Ozone is a serious air pollution problem in most large U.S. cities. In the Sacramento County metropolitan area, motor vehicles are a major source of ozone precursors, contributing approximately 59 tpd (68% of total) NOx and 59 tpd (49% of total) anthropogenic HC emissions. While the bulk of the HC emissions are from tailpipe exhaust, approximately 9.7 tpd (16%) are from evaporative emissions that occur during daytime heating of fuel delivery systems of parked vehicles. Evaporative emissions, as well as exhaust emissions during the first few minutes of engine operation (primarily NOx), are sensitive to local microclimate.
We describe here a set of preliminary measurements to quantify the moderating influence of tree canopy on parking lot microclimate via shading and evaporative cooling from leaves. These estimates are used to calculate potential temperature-dependent emissions reductions from parked vehicles using the California Air Resources Board MVEI7G model. Potential emissions impact of air temperature differences between parking lots and normally used weather stations is also evaluated.
Measurements. Two automated weather stations and instrumented passenger cars were located in unshaded and shaded portions of a parking lot in Davis, CA for a week in August 1997. Air temperature, solar and net radiation, wind speed and direction, and vehicle cabin and fuel tank temperatures were measured. Concurrent meteorological data from a California Irrigation Management Information System (CIMIS) station located approximately 5 km southwest of the study site were also utilized.
The unshaded parking lot was 1 to 1.5°C warmer than the CIMIS site from noon to midnight. This finding suggests that actual HC emissions may be underestimated when CIMIS data are used to model temperature-dependent emissions. Peak daytime air temperatures at the shaded parking lot averaged 1 to 2°C cooler than the unshaded site; fuel tank temperatures of the shaded car were 2 to 4°C cooler than fuel tank temperatures of the unshaded car. Larger temperature differences between fuel tanks of shaded and unshaded cars compared to air temperature differences between shaded and unshaded lots indicate that direct shading of the vehicle influenced fuel tank temperature (hence HC evaporation rates) as much as or more than the aggregate effect of trees on air temperature.
Modeling. Observed air temperature regimes at the Davis parking lot were used to design "base case" and "treatment" cases for hypothetical changes in parking lot tree canopy. These temperature regimes were used as input to the MVEI7G model to simulate vehicle emissions in Sacramento County. ROG emissions (reactive organic gases) were reduced by 2% (0.85 tpd) for an increase in canopy cover from 8% to 50%. NOx emissions from cooler engine starts were reduced by 0.1 tpd. Though modest, projected ROG reductions are equivalent to projected hydrocarbon emission reductions for existing Sacramento Metropolitan Air Quality Management District control measures for graphic arts, ethylene oxide sterilizers, alternative fuel stations and waste burning (totaling 0.89 tpd). Projected NOx emission reductions (0.1 tpd) were equivalent to reductions projected from the district's light-duty vehicle scrappage program (0.1 tpd).