Study on pollutant characteristics near a roadway: Measurement, modeling, and instrument development.

摘要

Anthropogenic emission is an important factor affecting global climate change and human health. In a typical urban area, large fraction anthropogenic emissions are from traffic exhaust. Therefore, it is essential to experimentally study vertical and horizontal pollutant distributions for completely understanding pollutant characteristics, transport, and distributions near roadways.;Before developing a dispersion model, the pollutant dispersion due to two-way traffic effect was studied. The vorticity developed by the two-way traffic interaction induced a new drag force. Comparing the regular viscous drag force form, an equivalent drag coefficient was introduced and involved into the turbulent kinematic energy (TKE) parameterizations. The formula was validated by a field measurement.;Aiming at developing a parameterized Gaussian line source model without over-prediction issue near roadways, the traffic produced turbulence (TPT) including the newly developed two-way traffic model was incorporated into the new dispersion model. In this model, the TPT effect was strengthened near roadways in comparison to the popular line source model, CALINE4. Using above field measurement results, the predictions made by the newly developed model were compared with the results from CALINE4. By this way, different dispersion mechanisms were demonstrated. At this simple terrain, the TPT influence range and the extent evolving particle dispersion were evaluated. The TPT dominates particle dispersion at the site 15 m to the highway and is negligible at the site 100 m to the highway. The model is also developed to predict pollutant concentration at upwind site when the wind speed is low. The applicability of the model was demonstrated by comparing the prediction with the measurement at different wind speed, direction and sites.;Finally, couple instruments were developed for large scale field measurement on particle concentrations. A battery-powered low-cost electrometer was developed for measuring low current with femto-ampere (fA. 10-15 A) to obtain ambient particle number concentration. A differential mobility analyzer (DMA) was modified to improve its resolution by replacing with a new base. The performance of the DMA with new base was compared with the old base used for TSI 3071. Finally, the sheath flow rate limitation of the DMA was studied.;The field experiment was conducted in a flat terrain near a highway. A 30-foot height tower was deployed to obtain vertical pollutant sampling at nine different heights. Five identical water condensation particle counters (WCPCs, TSI 3786) were used to simultaneously measure particle number concentrations at five different heights. The particle size distribution was measured in real time by fast mobility particle sizer spectrometer (FMPS, TSI 3091). CO was also measured. The pollutant concentration distributions were shown in different wind speed and direction ranges. Three measurement sites (15, 50, and 100 m to the highway) were used to demonstrate pollutant distributions in horizontal direction. The vertical profile at each site is a function of wind speed and direction. Especially at the site 15 m to the highway, traffic related particles were detected at upwind conditions. Particle concentration plume profile and evolution are studied as a function of wind speed and distance to the highway. The correlations between traffic volume and particle concentration at different heights and measurement sites were studied.