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Measurements of ozone vertical profiles are valuable for the evaluation ofatmospheric chemistry models and contribute to the understanding of theprocesses controlling the distribution of tropospheric ozone. The longestrecord of ozone vertical profiles is provided by ozone sondes, which have atypical frequency of 4 to 12 profiles a month. Here we quantify theuncertainty introduced by low frequency sampling in the determination ofmeans and trends. To do this, the high frequency MOZAIC (Measurements ofOZone, water vapor, carbon monoxide and nitrogen oxides by in-service AIrbusairCraft) profiles over airports, such as Frankfurt, have been subsampled attwo typical ozone sonde frequencies of 4 and 12 profiles per month. We foundthe lowest sampling uncertainty on seasonal means at 700 hPa over Frankfurt,with around 5% for a frequency of 12 profiles per month and 10% for a 4profile-a-month frequency. However the uncertainty can reach up to 15 and29% at the lowest altitude levels. As a consequence, the sampling uncertaintyat the lowest frequency could be higher than the typical 10% accuracy ofthe ozone sondes and should be carefully considered for observationcomparison and model evaluation. We found that the 95% confidence limit onthe seasonal mean derived from the subsample created is similar to thesampling uncertainty and suggest to use it as an estimate of the samplinguncertainty. Similar results are found at six other Northern Hemispheresites. We show that the sampling substantially impacts on the inter-annualvariability and the trend derived over the period 1998–2008 both inmagnitude and in sign throughout the troposphere. Also, a tropical case isdiscussed using the MOZAIC profiles taken over Windhoek, Namibia between 2005and 2008. For this site, we found that the sampling uncertainty in the freetroposphere is around 8 and 12% at 12 and 4 profiles a month respectively

Impact of sampling frequency in the analysis of tropospheric ozone observations