Changes in mixing ratio and isotopic composition of CO2 in urban air from the Los Angeles basin, California, between 1972 and 2003

Atmospheric CO2 mixing ratios and C and O isotopic compositions are reported for the Los Angeles basin in southern California, a region renowned for its air pollution. Air samples collected midday on the Caltech campus in Pasadena, California, contained similar to 30 ppm more CO2 in 1998-2003 than in 1972-1973 (averaging 397 ppm in 19982003 and 366 ppm in 1972-1973) compared to a 47 ppm change in background air CO2, yet the ranges of the carbon and oxygen isotopic compositions remained essentially constant. Because the 1998-2003 data show a significant progression through time, analysis was done on data from 2002 to 2003 complete calendar years (CO2 mixing ratios increased 41 ppm between 1972 and 1973 and 2002-2003). Both 1972-1973 and 2002-2003 data sets display significant correlation between delta C-13 and 1/[CO2] with local CO2 source end-member delta C-13 values of -30.9 +/- 0.5 parts per thousand for 1972-1973 and -29.9 +/- 0.2 parts per thousand for 2002-2003 (1 sigma errors). Mass balance calculations explain that this apparently coincidental similarity reflects a change in the relative proportion of natural gas and petroleum products burned in the region combined with a change in the origin, and thus isotopic composition, of the petroleum burned. The delta C-13 of the average CO2 inventory in Pasadena can be explained by local addition to background air of 38 +/- 4 ppm CO2 in 1972-1973 and 29 +/- 3 ppm in 2002-2003 from anthropogenic sources, in seeming contradiction to the known increase in CO2 emissions between these two time periods.