Sources and Transport of Black Carbon at the United States-Mexico Border near San Diego-Tijuana

Black carbon (BC), a major component of soot, is a byproduct of incomplete combustion of carbonaceous fuels, such as diesel and biomass. It has significant climate effects due to its ability to absorb solar radiation across a wide spectral range; the magnitude of this radiative forcing is estimated to be second only to carbon dioxide. It also has worrisome health effects due to its high porosity and ultrafine size that allow it to adsorb other potentially carcinogenic species in the vapor phase and easily deposit into the respiratory system. Reducing BC emissions and impacts requires an improved understanding of its sources and their spatial and temporal distributions. This problem is of special interest at the US-Mexico border, where air quality is poor and the potential for cross-border transport creates an imperative for binational cooperation in designing air quality management strategies. The overall objective of this research is to assess emissions of BC and its transport in the USMexico border region. The specific goals of this research are to characterize the spatial and temporal variability in BC concentrations and emissions in the border region, to identify potential source areas of BC emissions, and to characterize the cross-border transport of BC and assess its impact on local and regional air quality. As part of the Cal-Mex 2010 field campaign in the border cities of San Diego and Tijuana, we measured BC concentrations at three locations in Mexico and one in the US. BC was measured continuously with an aethalometer, and these measurements were used in conjunction with particle dispersion simulations conducted with the iii Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Three-dimensional wind fields from Weather Research and Forecasting (WRF) model output, supplied by the Environmental Protection Agency (EPA), were used to drive HYSPLIT. Probabilistic analysis of HYSPLIT modeling results produced residence time analysis and concentration field analysis plots, which can be used to assess sources and transport of pollutants. BC concentrations at Parque Morelos, the campaign’s supersite, averaged 2.1 μg m and reached a maximum value of 55.9 μg m near midnight. This average value is comparable to that found in large American cities like Los Angeles and similarly sized Mexican cities like Mexicali. The maximum value is extremely high for anywhere in the world, and similar events were observed on other nights, too, possibly due to illicit industrial emissions. Strong diurnal patterns in BC were observed at all four sites, with peak hourly concentrations occurring in the morning rush hour around 8:00. BC was strongly correlated with carbon monoxide, a tracer for gasoline-powered vehicle emissions, at all three of the Mexican sites, where R values were 0.71-0.86, but was weakly correlated at the American site, where R was 0.13. Modeling results were assessed by both wind roses and residence time analyses and indicated that winds were typically westerly, due to onshore flow typical in this region, except at Parque Morelos where they were southerly. The consistency of the winds typically resulted in concentration field analyses that mirrored residence time analyses. Generally, BC in Tijuana appeared to originate within the city, as backward simulations showed transport from the US into Mexico at only one site. The majority of the backward and forward trajectory analyses indicated, however, that there was often transport from Tijuana into the US, sometimes directly from the south but often entering in a northeasterly direction east of San Diego-Tijuana and sometimes as far east as Imperial County at the eastern edge of California.