Accurate measurements of ozone absorption cross-sections in the Hartley band

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 × 10 −18 cm 2 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 9.48×10 −18 , 10.44×10 −18 and 11.07 × 10 −18 cm 2 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.

Updated ozone absorption cross section will reduce air quality compliance

“…Most cross sections are lower than that reported by Hearn (1961). The Viallon et al (2015) cross section is the lowest of all of the reported values. This 1.8 % change in the ozone absorption cross section appears to be modest.…”

“…Recent re-measurement of the absorption cross sections by Viallon et al (2015) find a cross section of (11.27 ± 0.097) × 10 −18 cm 2 molecule −1 (mean ±2σ ) that is 1.8 % lower than the Hearn (1961) evaluation. If the Viallon et al (2015) absorption cross section is officially adopted, this will imply that the ozone observations in ambient air are systematically 1.8 % higher than previously reported. Figure 1 illustrates all available ozone absorption cross sections at 253.65 nm and their uncertainties based on a compilation by Orphal (2002).…”

“…Figure 1 illustrates all available ozone absorption cross sections at 253.65 nm and their uncertainties based on a compilation by Orphal (2002). The absorption cross sections measured by Hearn (1961) (maroon) and Viallon et al (2015) (grey) are highlighted with thick lines. Most cross sections are lower than that reported by Hearn (1961).…”

“…Photometric ozone measurements rely upon an accurate value of the ozone absorption cross section at 253.65 nm. This has recently been re-evaluated by Viallon et al (2015) as 1.8 % smaller than the accepted value (Hearn, 1961) used for the preceding 50 years. Thus, ozone measurements that applied the older cross section systematically underestimate the amount of ozone in air.…”

Fundamental Review to Ozone Gas Sensing Using Optical Fibre Sensors

“…Redefinition of the value of ozone gas absorption cross section in the UV for accurate measurements of ozone gas [90] and preservation of linearity of BearLamberts law by measuring ozone gas concentration at an alternate sampling wavelength of 279.95 nm [85,91]. Ozone gas measurement in the visible spectrum using LED as a light source at 605 nm [92] and sensitivity enhancement through light propagation at incident angle [93].…”