We present a time-dependent photochemical model of tropospheric ozone. In accordance with our previous calculations (Chameides and Walker, 1973) we treat ozone as an active chemical constituent with a local abundance that is determined by photochemical processes rather than by transport processes. The local abundance of odd nitrogen is shown to play a major role in determining the intensity of photochemical production and destruction of ozone, a large local abundance of NOx(=NO+NO2) leading to large diurnal ozone variations. The photochemical model is shown to reproduce the diurnal variations of ozone and nitrogen dioxide observed near the ground at Research Triangle Park in North Carolina.