The nitrate radical, NO3, has been identified and measured for the first time in the polluted troposphere using long path (1970 and 750 m) differential optical absorption spectroscopy at two sites in the Los Angeles basin. NO3 concentrations of up to 355 ppt were measured using the strong NO3 absorption bands at 623 and 662 nm. During pollution episodes from September 11 to September 19, 1979 concentrations increased sharply after sunset and peaked about one hour later at ~20:00 (PDT). IN many other cases peak concentrations were much lower and sometimes below the detection limit of several ppt. Possible sinks for the NO3 radical under polluted conditions are considered, including reaction with NO, reaction with organic species, and the hydrolysis of N2O5 for which a new upper limit rate constant is derived.