Kinetic studies have been made of the CH3CHO-NO3 reaction in dilute mixtures of CH3CHO,NO2, and N2O5 in synthetic air at 700 torr and room temperature. During the course of the reaction, reactant and product analyses were made in situ, using long path length Fourier transform infrared spectroscopy. The major products of the reaction for these conditions are HONO2 and CH3COO2NO2. All evidence suggests that the first step in the reaction sequence, NO3+CH3CHO→HONO2+CH3CO, proceeds with a rate constant, k=(2.1¿0.4)¿10-15 cm3 molecule-1 s-1 at 299¿1 K. These results, combined with other reaction rate data pertinent to tropospheric chemistry, are used to computer simulate the homogeneous chemistry of the polluted troposphere and to evaluate theoretically the nature of the nighttime chemistry.