Research On Characteristics And Impacts Of Urban Cyclists’ Exposure To Fine Particulate Matter

Traffic-related pollution has become the primary source of airborne particles in urban environment due to rapid increases of on-road vehicles.As a popular commuting mode in Chinese cities,cyclists may exposed to the high risk of particles pollution in the on-road traffic microenvironment.Nevertheless,there are a limited number of studies examining characteristics and determinant factors of cycling exposure to airborne particles.This paper provides a comprehensive study including concentration levels,physical and chemical characteristics,source apportionment of fine particles in cycling exposure environment,and impacts of traffic,street characteristics and meteorological conditions on the cycling exposure based on field measurements and sampling work on/in typical urban roads/streets in Fuzhou,China.The overall objective of this work is to provide strategies to improve the cycling environment and reduce the exposure risks of cycling commuters through Investigations above.The main contents and conclusions of this paper are as follows:1.Characteristics of urban cycling commuting exposure to fine particulate matters.（1）Comparisons of exposure concentration and dose between four transportation modes were conducted on selected commuting routes.The results showed that cyclists were exposed to highest PM_2.5（30.0μg/m³）and PM₁₀（45.1μg/m³）concentrations,and most frequently encountered some higher peaks during cycling period.During the winter,the average exposure concentration of PM_2.5 cycling commute exceeds the national secondary standard in China（35μg/m3）,and the longer commuting time and higher respiratory rate caused the average total exposure dose of cyclists were 2,3 and 5 times higher than bus,car and subway respectively.（2）The Pearson correlation results showed that ambient background concentration had a strong significant association and contribution to cycling exposure concentration.In order to eliminate the"masking effect"of ambient background concentration,Relative Exposure Concentration（REC）was proposed after removing the background component,which was employed as the indicator to examine the differences in individual exposure concentration levels in cycling microenvironment on-road level and investigate impacts other than the background concentration.The diurnal and seasonal comparison indicated that REC PM10was generally higher than REC PM2.5;the REC in winter was significantly higher than in summer,and the REC during the morning rush hours was slightly higher than the evening.The higher background concentration could promote the nucleation and accumulation of particulate matter in the on-road traffic environment,which caused higher cycling REC.Rainy summer with higher wind speed,and windy evening rush hour,which could benefit the dispersion of fine particles.（3）PM_2.5 samples were collected at the height of cyclists breathe level along the fivetypical commuting routes water-soluble ions,carbon components,and heavy metal elements and their morphological characteristics of samples were analyzed.Based on heavy metal concentrations,the health risk assessment for cyclists was conducted.Positive matrix factorization（PMF）was employed in source apportionment of fine particles in the cycling exposure environment based on the contents of chemical components in samples.The results showed that the water-soluble ions in the cycling environment were mainly secondary ions such as NH₄⁺,NO₃^-and SO₄^2-,and the ratios of NO₃^-/SO₄^2-exceed 1;which indicated that the water-soluble ions in the cycling exposure environment mainly contributed by traffic sources.Urban streets with dense trees could cause the formation and accumulation of secondary particles such as ammonium nitrate.The content of organic carbon（OC）was two times higher than elemental carbon（EC）.The higher proportion of OC3,OC4,and EC1（86%）explained that carbon component of cycling exposure was mainly secondary pollution and vehicle exhaust emissions contributed the most to the carbon component,whereas together with road dust,diesel counts and poor ventilation were also the main factors to promote carbon component pollution.The traffic related elements such as Cd,Cu,Pb,and Zn were highly enriched in PM_2.5（over or close to 1000μg/g）.The concentration levels of Mn and Cr had potential non-carcinogenic risks to cyclists,whereas Ni,As had potential carcinogenic risk and Cr presented a significant carcinogenic risk.The morphological characteristics showed that particles were mainly finer soot particles,followed by irregular mineral particles,indicating that the fine particles cyclists exposed were more influenced by vehicle exhaust and road dust.The result of PMF modeling showed that there were five categories sources including road dust（30.4%）,secondary particle sources（23.3%）,vehicle exhaust（23.1%）,coal-burning（18.2%）and industrial sources（5.0%）.The combined contribution（53.5%）related to road traffic of dust and vehicle exhaust was significantly higher than the background.2.Impacts factors of urban cycling commuting exposure to fine particulate matters.