The Impact of Roadside Barriers and Buildings on Near Road Concentrations of Vehicle Emissions

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, December 2015
Dr. Akula Venkatram Chairperson

Exposure to elevated concentrations of vehicle emitted pollutants is associated with negative health effects. Elevated concentrations are typically found within several hundred meters of high traffic roads, where atmospheric dispersion has not sufficiently diluted pollutants.

Tall  buildings  next  to  roads  reduce  dispersion,  thereby  creating  pollutant  hot  spots and  increasing  exposure  to  vehicle  emissions  for  city residents.  Roadside  barriers  enhance dispersion  of  roadway  emissions  and  thus  can  be  used  to  mitigate  elevated  concentrations next to large roads. The work in this thesis develops semi-empirical dispersion models that are  useful  for  estimating  near  road  concentrations  of  vehicle  emissions  when  there  are buildings or barriers next to the road.

Dispersion models that account for the effect of near road barriers on concentrations are  developed  and  evaluated  with  data  from  a  wind  tunnel  and  a  field  tracer  study.  The model  evaluation  shows  that  the  primary  effect  of  roadside  barriers  is  enhancement  of  the vertical mixing by an amount proportional to the barrier height. Additionally, turbulence is enhanced in  the barrier's wake, resulting in  more rapid  growth  of the pollutant plume. The models  perform  well  during  neutral  and  stable  atmospheric  conditions.  During  unstable conditions  the models  overestimate  concentrations.  A model that  accounts  for reduction of the mean wind speed in the barrier wake is unbiased for all stabilities.

Models of the impact of tall buildings next to the road on near road concentrations of vehicle  emissions   are  developed.  The  models   are  evaluated  with   data  from   field measurements conducted in Los Angeles and Riverside counties, CA, and with data from an urban  area  in  Hannover,  Germany.  The  study  specifically  investigates  dispersion  in  cities with  significant  building  height  variability.  Model  evaluation  shows  that  vertical  turbulent transport  dominates  dispersion  in  cities.  The  primary  variables  governing  near  road concentrations of vehicle emissions in cities are the ratio of area weighted building height to street  width  and  the  vertical  averaged  standard  deviation  of  vertical  velocity  fluctuations. The model informs design of transit oriented developments, dense residential areas located in  close  proximity  to  transportation  infrastructure,  which  are  used  to  reduce  pollution  and greenhouse gas emissions due to transportation.