the atmosphere, a so called secondary particles. We want to know more about the sources of secondary particles as we can make reasonable estimates of primary particle sources. Particles are known to be bad for air quality and a reduction in levels would be of great benefit. In order to make these measurements we will release small amounts of organic molecules that react with oxidants such as HO and some molecules that don't react with anything. We have tagged the reactive molecules so we can tell them apart from ones that are there already into the city and will measure the levels of both reactive and inert species downwind of the release point. As these molecules disperse their level will drop because of dilution and the inert species will tell us the dilution rate, the reactive ones will drop even more as they will not only disperse but also react. By using these two pieces of information we can estimate their chemical removal rates and hence the amount of HO present. Other measurements of pollutants and meteorology will be made at the same time and we will then be able to estimate how quickly these pollutants are removed. Computer models that contain our current knowledge of urban air quality will be compared with all the measurement data and we will then be able to test a number of hypotheses for the missing source of HO and other oxidants. We will also carry out studies at night where HO levels are very low and a different oxidant called the nitrate radical (NO3) takes over. We have an instrument that can make measurements of NO3 and so we can compare the measurement of NO3 in one location with the NO3 experienced by pollutants as they pass through the city. In this way a detailed comparison can be made