（1）Fixed-sites monitoring at the breathing height of cycling commuters were conducted on selected typical commuting roads to investigate the impact of meteorological factors,especially wind speed and wind direction on cycling the relative exposure concentration（REC）of PM_2.5 and PM₁₀.By the statistics of REC and optimization of monitoring intervals,non-meteorological factors such as background concentration,traffic,and building physical characteristics were relative stabilized to improve the identification of meteorological factors.The results showed that wind speed had the most significant impact on REC explaining the variation of REC（15-63%）.There was no significant correlation between temperature and humidity and REC.Higher REC（>6μg/m³）mainly occurred with low wind speed（<1.5 m/s）.Parallel wind could better benefit the dispersion of particles.Vertical wind with medium speeds（<3.4 m/s）could increase PM concentration on the leeward side of street canyon by vortex effects,whereas vertical wind with lower speed（<1.5 m/s）could increase PM concentration on the windward side of open roads.（2）Mobile roadside measurement were deployed to monitor PM concentrations in roadside pollutants concentrations in both of typical wide and narrow streets in urban area.The impacts of traffic volume and fleet composition between street segments on REC were analyzed.The influence of changes in Physical Street features（built environment features and roadside vegetation）on the spatial variation of REC within each street segment were evaluated by multivariable models.The results indicated that higher RECs（>4μg/m³）not only appeared in wide streets with larger traffic volumes,but also in the narrow street with dense vegetation due to the accumulation of particles contributed from emission and road dust caused by diesel trucks.In multivariable models,the street aspect ratio H/W and buildings spacing distance（BSD）were the main factors in wide streets,which could explain explained 15-62%of the variation of REC.The leaf area index（LAI）and canopy coverage percentage（CCP）of the roadside vegetation explained 53%of the variation for REC PM_2.5and 34%for REC PM₁₀ in narrow streets with dense trees.When CCP exceed 40%and LAI was higher than 4-5,REC was significantly increased in narrow Streets,which indicated that trees restrained the dispersion of particulate matter.Conversely,REC remained at a low level under lower CCP（<40%）and LAI（<4-5）and trees could remove some particulate matters due to deposition and filtration effect.（3）The effectiveness of different types of roadside green belts（RGBs）in Fuzhou for reducing cycling exposure concentration was investigated by the statistics of the concentration changes before and after the RGBs.The results showed that the total average reduction rate of the RGBs on the fine particulate matters concentration was-3.8%to 9.7%.The‘combination of trees and hedges/shrubs’showed the highest reductions and the‘shrub hedges only’showed consistent reduction performance.Shrubs such as Duranta repens L.,Gardenia jasminoides and Loropetalum chinense var.rubrum have small and dense leaves with deep gullies,which could benefited PM-capturing and wind-firm capability with the increase of the height and density of hedges.The‘trees only’configuration had non-significant effect in reduced PM concentrations.Under the conditions of parallel wind and vertical wind（blowing to the green belt）with lower speed（0.3 m/s-1.5 m/s）,the effectiveness to reduce particles of hedges and mixed green belts of trees and shrubs performed better.3.Strategies for reducing the cycling exposure to fine particulate matters.Cyclists could select secondary arterial roads with low traffic flow and fewer diesel vehicles.The main road/street direction designed to be parallel to prevailing wind direction in urban planning,increasing the distance between buildings and the road aspect ratio,and reducing the terrace buildings along the street can help improve the ventilation of street canyon（especially on the leeward side）.Diesel vehicles should be restricted or reduced in urban street（especially in narrow streets with dense trees）.Strong pruning intensity was suggested for improving the ventilation of cycling environment.Daily cleaning of roads and branches and leaves of green belts could benefit reducing re-suspension of dust and planting‘combination of trees and hedges/shrubs’RGBs would be appropriate for reduction of exposure concentration.Avoid plant dense trees in narrow streets.In narrow streets dominated by banyan trees,the optimum range of CCP and LAI would be 40%and 4-5 respectively to obtain the best ecological and landscape benefits.