Project at a Glance

Title: Receptor modeling of transport of acidic air pollutants and oxidants to forested regions in the Sierra Nevada.

Principal Investigator / Author(s): Gertler, Alan W

Contractor: UC Los Angeles

Contract Number: A932-140

Research Program Area: Atmospheric Processes, Emissions Monitoring & Control

Topic Areas: Acid Deposition, Area Sources, Chemistry & Reactivity, Ecosystem Impacts, Impacts, Modeling, Vulnerable Populations


The major source area of acidic species and their precursors which has the potential for impact in the Sierra Nevada is the San Joaquin Valley (SJV). During the summer of 1990, the San Joaquin Valley Air Quality Study (SJVAQS) and Atmospheric Utility Signatures, Predictions and Experiments (AUSPEX) studies, two major, integrated field studies, were performed. These studies provided a basis for extending our knowledge of air pollution effects to regions outside the study area which are affected by emissions from that region. By taking the approach of "piggy-backing" on to the SJVAQS/AUSPEX studies, it was possible to determine the relationship between acidic species and their precursors emitted in the SJV source region and their eventual deposition at susceptible, forested receptor sites in the Sierra Nevada.

There were two main facets to the study. The first involved field measurements that are required as input for the receptor modeling task. The second involved the application of receptor models to apportion sources of atmospheric acidity. Sites included SJVAQS / AUSPEX sites at Sequoia and Yosemite, a site in the vicinity of Tehachapi, and a site on the western slope of the northern Sierra Nevada near Lake Tahoe (Blodgett Experimental Forest). Measurements of gaseous and fine particulate inorganic and organic acidic species were made on 14 days corresponding to the SJVAQS/AUSPEX intensive measurement days.

Observed PM2.5 concentrations were generally low, with a median concentration of 8 to 10 g/m3. Organic carbon and sulfate were the major components, accounting for more than 25% of the PM,, mass. Formic acid concentrations ranged from approximately 1 ppb to 40 ppb; those of acetic acid ranged from approximately 0.5 ppb to 13 ppb. Mean formic acid concentrations were 18 ppb at Tehachapi and between 12 and 13 ppb at the three other sites. Mean acetic acid concentrations ranged from 3.9 ppb at Blodgett to 8.0 ppb at Yosemite. Comparison of these results with previous studies indicate that carboxylic acid levels measured in the Sierra Nevada are higher than those measured in past studies and, in many cases, average concentrations ofboth formic and acetic acids observed during this study were greater than the maxima observed in other studies. Compared to levels of strong acids previously measured at Sierra Nevadan sites, organic acids appear to be significant contributors to the overall acidity. The high levels of organic acids measured during this program led to questions being raised regarding the current state of sampling and analysis methods for organic acids. The results of a review of the literature led to the conclusions that sampling is simple but preservation may be a problem, analytical methods are well developed but resolution problems remain, sources of primary emissions of organic acids are still not well characterized in most areas, and improvements-in sampling and analysis methods are required before a monitoring network for gaseous and hydrometeor-phase formic and acetic acids can be established.

The majority of the chemical species required for the model input of the Chemical Mass Balance (CMB) receptor model are well above the detection limits, with the sum of the species over PM2.5 mass ratios above 0.5 in most cases. Contributions to the average apportionments were different for all four sites. Local sources had the greatest impact on Blodgett with primary geological material contributing 28 % (1.6 0.7 g/m3) and primary motor vehicles contributing 22% (1.4 0.8 g/m3). Yosemite was dominated by vegetative burning (from both campfires and forest fires) with results from the CMB calculation showing 6.8 9.8 g/m3 (43 % of PM2.5 mass) vegetative burning for the 24 hr average. Motor vehicles contributed 15 % of PM2.5 mass during the same period. The mid-valley regional and secondary organic carbon sources are the major contributors at the Sequoia Lower Keweah site, which accounts for 55% of the PM10 mass. Tehachapi appeared to be impacted by emissions both from the SJV and SoCAB. The lower valley regional profile along with primary motor vehicle exhaust were the major contributors at the Tehachapi site, accounting for 31% and 20% of the PM respectively.

Secondary ammonium sulfate contributions are high, and vary from 15 to 25 g/m3 depending on the sampling period and day. Unlike any of the other sites, secondary ammonium nitrate accounted for 8% of the PM2.5 mass. For the PM10 averages at Sequoia and Yosemite, the major difference was the increased contribution of primary geological material. This source accounted for 43% of the mass at Sequoia and 40% of the mass at Yosemite.

The data were also analyzed by Principal Components Analysis (PCA) and the Source Apportionment by Factors with Explicit Restrictions (SAFER) multivariate receptor model. The data sets from each of the four sites showed five major principal components; two related to soil dust and wood smoke, and three related to secondary species: sulfate, nitrate and nitric acid. The SAFER model was then used to determine the range of possible source compositions of the sources of airborne particles. For example, at the Sequoia site, the data set was found to have only three factors: wood smoke, soil dust, and a source of organic carbon presumably from L transport. From the source compositions, it was determined that about 60% of the organic carbon was from transport.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

Stay involved, sign up with CARB's Research Email Distribution List