One of the interesting findings was on its height wise variation. The study found that ozone concentration increased with height. This was further supported by the spatial variation data, where two background sites (with usual low PM concentration) encountered relatively high ozone concentration. These two sites were at height than other sites.

Various international studies have indicated a high ozone concentration in Nepal, which is expected to rise in future also. Increasing number of vehicles in Kathmandu that has built up high concentration of Nitrogen Oxides (NOx) in air is favoring ozone formation.

This study, which was carried out using the Eco-badge test card (passive sampler) and Zikua card reader, has found ozone concentration on a similar pattern of spatial variation as in other air pollutants such as PM 10. Highest concentration of 27 parts per billion (ppb) for three days average was found at Putalisadak and lowest of 17 ppb at Matsyagau. The diurnal pattern of ozone concentration shows that higher concentration of ozone occurred during mid-days, 12:00 to 14:00 hrs.

The ozone concentration and temperature also shows some relation and correlation coefficient between them was found to be 0.61. Relationship of ozone concentration and temperature was studied to get idea on effect of temperature on ozone formation.

Study of vehicle number and ozone revealed that high ozone concentration was found during the vehicle rush hour. The ozone peak occurred after the vehicle peak as expected and correlation coefficient between ozone concentration and number of vehicles plying on the road found to be 0.938. Although there was sudden increase in vehicle numbers at 4:00 PM, the constant decrease in ozone concentration could be due to the reduced solar radiation level, which was not monitored in this study.

The 8-hour ozone monitoring at two different heights; 6.5 m and 2 m revealed that higher ozone concentration was found at higher level than the lower level. On an average, 8-hour ozone concentration was found to be 26.33 ppb at 6.5 m and 21 ppb at 2 m during three days of monitoring.

Diurnal and spatial variation of ozone is a manifestation of different conditions, particularly solar radiation, wind speed & direction, atmospheric stability, amount of precursor in the atmosphere represented by its sources. This study on ground level ozone in the Kathmandu valley indicated that Kathmandu valley is not currently under threat of high ozone concentration in terms of health impacts, using the World Health Organization (WHO) guideline value of ground level ozone of 50 ppb for 8-hour.

Although the concentration of ground level ozone was not found at the level to be harmful to the human health during the study period, there is high possibility of harmful level ozone concentration during summer time due to high solar radiation.

Government of Nepal, therefore, should take ground level ozone as a serious air pollutant and include in the current monitoring system as well as should formulate guideline value of 8-hour average ozone concentration.