Impact of sampling frequency in the analysis of tropospheric ozone observations

Abstract

Abstract. Measurements of ozone vertical profiles are valuable for the evaluation of atmospheric chemistry models and contribute to the understanding of the processes controlling the distribution of tropospheric ozone. The longest record of ozone vertical profiles is provided by ozone sondes, which have a typical frequency of 4 to 12 profiles a month. Here we quantify the uncertainty introduced by low frequency sampling in the determination of means and trends. To do this, the high frequency MOZAIC (Measurements of OZone, water vapor, carbon monoxide and nitrogen oxides by inservice AIrbus airCraft) profiles over airports, such as Frankfurt, have been subsampled at two typical ozone sonde frequencies of 4 and 12 profiles per month. We found the 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 4 profile-a-month frequency. However the uncertainty can reach up to 15 and 29 % at the lowest altitude levels. As a consequence, the sampling uncertainty at the lowest frequency could be higher than the typical 10 % accuracy of the ozone sondes and should be carefully considered for observation comparison and model evaluation. We found that the 95 % confidence limit on the seasonal mean derived from the subsample created is similar to the sampling uncertainty and suggest to use it as an estimate of the sampling uncertainty. Similar results are found at six other Northern Hemisphere sites. We show that the sampling substantially impacts on the inter-annual variability and the trend derived over the period 1998-2008 both in magnitude and in sign throughout the troposphere. Also, a tropical case is discussed using the MOZAIC profiles taken over Windhoek, Namibia between 2005 and 2008. For this site, we found that the sampling uncertainty in the free troposphere is around 8 and 12 % at 12 and 4 profiles a month respectively.

U.S. CH4 emissions from oil and gas production: Have recent large increases been detected?

“…Atmospheric variability ultimately plays a role in all of the detection issues we have discussed: the need for high sampling frequency, the difficulty of defining a background, and accounting for short‐term transport trends. The effect of atmospheric variability on trend detection has also been noted by other studies for ozone and carbon monoxide [ Saunois et al ., ; Strode and Pawson , ; Weatherhead et al ., , ]. Atmospheric modeling can be helpful because it can in principle account for transport variability.…”

The MACC reanalysis: an 8 yr data set of atmospheric composition

“…The differences seen in the biases with respect to ozonesondes and MOZAIC data are consistent with findings by Tilmes et al (2012), who showed that ozonesondes measure higher concentrations than MOZAIC data in the free troposphere over western Europe and North and South America, but agree to within the given error bars. Saunois et al (2012) investigated uncertainties as a result of different sampling frequencies and found these to be of the order 7-14 % in the free troposphere and larger above and below. These sampling frequency uncertainties have to be considered when comparing ozonesonde and MOZAIC data, which have very different measurement frequencies, and they are large enough to explain the differences between the ozonesonde and MOZAIC biases seen in Fig.…”

Global distribution and trends of tropospheric ozone: An observation-based review

“…This extensive database has been utilized by over 200 peer-reviewed publications (see list at http://www.iagos.fr/web/rubrique4.html) and provides detailed ozone climatological and trend information across much of the NH (Thouret et al, 2006;Logan et al, 2012;Zbinden et al, 2013). In the coming years, expansion of the program will allow the calculation of ozone trends in the boundary layer and free troposphere at many more locations around the world (Saunois et al, 2012). Another monitoring program that is providing long-term information on free-tropospheric ozone is The Southern Hemisphere Additional Ozonesonde Network (SHADOZ, http://croc.gsfc.nasa.gov/shadoz) (Thompson et al, 2003(Thompson et al, , 2012.…”

“…Hence, continuous hourly data records are ideal, since they can provide averages at any needed temporal resolution, but they are generally limited to surface observations. A regular program of ozonesonde or aircraft measurements can provide seasonal or annual averages, but they must be relatively frequent to effectively average over synoptic scale variability; Saunois et al, (2012) investigate the required frequency. Such a program must also consider time of day of measurements to avoid unwanted contributions to ozone variability from repetitive diurnal changes.…”