Molecular Level Understanding of New Particle Formation in the Urban Atmosphere: Contribution of Local Pollutants

空气中的颗粒物是由漂浮在空气中的微小固体物质或液滴组成的，它们太小了，肉眼无法单独看到。获得对这些颗粒的良好理解是重要的，原因包括以下几个：-呼吸高浓度对人类健康有害，与增加住院率和减少预期寿命有关。在全球范围内，由于接触空气中的颗粒物而死亡的人数超过疟疾和艾滋病死亡人数的总和;小颗粒物影响云的形成，它们在大气中的浓度通过影响反射回太空的阳光量而对气候产生直接和间接的影响;-空气中高浓度的颗粒物会导致能见度下降，并在污染的日子里出现雾霾。大气科学是指在大气中通过称为成核的过程从气体中形成新粒子。仪器的最新进展已经允许直接观察气体分子结合在一起形成新粒子的成核过程。到目前为止，这些过程主要是在反应室和清洁的山顶空气中在非常受控的条件下进行研究的。在严重污染的空气中发生的过程与这些不同，我们的建议是在污染的大气中测量颗粒成核过程，在城市中的组成是非常好的特点。这将使我们能够评估污染物排放对颗粒物形成过程的贡献，以及它们对城市大气中新颗粒物产生的贡献，以及与道路交通排放无关的过程。将在北京和巴塞罗那进行实地测量，这两个城市经常出现新的颗粒物形成，并将建立两个采样点，一个在主要高速公路的路边，另一个在背景位置。这些仪器将配备高度复杂的最先进的仪器，这些仪器将确定大气中新粒子形成的时间，并将确定凝聚成团簇以形成新粒子的分子的化学特性。这两个城市都有很高的日照强度（需要频繁的新颗粒形成），但污染气候差异很大。这项工作的结果将是更好地了解负责通过污染空气中的成核形成新颗粒的过程。这些知识将有助于更好地设计减少超细颗粒物浓度的缓解战略，并提供必要的知识，以改进气候预测模型，目前最大的不确定性之一与空气传播颗粒物在影响气候方面的作用有关。未来，预计污染物排放的减少和空气质量的改善可能会影响新颗粒物的形成。排放的颗粒物浓度降低将有利于增加新颗粒物的形成，而二氧化硫和有机化合物的减少预计将不利。需要设计良好的数值模型来预测未来对新粒子形成的净影响，从而预测城市粒子浓度和更偏远地区的云凝结核。

Airborne particles are made up of tiny specks of solid matter or liquid droplets floating in the air, too small to be seen individually by the naked eye. Gaining a good understanding of these particles is important for a number of reasons including the following:- breathing high concentrations is bad for human health, having been associated with increased hospital admissions and reduced life expectancy. More deaths occur globally due to airborne particle exposure than from malaria and AIDS combined; - small particles influence the formation of clouds, and their concentration in the atmosphere has a direct and indirect influence on climate by affecting the amount of sunshine reflected back to space; - high concentrations of particles in the air cause a loss of visibility and the hazes that can be seen on polluted days.One of the most pressing current scientific questions in atmospheric science is the formation of new particles in the atmosphere from gases by processes known as nucleation. Recent advances in instrumentation have allowed direct observation of nucleation processes as gas molecules join together to form new particles. Up until now these processes have been studied mostly under very controlled conditions in reaction chambers and in clean mountain-top air. The processes occurring in heavily polluted air are different from these and our proposal is to make measurements of particle nucleation processes in polluted atmospheres in cities where the composition is very well characterised. This will allow us to evaluate the contribution of pollutant emissions to particle formation processes and their contribution to new particle production in the urban atmosphere, as well as processes unconnected with road traffic emissions. Field measurements will be made in Beijing and Barcelona, cities where new particle formation occurs frequently, and where two sampling sites will be established, one at the kerbside of a major highway and the other at a background location. These will be equipped with highly sophisticated state-of-the-art instruments which will identify the times when new particles are forming in the atmosphere and will determine the chemical characteristics of the molecules which are condensing into clusters to form the new particles. Both cities have high sunshine intensity (needed for frequent new particle formation), but widely differing pollution climates. The outcome of the work will be a much better understanding of the processes responsible for new particle formation through nucleation in polluted air. Such knowledge will allow better design of mitigation strategies for reduction of ultrafine particle concentrations as well as providing the necessary knowledge to improve climate prediction models where currently one of the largest uncertainty relates to the role of airborne particles in affecting climate. In the future, it is expected that reductions in pollutant emissions and improved air quality may affect new particle formation. Lower concentrations of emitted particles will favour increased new particle formation, while reductions in sulphur dioxide and organic compounds are expected to be unfavourable. Well-designed numerical models are needed to predict the net future impacts upon new particle formation, and hence both urban particle concentrations and cloud condensation nuclei in more remote locations.