Abstract
Abstract. Ozone plays a crucial role in tropospheric chemistry, is the third largest contributor to greenhouse radiative forcing after carbon dioxide and methane and also a toxic air pollutant affecting human health and agriculture. Long-term measurements of tropospheric ozone have been performed globally for more than 30 years with UV photometers, all relying on the absorption of ozone at the 253.65 nm line of mercury. We have re-determined this cross-section and report a value of 11.27 x 10−18 cm2 molecule−1 with an expanded relative uncertainty of 0.86% (coverage factor k= 2). This is lower than the conventional value currently in use and measured by Hearn (1961) with a relative difference of 1.8%, with the consequence that historically reported ozone concentrations should be increased by 1.8%. In order to perform the new measurements of cross-sections with reduced uncertainties, a system was set up to generate pure ozone in the gas phase together with an optical system based on a UV laser with lines in the Hartley band, including accurate path length measurement of the absorption cell and a careful evaluation of possible impurities in the ozone sample by mass spectrometry and Fourier transform infrared spectroscopy. This resulted in new measurements of absolute values of ozone absorption cross-sections of 9.48 x 10−18, 10.44 x 10−18 and 11.07 x 10−18 cm2 molecule−1, with relative expanded uncertainties better than 0.7%, for the wavelengths (in vacuum) of 244.06, 248.32, and 257.34 nm respectively. The cross-section at the 253.65 nm line of mercury was determined by comparisons using a Standard Reference Photometer equipped with a mercury lamp as the light source. The newly reported value should be used in the future to obtain the most accurate measurements of ozone concentration, which are in closer agreement with non-UV-photometry based methods such as the gas phase titration of ozone with nitrogen monoxide.
Highlights
Introduction and aimsThe property of ozone to strongly absorb UV radiation, notably in the Hartley Band, and the relative ease of reproducing a mercury line at 253.65 nm has led to the value of the ozone absorption cross-section at this wavelength becoming important for global ozone atmospheric monitoring
Absolute measurements of the ozone cross-section at the same wavelengths have been determined with the smallest uncertainties published to date. This required a cryogenic ozone generator to be developed, together with a method based on cycles of evaporation–condensation of ozone so as to better evaluate the purity of gaseous samples on which UV absorption measurements were performed
A series of 20 repeat measurements resulted in an average path length of 48.33 mm, which was consistent with measurements of the cell length using a coordinate machine, which gave a value of 48.35 mm
Summary
The property of ozone to strongly absorb UV radiation, notably in the Hartley Band, and the relative ease of reproducing a mercury line at 253.65 nm (in air) has led to the value of the ozone absorption cross-section at this wavelength becoming important for global ozone atmospheric monitoring. The BIPM first developed a laser ozone photometer, capable of measuring ozone concentrations in the same range as the SRP but with improved accuracy, as described in Petersen et al (2012) This instrument was used to deduce new values of the ozone crosssection in the Hartley band, relative to the reference value obtained by Hearn at 253.65 nm. Absolute measurements of the ozone cross-section at the same wavelengths have been determined with the smallest uncertainties published to date This required a cryogenic ozone generator to be developed, together with a method based on cycles of evaporation–condensation of ozone so as to better evaluate the purity of gaseous samples on which UV absorption measurements were performed. This work is in agreement with recent measurement results that indicate that the historical conventional cross-section value used for surface ozone measurements is biased too high, resulting in an underestimation of ozone concentrations, which can be corrected by use of the value published in this work
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