资源环境科技发展态势分析平台

Local sources of particles and precursor gases have long been considered as the major control for the ground-level particle number concentration in an urban environment. Here we show the existence of two distinct sources. The first source was detectable during morning and afternoon rush hours and was defined by high black carbon concentrations. Particle number concentration inversely correlated with the local planetary boundary layer height. The particle size distributions were characterized by a wide range of modal diameters and did not exhibit detectable modal growth. This source was attributed to vehicular emissions. The second source yielded particle number concentration comparable to those during the rush hours and was detected six times over the 3-week measurement campaign. Small particles produced by this source were recorded during the midday after the diminishment of the rush-hour traffic effects. The particles exhibited prolonged modal growth over 8 hr, which may indicate a regional scale nucleation event. The data suggest that these particles were likely formed above the nocturnal boundary layer after sunrise and were subsequently transported to the surface through convective mixing. Overall, the nocturnal and convective boundary layer evolution was found to be closely associated with the of small particle event and the most important factor affecting the ground-level particle number concentration. Shallow nocturnal boundary layers trapped pollution near the ground leading to particle number concentrations over 10(4) cm(-3).

Plain Language Summary More than 80% of people living in urban areas are exposed to air quality levels that exceed the World Health Organization limits. Ambient air pollution made of high concentrations of fine particles is one of the greatest risks to health-causing more than three million premature deaths worldwide every year. Automobile emissions have been considered as the major source of particles in urban areas, followed by the contribution from other sources. Here we emphasize the importance of an additional, less well-understood source of particles. These particles may not be directly related to human emissions of pollutants and be likely formed above the ground. Large concentrations of these freshly produced particles can be brought to the surface by daytime mixing. Understanding the processes that form these particles is important because they appear during the midday when contributions from traffic are less. This study reemphasizes the importance of local meteorology for determining the ambient air pollution levels in a surprising way. Breaking the boundary layer usually leads to the dilution of traffic-related particles. However, here we show that breaking of the boundary layer can also bring particles formed aloft and subsequently can lead to surface concentrations comparable to those experienced during rush hour.