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Los Angeles Basin

It is well known that the concentration of CO2 in the atmosphere has been increasing over the past 150 years due to the introduction of by-products of human industrial and agricultural activities. Accompanying this increase in CO2 concentration has been a shift toward lower 13C/12C ratios, reflecting the biological sources of the carbon (such as the burning of fossil fuels and rain forests), which favor the light isotope. Given the importance of these changes for the earth's climate, considerable effort has been and continues to be directed toward characterization of the spatial and temporal variations in the concentration and isotopic ratios of CO2 in air, with particular attention to sites far from the sources of pollution that drive these changes.

However, in contrast to the substantial and systematic efforts devoted toward monitoring and characterizing the global distribution of CO2, there have been fewer studies of atmospheric CO2 in urban areas. These environments are of interest because they are significant sources of anthropogenic CO2 in the global system, and they represent ecosystems of interest and importance to the human populations that live in them.

Over the past seven years, Sally Newman, Sam Epstein (now deceased), and I have been measuring the concentration and isotopic composition of CO2 and H2O in air collected on the Caltech campus, supplemented by other collections around southern California. Situated in the Los Angeles basin, a major urban center well known for its problems with air quality over many decades, these measurements provide information on the magnitude of variations in atmospheric CO2 and the factors leading to these variations. Our data show excellent correlations between concentration and δ13C, and comparison to unpublished measurements by Sam Epstein from 1972-1973 confirm that the area has had an increase in average CO2 concentration of ~43 ppm and that the heaviest end member has become lighter by ~0.5‰ over the past 30 years. Both of these observations reflect the increased amount of fossil fuel combustion products that have been added to the atmosphere during this time. Unlike the seasonal variations in CO2 concentrations that have been observed at sites at our latitude dominated only by oceanic influences, the data for the Caltech campus shows no seasonal variation due to vegetation because the pollution effects overwhelm it.

The observed variations in concentration and isotopic ratios of CO2 in Pasadena air are well explained by a simple mixture of clean air and a CO2-rich pollutant end member. We have analyzed several candidates for the "pollutant" end member, including automobile exhaust, exhaust from burning natural gas, and human breath exhalation. The trend for the air samples in Pasadena mainly corresponds with automobile exhaust. The pollutant end member has changed very little over the past 30 years, due to changes in the provenance of oil consumed in this region (increased consumption of light oil from the North Slope of Alaska) that compensated for the change in proportions of fuel types burned.

Keeling plot — δ13C (‰) versus 1/CO2 (ppm-1)

a. 2002-2003 and 1972-1973 Pasadena data sets, showing the change in CO2 concentration and isotopic composition through time compared with 25-40°N NOAA-CMDL oceanic sites and Keeling et al. (1979) northern hemisphere mean compositions for 1956 and 1978. Also shown are data from southern Californian clean air sites – Catalina, Mt. Wilson, Mojave Desert, Sequoia National Park, and overlooking the beach on Palos Verdes Peninsula.
b. The same Pasadena data sets plus data for possible pollutants.