特长隧道内机动车排放PM2.5和NO2的特征及对能见度的影响

Particulate matter emitted by motor vehicles, belongs to PM2.5 regime which aerodynamic equivalent diameter is smaller than 2.5 μm. The vehicular fine particles not only are harmful for human health and contaminate urban ambient air quality, but also are the most important extinction substances to reduce atmospheric visibility due to the particle size close to the visible light spectrum section and complex chemical compositions with carbon element. Besides, the ultrafine particles from traffic are also one of the important factors to form urban fog haze, a phenomenon indicating air pollution with visual range less than 10 km and relative humidity below 80%. The team of this project will study the distribution characteristics of the vehicular pollutant concentrations and establish emission models of vehicular PM2.5 based on the measurements inside the Qinling Zhongnanshan road tunnel. The extinction characteristic of NO2 will be explored based on the data measured in the tunnel and the Rayleigh scattering theory. According to Mie scattering theory, light propagation and intrinsic attenuation characteristics for vehicular particles with different size sections and chemical compounds will be studied. A theoretical model between the PM2.5 from vehicles in the tunnel and comprehensive extinction coefficients will be established. And a methodology for calculating the impact of PM2.5 emissions from vehicles on visibility impairment will be proposed. The study is trying to explore the light attenuation mechanism of pollutants emitted from vehicles. Therefore, the results of this study has an important scientific significance and application value for improving tunnel safety, assessing urban air pollution and explaining formation of haze etc.

机动车排放的颗粒物属于PM2.5（空气动力学粒径<2.5μm）的范畴。机动车排放的细颗粒物不仅危害人体健康，污染大气环境，更因其粒径靠近可见光谱段和复杂的含碳成分，成为衰减大气能见度的重要消光物质。此外，机动车排放的超细颗粒物也是形成城市雾霾（能见度<10km,相对湿度<80%）的重要因子。本项目拟在秦岭终南山特长公路隧道内，测定机动车污染物的浓度与分布规律，研究颗粒物排放特性，建立排放模型。基于实测数据与瑞利散射理论，探讨隧道内NO2的消光特性；基于Mie散射理论，研究机动车排放物的不同成分与不同粒径段颗粒物群的消光特性；建立隧道内综合消光系数与机动车污染物排放量之间的理论模型，提出道路交通PM2.5排放量影响大气能见度的计算方法。本研究探索机动车排放污染物对光的衰减机理，在提高隧道安全、评估城市大气污染和解释雾霾成因等方面具有重要的科学意义和应用价值。

道路机动车排放是城市空气污染物的重要来源之一。机动车排放的细颗粒物（PM2.5）和氮氧化物（NO2）等污染物不仅危害人体健康，而且是衰减大气能见度的重要污染物质。机动车排放的颗粒物作为云凝结核会影响云的形成与降水分布，从而影响区域或全球气候变化。国外学者通过隧道内机动车排放颗粒物与气态颗粒物浓度实测数据，以及测试隧道内消光系数，建立不同粒径颗粒物质量浓度与能见度的关联式。我国学者也曾对两条山岭隧道能见度与一些关联因子之间的影响进行过实测研究。但目前国内外研究中，从宏观的角度机动车污染物排放是如何影响能见度，以及从微观层次机动车排放的颗粒物粒径分布和NO2等消光成分影响能见度的机理依然未知。.本项目以秦岭特长公路隧道为实验环境，测试我国在用机动车排放的颗粒物浓度、粒径分布、化学成分以及NO2浓度等，研究确定了机动车排放的不同粒径段颗粒物群和NO2对光的散射和吸收函数关系，获得了尾气污染物的微观光衰减规律，建立了机动车污染物的消光机理模型，计算了消光系数。基于隧道内同步观测的交通量、能见度、气象条件、污染物浓度的时间序列，研究了机动车排放颗粒物粒谱对能见度的影响特性，提出了评价机动车排放PM2.5对灰霾形成的评估模型与计算方法。.研究结果显示秦岭隧道内距入口超过2km处颗粒物的综合密度为1.95g/cm3，综合复折射指数为1.619-0.087i；核膜态颗粒物的消光以EC的光吸收作用为主，积聚模态颗粒物对PM2.5消光系数的贡献率为98.73%；重构IMPROVE方程，隧道环境中颗粒物的散射和吸收对总消光系数的贡献率为67.8-76.9%和12.4-15.6%，PM2.5中OC、EC和无机盐对bext_总的贡献率分别为44-60.4%、19.3-35.4%和0.3-5.6%，NO2光吸收和空气分子的Rayleigh散射作用对bext_总的贡献率分别为8.8-16.3%和0.6-1.4%。.研究成果对城市区域环境内机动车排放污染物影响大气能见度、灰霾形成机理提供理论指导；对实施区域性“治污降霾，保卫蓝天”行动计划，制定控制机动车颗粒物衰减及NO2减排对策提供技术支撑；对改善公路隧道内空气质量，通风与照明设备优化提供指导意义。