The California-Mexico border region suffers from high levels of air pollution due to a large and expanding population, industrial growth, transportation and agriculture. In this project, researchers will investigate cross-border transport of pollutants and their impacts on regional air quality and climate through the collection and analysis of field data. The observational component of the work includes ground-based measurements along the California-Mexico border region, utilizing a combination of a central fixed site that will house state-of-the-science instruments to measure gases, aerosols, radiation and meteorological parameters and a mobile laboratory that can measure surface-atmosphere exchange fluxes using eddy covariance. The study will be complemented by measurements provided by Mexican colleagues at the fixed site and several mobile units for criteria pollutants and meteorological parameters. The main scientific objectives are to (1) characterize the emissions from major sources in the California-Mexico border regions, (2) determine the spatial and temporal variability in anthropogenic emissions of greenhouse gases and traditional air pollutants, and (3) elucidate the transport and transformation of these emissions and assess their impact on local and regional air quality and climate. A planned NOAA and California Air Resources Board study will provide additional data to aid in the analysis of these subjects.
The Intellectual Merit of the project is to assess the sources and processing of particulate matter and its precursors in the border region and their effects on regional air quality and climate. The analysis tasks will much better define the interplay of emissions and secondary particle formation, the diurnal evolution of fine particulate matter chemical composition under degraded air quality conditions, atmospheric processing of aerosols, particularly soot-containing particles, and their transport and transformation.
The Broader Impacts of the research include the direct involvement of graduate students and postdoctoral researchers in the tasks, and improved collaboration between U.S. and Mexican scientists and students. Results from the study will be directly presented to California and Mexican Government officials and will inform the development of air quality management strategies to reduce adverse human health, ecosystem, and climate impacts.
This Project involved one of four collaborative awards from NSF to support a field study conducted along the California-Mexico (Cal-Mex) border region. The overall goal is to characterize the sources of primary and secondary particulate matter and precursor gases in the border region, their transport and transformation, and the impact of these pollutants on regional air quality and climate. The collaborative teams (Molina Center, Texas A&M, University of California San Diego, and Virginia Tech) pursued the project goals by performing several tasks, which included defining science questions and setting measurement strategies, executing measurement plans, analyzing measurements obtained, developing emissions and meteorological input data for modeling studies, evaluating the data using meteorological and photochemical models, and preparing manuscripts for publication. The measurement phase of the Cal-Mex 2010 Campaign took place in May-June 2010 involving a combination of a central fixed site that housed state-of-the-science instruments to measure gases, aerosols, radiation and meteorological parameters and a mobile laboratory that measured surface-atmosphere exchange fluxes. The field study was complemented by measurements provided by Mexican researchers at the fixed site and several mobile units for criteria pollutants and meteorological parameters. Supplemental funding was provided by California Air Resources Board, Mexican Ministry of the Environment, and the Molina Center. The findings from Cal-Mex 2010 have improved our understanding of the importance of different emissions sources (urban, biomass burning, natural) that are unique along the border region such as San Diego-Tijuana and the important processes in aging urban air. Four representative plume transport patterns were identified during Cal-Mex 2010 as "plume-southeast", "plume-southwest", "plume-east" and "plume-north"; this is useful for linking meteorological conditions with observed changes in trace gases and particular matter. Criteria pollutant average concentrations were relatively low during most of the field study. Time series analyses show that CO has dropped substantially in the past few years in both San Diego and Tijuana, whereas O3, SO2, and NO2 show much smaller reduction rates. SO2 decreased significantly in 2003 after natural gas was introduced in the Rosarito power plant. Measurements and modeling results suggest the need to further refine the local emission inventory. Comparison of O3 levels among the monitoring stations in the San Diego-Tijuana Air Basin shows that O3 levels are higher in stations at high altitude. Sensitivity analysis of ozone formation in San Diego-Tijuana air basin indicates that it is VOCs-limited. Model study also illustrates the importance of biogenic emissions in enhancing ozone formation in the mountain region, and the interaction of the trans-boundary transport of emissions between California and Baja California that reduces afternoon surface O3 in the border region. Source apportionment indicated four major VOC sources: industrial emission, gasoline vehicle exhaust, diesel vehicle exhaust and aged plumes. Highest contribution was from industrial sources. Measured average PM concentrations were below Mexico air quality standards. Comparison of monthly average concentrations indicates that Cal-Mex 2010 corresponded to the season with the lowest PM concentration. Black carbon concentrations were more than two times higher in Tijuana compared to San Diego or the international border crossing. Black carbon mass concentrations were attributable to nighttime urban burning activities and diesel vehicles. Simulations and measurements suggest that 60% of the oxygenated organic aerosol observed in Tijuana may have originated from Southern California Air Basin. Transportation sector in Tijuana, especially diesel vehicles, contributes significant amount of black carbon, SO2, NOx, as well as primary formaldehyde (HCHO). Analysis indicates that HCHO plays a dominant role in regulating OH radical budget and thus degradation of other primary air pollutants and formation of secondary pollutants, including ozone and secondary organic aerosols. An effective control of the mobile sources can potentially reduce the photochemical reactivity in this region, as well as black carbon emission, which not only poses health risks to the local residential population, but also impacts the regional environment and climate. Documented findings from this project have been presented at international conferences and published in a special issue on US-Mexico Border Study in Atmospheric Environment. All data sets and publications are available to the scientific community. Scientific findings and policy implications as well as future research needs have been communicated to the relevant Mexican and US government officials. Results from this collaborative project were included in the new Tijuana air quality management program (PROAIRE in Spanish) recently published by the Government of Baja California. More than a dozen post-doctoral associates, graduate and undergraduate students from the four US institutions as well as from participating Mexican institutions were involved in the field measurement, data reduction and analysis, and co-authored appropriate reports and presentations. The US-Mexico collaboration has provided opportunity for US students and postdocs to work with students and scientists from Mexican institutions, as well as collaboration with Mexican government officials. Thus, Cal-Mex 2010 provides an excellent opportunity to build capacity for research, education, and policy and contributes to international exchange.
