Project at a Glance
Title: Regional source-receptor relationships for atmospheric acidity and acid deposition in California
Principal Investigator / Author(s): Karamchanadani, Prakash
Contractor: ENSR Consulting and Engineering
Contract Number: A032-189
Research Program Area: Atmospheric Processes
Topic Areas: Acid Deposition
This report describes the results of a database management and semi-empirical modeling study that was performed to evaluate regional source-receptor relationships (SRRs) for atmospheric acidity and acidic deposition in California. The objectives of the study were to quantify the contributions of the various source regions in California to acidic deposition at selected receptors in the state and to estimate the uncertainties in the derived values.
The study consisted of the following tasks:
* acquiring precipitation chemistry and dry deposition databases from the California Acid Deposition Monitoring Program (CADMP), as well as meteorological and emissions databases for the years of interest (1984 through 1989);
* incorporation of the CADMP data into a dBase IV Database Management System (DBMS) and limited validation of the data;
* preparation of annually and seasonally averaged 'modeling databases for ENSR's semi-empirical statistical acid deposition model (STATMOD);
* adaptation of STATMOD for application to California;
* application of STATMOD to compute annually and seasonally averaged precipitation and ambient concentrations of sulfate and nitrate and evaluation and optimization of the model using CADMP data;
* computation of source-receptor relationships for selected receptors;
* uncertainty analysis of the derived source-receptor relationships, based on the uncertainty of the model parameters; and sensitivity analysis of SO2, and NOX, emission controls on acidic deposition at sensitive receptor locations.
The model performed well in estimating nitrate concentrations in rain and air, but sulfate concentrations in rain and air at several receptors were consistently underestimated. Total sulfur (SO2, + sulfate) concentrations were also underestimated, suggesting that the SO2, emissions used in the simulations were either inaccurate or incomplete. Assuming that there were no major inaccuracies in the anthropogenic SO2, emissions, this indicated that the model underestimated sulfur concentrations in rain and air because SO2, emissions from non-anthropogenic sources (e.g., sea salt, or wind blown soil dust) were not included in the inventory. A correction for the sea salt contribution improved the performance of the model. However, the model still tended to underestimate the observed sulfur concentrations. A statistical analysis using observed calcium concentrations in rain was performed to demonstrate that model performance could be improved further by estimating the contribution of wind blown soil dust to sulfur concentrations in rain and SO2 and sulfate concentrations in air. The source-receptor relationships for a composite year and composite seasons were developed in the form of plots showing the relative contribution of the various source regions in California to acidic deposition at selected receptor locations, as well as zones of influence for each receptor. Ten receptors from various urban, rural, and remote regions of the state were selected to create a representative sample for the analysis. The source-receptor analysis showed that the contribution of local sources dominated the total sulfur (SO2 + sulfate) dry deposition at receptors located in urban regions or close to large sources. The zone of influence for total sulfur dry deposition at these receptors was usually less than 100 km, i.e., dry sulfur deposition at these receptors could be attributed mainly to sources that were less than 100 km away from the receptors. At remote receptors, there was some evidence of long-range transport in the dry deposition of sulfur. The zone of influence for these receptors was of the order of 200 km. Long-range transport played a larger role in total sulfur wet deposition as compared to dry deposition, even at receptors in the vicinity of large sources. The zone of influence for sulfur wet deposition was higher than that for sulfur dry deposition at all receptors. At some receptors, the wet deposition zone of influence was more than twice the dry deposition zone of influence. Long-range transport was found to be a factor in nitrogen dry deposition, even at receptors that were located close to high NOX, sources. The zones of influence for dry deposition of nitrate were always larger than the zones of influence for sulfur dry deposition, indicating that nitrate dry deposition has a more regional nature than sulfur dry deposition. The zones of influence for the wet deposition of sulfur and nitrate were comparable. The total (dry + wet) nitrogen deposition was larger than the total sulfur deposition at most receptor locations. These results are consistent with the relative NOX, and SOX, emission rates in California. Sensitivity studies were conducted to determine the sensitivity of the derived source-receptor relationships to changes in the model parameters within their expected range of values and to determine the response of the model to changes in NOx and SOx emissions. The Latin Hypercube Sampling (LHS) technique was used to construct 100 parameter sets. The simulations were performed for a composite year. The sensitivity studies showed that the estimated sulfur concentrations in rain were relatively insensitive to changes in the model parameters. The standard deviation was less than 10 percent for all the receptors. The estimated ambient sulfate concentration was more sensitive to changes in the model parameters than sulfur in rain. However, the standard deviation of the estimated values was still small (less than 15%) for all the receptors. Nitrate concentrations in both rain and air were more sensitive to the input parameters. For many of the receptors, the maximum estimated concentrations were about two times larger than the minimum estimated concentrations. The relative contributions of the source regions to total sulfur and nitrate deposition at various receptors were found to be insensitive to the changes in STATMOD parameters. The effect of five different emission control scenarios on acidic deposition in California was examined. The results of the emission control scenario studies showed that a 50 percent reduction in statewide SOX, emissions did not always result in a 50 percent reduction in the dry, wet or total sulfur deposition. On the other hand, the response to a 50 percent reduction in NOX, emissions across the entire state was linear. Emission controls in individual source regions showed that larger reductions were estimated at receptors that were directly influenced by these source regions as compared to receptors that did not receive large contributions from these sources. The results were slightly different for sulfur and nitrogen, particularly at receptors located close to the source regions where emissions were reduced in our simulations. The differences in the results for sulfur and nitrogen can be attributed to the differences in their zones of influence for dry deposition, and the fact that wet scavenging of sulfur is limited by the availability of oxidants.
For questions regarding this research project, including available data and progress status, contact: Heather Choi at (916) 322-3893
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