These altitudes, just below the tropopause , should usually be ozone rich because of its proximity to the stratosphere. In spring, the maxima occurs predominantly at the 3-4 km altitude, above the boundary layer. Possibly due to the stronger convection, summer finds the ozone maxima occurring higher up, at about 4-7 km. In autumn the maxima are located most frequently between 1-3 km. These near surface maxima probably have their origins in local pollution.
Winter again sees the maxima occurring most often in the boundary layer.
The variation with season demonstrates the influence of meteorology on where in the troposphere the maxima would occur. In terms of the concentration, the majority of the ozone maxima has a concentration that is in the 50 to 60 ppbv range, but concentrations as high as 130-140 ppbv are also found. The frequency of occurrence decreases monotonically with concentration. Spring in Hong Kong is the season in which the largest concentration, 130-140 ppbv, is found. Summer, on the other hand, finds no maxima above 70-80 ppbv.