The time dependence of the NO3 and HONO profiles observed by Platt et al. (1980a,b) in the nighttime urban atmosphere can be simulated by computer methods. With the assumption that the reaction, HO2+NO2→HONO+O2 (21) occurs at the maximum value of the rate constant observed by Howard (1977), k21≤3¿10-15 cm3 molec-1 s-1, the time dependence of the 3>, 3>, and 2> data can be fitted reasonably well. According to this hypothesis the HONO and NO3 chemistry are linked in that major nighttime sources of HO2 radicals involved in (21) are derived from two of the several decay reactions of the NO3 species-reactions with formaldehyde and acetaldehyde.