Chlorine atoms are generated in the marine boundary layer, including coastal urban areas, through heterogeneous reactions of sea-salt particles. However, there are no measurements of tropospheric chlorine atoms, and few measurements of their precursors due to the difficulty in specifically identifying and measuring parts per trillion levels of these highly reactive species. We report here studies which suggest an alternate approach for elucidating the role of chlorine atoms in the chemistry of coastal urban areas: the identification and measurement of unique chlorine-containing products from the reactions of Cl with organics, in this case 1,3-butadiene, in air. The reaction of chlorine atoms with 1,3-butadiene (19-61 ppm), which is emitted by motor vehicles, was studied at 1 atm pressure in air at room temperature in both the absence and presence of NO (0--20 ppm). Cl2 (4--16 ppm) was photolyzed to generate chlorine atoms, and the products were identified and measured using gas chromatography-mass spectrometery (GC-MS) and long path (40 m) Fourier transform infrared spectroscopy (FTIR). We identified 4-chlorocrotonaldehyde (CCA) as a unique chlorine-containing product from the Cl atom reaction with 1,3-butadiene in both the presence and absence of NO. The yield of CCA was (29¿7)%(2&sgr;) in the absence of NO and (65¿20)%(2&sgr;) in its presence. The yield of chloromethyl vinyl ketone (CMVK), another chlorine-containing compound from this reaction, was measured to be (19¿4)%(2&sgr;) in the absence of NO; it was not observed in its presence. The reaction system was modeled using ACUCHEM to clarify the reaction paths and to allow extrapolation to ambient air conditions. These studies predict that CCA will be formed in coastal urban areas in the early morning hours at concentrations of ~100 ppt, which should be readily detectable by GC-MS and GC with electron capture detection (ECD). Some studies were also carried out on the isoprene reaction, and some potential unique products of its reaction with chlorine atoms are suggested as candidates for field investigations. ¿ 2001 American Geophysical Union