Measurement and Deduction of Emissions of Short-lived Atmospheric Organo-chlorine Compounds

Atmospheric studies of halogenated organics have centered on long lived halocarbons due to their effect on stratospheric ozone. Now that controls have been put in place to curb emissions of longer lived halocarbons through the Montreal Protocol, and speculation about the safety of many short-lived chlorinated organic molecules has been raised, there has been more consideration given to the efforts aimed at determining the levels of human exposure to all types of halogenated organics. Most previous studies of reactive chlorine compounds have focused solely on quantifying their ambient levels in urban and rural regions. However, for many of these organo-chorine molecules a detailed knowledge of emissions levels, transport, and final environmental disposition still does not exist. The present work was designed to aid in understanding the emissions patterns for several reactive halogenated organic compounds including trichloromethane (chloroform, CHCl3), trichloroethene (TCE, CHClCCl2), and tetrachloroethene (perchloroethylene, CCl2CCl2). A high temporal frequency (hourly) measurement campaign in Nahant, Massachusetts (approximately 10 km northeast of Boston) provided automated gas-chromatographic measurements for these species as well as the somewhat more stable 1,1,1 trichloroethane (methyl chloroform, CH3CCl3). Cryogenic preconcentrations, daily calibrations, and weekly linearity tests insure high precision (≤5%) measurements using electron capture detection. Calibration gases used for these tests, initially manufactured at MIT, have been corrected by intercomparison with gas standards used by the AGAGE program (produced at Scripps Institution of Oceanography) as well as those used at the National Center for Atmospheric Research. The absolute accuracy of our corrected MIT standard is estimated to be ≤10%. Over 12,000 measurements of the selected species were made between March, 1998 and January, 1999. These data show wide variability for the shortest lived species ranging from our detection limits (4.5 ppt for trichloroethene, 4.2 ppt for tetrachloroethene, and 7.8 ppt for trichloromethane) up to several hundred ppt during periods of local pollution.