Halocarbon emissions estimated from Advanced Global Atmospheric Gases Experiment measured pollution events at Trinidad Head, California

The emissions of halogenated gases from the West Coast region of the United States are estimated from measurements from 1995 to 2003 at the Advanced Global Atmospheric Gases Experiment site at Trinidad Head, California. The emissions estimation procedure uses pollution events combined with population densities integrated along back trajectories, and the estimates are constrained by independent estimates of CH4 and N2O emissions from the U. S. West Coast region. The best fit, average emissions of CH4 and N2O and the average chloroform emissions in California, Oregon, and Washington combined from 1996 to 2002 are 44, 3.7, and 0.07 kg person(-1) yr(-1), respectively. The emissions per person of CFC-11 (CCl3F), CFC-2 (CCl2F2), CFC-113 (CCl2FCClF2), and methyl chloroform (CH3CCl3) from California in 1996-1998 are calculated to be factors of approximately 2.2, 1.3, 0.7, and 1.6, respectively, less (more for CFC-113) than those reported for the northeastern United States by Barnes et al. (2003). The emission per person of all these gases in the U. S. West Coast region decreased from 1998 to 1999 by a factor of 2 or more, but from 1999 to 2002 the estimated emissions of all four gases have remained fairly constant and are 0.016, 0.048, 0.002, and 0.006 kg person(-1) yr(-1), respectively. The methyl chloroform estimates suggest a delay of up to 1 year in the decline of the emissions from 1996 to 1998, but otherwise, and in 1999-2000, in contrast to the Millet and Goldstein (2004) results, they are in agreement with the average methyl chloroform emissions per person for the United States based on the UNEP country by country consumption figures (A. McCulloch, private communication, 2004). Averaging the Trinidad Head and the Barnes et al. (2003) per person estimates and multiplying by the U. S. population suggests average methyl chloroform emissions in the United States of 18 Gg yr(-1) in 1996 to 1998. In 2001-2002, if the ratio of the emissions per person in these two regions was the same as in 1996-1998, we estimate U. S. emissions of 2.2 Gg yr(-1), which is one half of the Millet and Goldstein (2004) estimate.