Urban Measurements of Monohalogenated Alkanes in Southern California

Surface air samples were analyzed for chloromethane (CH₃Cl), bromomethane (CH₃Br), chloroethane (C₂H₅Cl), bromoethane (C₂H₅Br), and n-bromopropane (CH₂BrC₂H₅) after being collected from Los Angeles County starting in early 1999 and Riverside County starting in mid-1999. These urban measurements provide a comparison to measurements of the halogenated methanes and ethanes observed in unpolluted background tropospheric air along the California coast. Also, the study will show the change of these trace gas concentrations as air masses move inland. Moreover, they provide the first data set of atmospheric n-bromopropane, also known as n-propyl bromide. N-bromopropane is currently replacing chlorofluorocarbons (CFC's) and hydrochlorofluorocarbons (HCFC's) in certain applications.

Samples were taken at regular weekly intervals. A Silco-Steel ^TM manifold system using a portable pump pressurized the samples into electropolished, stainless steel canisters. Within two days of collection, samples were analyzed by GC/MS in single ion mode. A combination of retention time, mass to charge ratios, and the known natural isotopic abundances were used for identification. Additionally, a reduction gas analyzer determined the mixing ratios of carbon monoxide, a tracer indicative of combustion and urban air. The carbon monoxide concentrations were then correlated to the trace gas concentrations measured. Lastly, a simple trajectory model will be used to show the likely paths air masses from the coast will follow.