We have determined four hour average concentration obtained at 2:00, 8:00, 14:00, and 20:00 respectively in a day. Is it reasonable to calculate the daily mean value of SO2 using the average value of four hour average concentration?
I don´t think so, because if in the day you made the measures, was to windy? or there were not wind at all? It will depend also of the temperature of the day, and also depend of the season you make the measures.
For a sulphuric acid plant, you probably would do continuous SO2 monitoring.
For ambient conditions, time of day, wind velocity and direction can have an effect as mentioned by Marcelo. So it really depends on what you are monitoring
The time you have mentioned shows 06 hours gap but the duration you have mentioned is 04 hours. Apparently, you have kept two hours gap for each observation of four hours span in a day.
In fact, average value of 06 concentration values of 04 hours each is appropriate for daily average. This should be considered for ambient air, not for fugitive emission estimation in work zone.
This one process could eventually be used for Saturdays and Sundays where emissions are lower. As for the week days I find very little measurement time. In the minimum should be 2 at 2 hours
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