A UV reference photometer for atmospheric trace gases : ozone and VOCs
Accurate determination of absolute absorption cross sections for molecules of atmospheric interest is crucial for retrieving molecular concentrations from remote sensing measurements. The value of the absolute absorption cross section of ozone at 253.65 nm currently recommended as a reference for ozone concentration measurements has 2 % accuracy and presents possible discrepancies with regard to more recent studies. Therefore, we have designed a UV photometer operating at 253.65 nm in order to obtain absorption cross sections with accuracy better than 1 %. The photometer that we constructed during the thesis was tested on styrene, which is an aromatic volatile organic compound (VOC). VOCs as well as ozone are tropospheric pollutants with strong absorptions in the UV. The measurement of the absorption cross section of styrene is not only an ideal performance test; it also provides an important improvement over the previous value that suffers from an uncertainty in the several 10 % range. The strong absorptions of styrene and ozone require measurements to be performed at low pressures (<1 Torr) where non-linearities in pressure measurements can arise. In order to correct for these non-linear effects due to heating of the capacitive pressure gauges, the thermal transpiration of styrene has been determined. We therefore realized the first measurement of thermal transpiration for an aromatic molecule. Owing to a rigorous examination of uncertainties and corrections, we have shown that the accuracy of the photometer was below 1 %. The absorption cross section of styrene was measured to be 1.521E-17 cm^2 with an uncertainty of 0.6 % at the 95 % level of confidence.