AbstractSeveral gaseous pollutants show marked diurnal variations, with daily maxima and nightly minima of concentration. The nocturnal decrease is held to be due to deposition at the ground, with isolation of the lowest layers of the atmosphere from the air above by the nocturnal inversion. In the morning, deep mixing is re‐established and brings material down to the surface from higher layers. Ozone exhibits this behaviour clearly and is convenient to study as an example, but it has been suggested by some authors that changes in the rates of gas‐phase reactions that produce and destroy ozone are responsible for the daily cycle of concentration. We have studied the destruction of ozone at the surface and conclude that this process can probably explain the nocturnal decline of concentration near the ground.Deposition rates for ozone were deduced from profiles of concentration, windspeed and temperature measured over grassland in southern England. The mean deposition velocity was 0.58cms−1by day and 0.29cms−1by night. The aerodynamic and surface components of resistance both increased at night, but the behaviour of the surface resistance was not consistent with stomatal control.Observations of the nocturnal decrease of ozone concentration at the same site are reported. A computer simulation of the behaviour of ozone, with a description of the nocturnal boundary layer due to Wyngaard, duplicated the major features observed, and a procedure for predicting the height of the mixed layer and the rate of decline of ozone concentration at the ground gave good agreement with observations on many occasions.The nocturnal behaviour has important consequences for the measurement of mean concentrations in the atmosphere, and for long‐range transport. Observation of the daily cycle of concentration may indicate the likely importance of the surface sink for m