Chlorine atoms may play an important role in the tropospheric chemistry of coastal areas and the Arctic at polar sunrise. However, the sources of chlorine are not well established and measuring its precursors in a specific manner is difficult. A potential approach to probe Cl production in the troposphere is to identify and measure chlorine-containing products unique to the reactions of Cl with organics in air, e.g. 1,3-butadiene, which is emitted from motor vehicles. We present here product studies of the reaction of Cl with 1,3-butadiene in the presence and absence of NO in 1 atm air at room temperature using FTIR, API-MS and GC-MS. 4-Chlorocrotonaldehyde (CCA) is identified as a unique chlorine-containing product from this reaction, with a yield of (28¿8)%(2&sgr;) independent of the presence of NO. CCA may therefore be useful in field studies as a marker for Cl chemistry in urban coastal areas. ¿ 2000 American Geophysical Union