Abstract
Abstract. Estimates of the rate of production of excited oxygen atoms due to the photolysis of ozone (J(O1D)) have been derived from radiation measurements carried out at Cape Grim, Tasmania (40.6° S, 144.7° E). The individual measurements have a total uncertainty of 16 % (1σ). These estimates agree well with model estimates of clear-sky photolysis rates. Observations spanning 2000–2005 have been used to quantify the impact of season, clouds and ozone column amount. The annual cycle of J(O1D) has been investigated via monthly means. These means show an interannual variation (monthly standard deviation) of 9 %, but in midsummer and midwinter this reduces to 3–5 %. Variations in solar zenith angle and total column ozone explain 86 % of the observed variability in the measured photolysis rates. The impact of total column ozone, expressed as a radiation amplification factor (RAF), is found to be ~ 1.53, in agreement with model estimates. This ozone dependence explains 20 % of the variation observed at medium solar zenith angles (30–50°). The impact of clouds results in a median reduction of 30 % in J(O1D) for the same solar zenith angle range. Including estimates of cloudiness derived from long-wave radiation measurements resulted in a statistically significant fit to observations, but the quality of the fit did not increase significantly as measured by the adjusted R2.
Highlights
It is widely recognised that the chemistry of the clean troposphere is driven by a few key oxidising species, with a major contributor being the hydroxyl radical (OH) (Crutzen, 1974)
As the Cape Grim UV data set includes both the diffuse and global irradiance, Eq (5) can be used, as the direct beam irradiance can be derived from the difference between the global and diffuse component
Several days exist where the irradiance appears to vary smoothly but with differences of up to 10 % at solar noon. This could be due in part to the limited measurement range (Sect. 3.2), a calibration issue that occurs at these solar zenith angles, aerosol, ozone column estimate errors or due to clouds
Summary
It is widely recognised that the chemistry of the clean troposphere is driven by a few key oxidising species, with a major contributor being the hydroxyl radical (OH) (Crutzen, 1974). It has long been realised that changes in the amount of OH in the atmosphere could have a profound effect on global air quality, and there has been a long-term effort to develop techniques to measure the key chemical species (Heard and Pilling, 2003). There are several techniques in use for such measurements, including fluorescence, UV absorption and mass spectrometry (Heard and Pilling, 2003), long-term measurement sets are rare (Rohrer and Berresheim, 2006; Berresheim et al, 2013)
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