Study On Characteristics Of Roadside Particulate Matter With Different Patterns In Traffic Flow

The impacts of vehicle emission on ambient air quality have been of great concernbecause of the rapid increase of vehicle counts in megacities. Particulate matter hasbeen the primary air pollutant in Beijing over the past decade. The pollution level isoften found to be more severe in roadside microenvironment than in other urbanbackground areas. Therefore, the correlation analysis between traffic structure and thecharacteristics of particulate matter in the roadside microenvironment could sever as thescientific basis of future decision-making for urban traffic development and airpollution control in megacities in China.In this paper, a typical roadside site (North4thRing Road) and an urbanbackground site (Miyun) were selected; and comprehensive monitoring were conductedin2008~2009with normal traffic structure and special traffic structure regulated by thetemporary traffic control measures implemented during the Beijing Olympic Games(August,2008). Size-resolved mass concentration and number distribution weremeasured using continuous aerosol monitors. Chemical mass balance and toxicequivalency were also calculated for roadside particulate matter. Based on those data,the main sources for roadside airborne particles were identified. Source apportionmentresults with normal and Olympic traffic structures were compared in order to assess theenvironmental improvement effects from different traffic control measures during theOlympic period.With the special traffic structure in August2008, the reduction of average hourlymass concentrations of PM10、PM2.5and PM1at the roadside site were12.7%,49.3%and55.4%, respectively, compared to the non-Olympic summer period (June2008andAugust2009) with normal traffic. Larger reduction of PM2.5and PM1represented thatthe restriction of traffic emissions contributed significantly on the decrease of ambientPM1at roadside site. Bimodal distribution was identified in the diurnal profiles ofparticle mass concentration during the non-Olympic period, with high concentrations inmorning and evening rush hours. This bimodal distribution became flatter during theOlympic period because of the regulation of traffic flow. The average numberconcentration of ultrafine particles at the roadside site was0.55×104particle/cm3duringthe Olympic period, with a52.2%reduction compared to normal traffic exposure. Nucleation mode particles from gasoline vehicle exhaust and homogeneous formationdeclined more than accumulation mode particles. High nucleation mode particle numberconcentration in rush ours, which were found in normal traffic conditions, weredisappeared during the Olympic period, which indicated the significant reduction ofparticle number concentration from vehicle emission.According to the chemical mass balance for roadside particles, carbonaceouscomponents contributed the largest fraction to the total mass of PM10, PM2.5and PM1.During the Olympic period, larger decreases of OC and EC were found at the roadsidesite compared to the background site. Furthermore, the average OC/EC ratio wassignificant higher than that measured with normal traffic structure, which should beattributed to the prohibition of diesel fleet and high-emission vehicles (known as“yellow label vehicles”). The total particulate PAHs calculated under Olympic trafficcondition were59%lower compared to the non-Olympic summer period. The diurnalvariation of vehicle-generated PAHs was also weakened. The restriction of dieselvehicle emission was found contribute the largest fraction for the reduction PAHs levelduring the Olympic period. The total toxic equivalency of particulate PAHs at roadsidesite also decreased with the Olympic traffic structure; however, the reduction was lowerthan that calculated for PAHs mass concentration. Benzo[a]pyrene was found to be thepriority pollutant for the control of PAHs toxicity.Multiple linear regression analysis suggested that the pollution control measureduring the Olympic period accounted for a larger portion of the total variation of PM2.5concentration than meteorological parameters. Source apportionment for PM2.5revealedthat the contribution of vehicle emission decreased during the Olympic period, whichindicated that the reduction of vehicle particle emission was more strengthenedcompared to other sources. For roadside particulate PAHs, diesel vehicle emission wasthe leading source in summer period with normal traffic. During the Olympic period,the contribution of diesel vehicle exhaust significantly decreased. Based on abovediscussion, the restriction of traffic counts and vehicle emission significantlycontributed to the reduction of roadside ultrafine particle concentration and particulatePAHs level.