Fine particulate matter (PM) affects public health, visibility, climate, and influences ecosystem productivity and species diversity. Diesel engines are an important source of air pollution and will face a variety of new regulations, so emissions from these vehicles are expected to undergo changes over the next decade that will have important effects on primary PM emissions, especially black carbon (BC) emissions, as well as nitrogen oxide (NOx) emissions and therefore secondary pollutants such as ozone and PM nitrate. Analysis of observed and modeled air quality responses to changes in diesel engine emissions provides insights into the relative importance of diesel emissions and the effects of future emission controls.
Yearlong records of online measurements of black/elemental carbon at Fresno, St. Louis, and Pittsburgh were analyzed as part of this research. Diesel truck activity decreases substantially on weekends, and the pollutant time series were analyzed to look for corresponding reductions in ambient BC concentrations. Significant weekend BC reductions of 22±6 and 25±5% were found at Fresno and St. Louis, respectively. Smaller reductions were observed at Pittsburgh. Continuous measured records of fine particulate nitrate were also analyzed over yearlong periods at the three locations mentioned above and at Claremont, CA, an inland location within the Los Angeles basin. Reductions in PM nitrate were observed on Sundays and Mondays, indicating a delayed response to NOx emission reductions that occur on Saturdays and Sundays. Nitrate reductions of 23±12, 29±23, and 16±9% were observed on Mondays at Fresno, Claremont, and St. Louis, respectively, relative to 7-day moving averages.
Exhaust emissions from diesel-powered construction equipment are typically estimated using statistical models. Both the emission estimates and the underlying engine activity estimates are subject to large uncertainties that are not routinely quantified. A fuel-based inventory of construction equipment emissions for California was developed and showed NOx and exhaust PM emissions to be 4.5 and 3.1 times smaller, respectively, than official emission inventory estimates developed by California Air Resources Board staff. Also, a revised description of the spatial distribution of diesel engine activity based on construction permit data showed construction activities had moved on from older housing development projects along the coast to new locations further inland in Southern California. Updating the construction inventory had significant effects on air quality model predictions for NOx, BC, and ozone.
A gridded Eulerian model was used to assess weekend effects on particulate matter and compare model predictions weekly nitrate cycles to the observational analysis described above. The model incorporated the new construction inventories described above. This model was further employed to analyze the air quality effects of new regulations on in-use diesel trucks.
The model was run over two seasons, and a baseline scenario was compared to a scenario including weekday emissions substituted for weekend emissions. The model analysis showed similar weekend reductions of BC to changes at ground-based observation sites in southern California. Unlike BC, particulate nitrate is a secondary pollutant with non-linear and non- intuitive dependence on precursor emissions. Analysis of particulate nitrate effects was also challenging because observed weekend effects were smaller than for BC, and meteorological variability made the signal harder to discern. Both modeled and observed reductions in nitrate were found on summer Mondays at Claremont (inland site). Process analysis showed that in some locations weekend NOx reductions could lead to higher nitric acid production and higher nitrate levels, as there are factors that offset the effect of lower weekend NOx on the rates of both daytime (OH+NO2) and nighttime (via N2O5) pathways to nitric acid formation. An important difference between modeled nitrate and observed nitrate was found during the fall season: modeled nitrate increased on Monday.
The effects on future air quality of new regulations requiring the retrofit of in-use heavy-duty diesel trucks and buses to meet stringent PM emission requirements were evaluated. By 2014, the in-use retrofit rule is predicted to reduce average ambient BC concentrations in southern California by 12±2 and 14±2% during the summer and fall, respectively, relative to a baseline scenario that included emission decreases due to fleet turnover effects but no retrofits. Primary NO2 emissions are predicted to increase with greater use of oxidative particle filters, however, ambient NO2 concentrations are not predicted to increase as parallel, but less stringent, retrofit requirements reduce total NOx emissions. Increases in ambient ozone and particulate nitrate concentrations were predicted to occur within the Los Angeles basin, especially during the fall season, due to an increase in the NO2/NOx emission fraction and reduced total NOx emissions.