Abstract
Abstract. Ozone soundings from nine Nordic stations have been homogenized and interpolated to standard pressure levels. The different stations have very different data coverage; the longest period with data is from the end of the 1980s to 2014. At each pressure level the homogenized ozone time series have been analysed with a model that includes both low-frequency variability in the form of a polynomial, an annual cycle with harmonics, the possibility for low-frequency variability in the annual amplitude and phasing, and either white noise or noise given by a first-order autoregressive process. The fitting of the parameters is performed with a Bayesian approach not only giving the mean values but also confidence intervals. The results show that all stations agree on a well-defined annual cycle in the free troposphere with a relatively confined maximum in the early summer. Regarding the low-frequency variability, it is found that Scoresbysund, Ny Ålesund, Sodankylä, Eureka, and Ørland show similar, significant signals with a maximum near 2005 followed by a decrease. This change is characteristic for all pressure levels in the free troposphere. A significant change in the annual cycle was found for Ny Ålesund, Scoresbysund, and Sodankylä. The changes at these stations are in agreement with the interpretation that the early summer maximum is appearing earlier in the year. The results are shown to be robust to the different settings of the model parameters such as the order of the polynomial, number of harmonics in the annual cycle, and the type of noise.
Highlights
Tropospheric ozone is a short-lived trace gas with a lifetime of 3–4 weeks on average and a following strong temporal and spatial variability
The results show that all stations agree on a well-defined annual cycle in the free troposphere with a relatively confined maximum in the early summer
While the photo-chemical processes dominate globally, model studies (Wespes et al, 2012) indicate that in the Arctic anthropogenic pollution from the Northern Hemisphere is the dominant source of ozone from the surface to 400 hPa and that the stratospheric influence is the main contribution at pressures less 400 hPa
Summary
Tropospheric ozone is a short-lived trace gas with a lifetime of 3–4 weeks on average and a following strong temporal and spatial variability. In the free troposphere they found a statistically significant increase of 11 % in this period with largest values in January to April, the period of greatest inter-annual variability They attributed the observed change to the combined increase in the stratosphere– troposphere exchange and the transport of precursors towards the higher latitudes. The present study includes recent ozonesonde measurements obtained in the period from the early 2000s to 2014, which have not been analysed in details before This results in a 27-year data set for the longest record. The Bayesian approach gives us mean values and uncertainties of the parameters and on derived quantities such as temporal differences and annual cycles This approach naturally handles strongly irregular sampled time series including extended periods without data and is favourable for the analysis of ozone time series
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