Calibration of atmospheric trace gas detectors using a purpose built mixing unit

Abstract

Two consecutive months of ozone, carbon monoxide and sulphur dioxide measurements together with meteorological parameters have been carried out at the background GAW station located at Giordan lighthouse (36° 4' 22" N, 14° 13' 911 E) on the Northwestern tip of Gozo. A two channel mixing unit was built to carry out the calibration of the carbon monoxide and sulphur dioxide analysers. The whole systematic calibration set-up is controlled automatically by a program that is stored in a data logger. The data from the data logger is downloaded on to a computer hard drive located at Xewkija station where a normal Haynes modem controls the collection of data. From evaluation of the data it was observed that ozone has a clear diurnal variation with a maximum in the mid-afternoon. Higher ozone concentrations were recorded when the wind was blowing from the Northern hemisphere. Ozone mixing ratio showed a poor correlation with relative humidity at any time of the day. Since the CO and S02 are both primary pollutants, these atmospheric trace gases have a good correlation at any time, independent of the wind direction and with a diurnal maximum early in the morning. When the prevailing wind was from the South East (between 130° and 160°) higher CO and S02 mixing ratios were recorded because air parcels arriving from Malta carry anthropogenic emissions towards Giordan lighthouse. Also, from this particular wind direction ozone showed a clear anti correlation with these two primary trace gases. This phenomenon is related to the short distance between the two islands because there is not enough time for the photochemical process to take place.