Research Projects

Project at a Glance

Project Status: complete

Report Published February 1983:

Title: Investigation of the role of natural hydrocarbons in photochemical smog formation in California

Principal Investigator / Author(s): Winer, Arthur M

Contractor: University of California Statewide Air Pollution Research Center

Contract Number: A0-056-32


Research Program Area: Atmospheric Processes


Abstract:

A major uncertainty in present emission inventories for California's urban airsheds is the lack of data concerning reactive organics (ROG) from vegetation; the contribution of such emissions to formation of photochemical air pollution has been the subject of considerable controversy. In particular, an experimentally determined,"airshed-specific" estimate of such emissions has not been available for the California South Coast Air Basin (CSCAB) as an input to the Basin's Air Quality Management Plan.

In order to acquire the necessary data for the CSCAB, we designed and implemented a three-tiered, stratified, random sampling approach, which could be applied to other airsheds as well. This approach involved: (a) quantitative assessments of the area coverage of vegetation in randomly selected sample areas within the CSCAB urban area, using a combination of high altitude (NASA U-2) and low altitude, high resolution, color infrared imagery; (b) field determinations of the distribution and green leaf mass fo the trees, shrubs and ground covers found in the selected sample areas; and (c) experimental measurements of emission rates of isoprene and monoterpenes from the most abundant natural and ornamental species found in the CSCAB. From these coordinated studies several significant data bases were generated which were not previously available. These include:

* Experimentally determined rates of emission of isoprene and monoterpenes for more than 60 plant species indigenous to Southern California.
* The first detailed survey of vegetation species composition and distribution in the urban portion of the Los Angeles Basin.
* Development of leaf mass constants for ~50 of the species identified in the study area.
* Acquisition of low altitude, high resolution color infrared imagery for 20 randomly selected areas of the Basin.
* Estimates of total green leaf mass and total percent cover by vegetation in the study area.

Using these data, estimates of emission strengths of isoprene and monoterpenes were derived for a 2600 km2 urban area. These were combined with results for the native coastal sage and chaparral communities in the foothills surrounding Los Angeles. The total inventory for isoprene and monoterpenes emitted from vegetation in the 4500 km2 study area during a summer day ranged from ~25 to ~80 tons day -1 depending upon the specific data analysis employed. The higher value and a "worst case" upper limit of 93 tons day-1, were obtained by assuming that all plants were emitters of both isoprene and the monoterpenes at levels corresponding to the detection limit of our chromatographic techniques. Since the study area encompassed 69.4% of the total anthropogenic ROG emitted in the Basin (1693 tons day -1 an average summer weekday) these values for hydrocarbons emitted by vegetation may be compared to ~1200 tons day -1 of ROG emitted from anthropogenic sources in the same area.

So that detailed computer modeling of the impacts of ROG emissions from vegetation can be made, our emissions inventory is available in a format consistent with the grid system employed for the emission inventory assembled for anthropogenic sources in the 1982 AQMP for the CSCAB. While detailed airshed modeling was beyond the scope of this program, we have carried out "EKMA"-type calculations. The results suggest that isoprene and monoterpene emissions in the CSCAB will contribute no more than, and probably much less than, a few tens of a part per billion (ppb) of O3, under conditions which correspond to the production of several hundred ppb O3 from anthropogenic ROG.


 

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