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

空气中的颗粒是由漂浮在空气中的微小的固体物质或液滴组成的，它们太小了，肉眼无法单独看到。对这些颗粒物有很好的了解是很重要的，原因包括以下几点：-吸入高浓度空气对人类健康有害，因为这与住院人数增加和预期寿命缩短有关。全球因暴露在空气中的颗粒物造成的死亡人数比疟疾和艾滋病造成的死亡总和还要多；-微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太阳光对气候产生直接和间接的影响；-空气中高浓度的颗粒物导致能见度丧失，在污染的日子里可以看到雾霾。当前大气科学中最紧迫的科学问题之一是通过称为成核的过程从气体中形成新的颗粒物。随着气体分子结合在一起形成新的粒子，仪器的最新进展使人们能够直接观察成核过程。到目前为止，这些过程主要是在非常受控的条件下，在反应室和清洁的山顶空气中进行研究的。在严重污染的空气中发生的过程与这些不同，我们的建议是在成分特征非常好的城市中测量污染空气中的颗粒物成核过程。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。现场测量将在北京和巴塞罗那这两个新颗粒形成频繁的城市进行，并将在这两个城市建立两个采样点，一个在一条主干道的路边，另一个在背景位置。这些设备将配备高度先进的仪器，这些仪器将识别新粒子在大气中形成的时间，并将确定凝聚成簇形成新粒子的分子的化学特征。这两个城市都有很高的日照强度(需要经常形成新的颗粒)，但污染气候截然不同。这项工作的结果将是更好地理解在受污染的空气中通过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，并提供必要的知识来改进气候预测模型，目前最大的不确定性之一与空气中颗粒物在影响气候中的作用有关。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。排放的颗粒物浓度较低将有利于新颗粒物的形成，而二氧化硫和有机化合物的减少预计将不利于新颗粒物的形成。需要设计良好的数值模式来预测未来对新粒子形成的净影响，从而预测更偏远地区的城市粒子浓度和云凝结核。

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.