Uncertainties in surface ozone trend at hohenpeissenberg

It is well-known that early surface ozone measurements using the iodometric method suffered from both negative and positive interferences by reducing and oxidizing gases, notably SO₂ and NO₂, respectively. These interferences could be serious in areas close to emission sources. Using the important Hohenpeissenberg Observatory surface ozone data series as an example, we demonstrate that the uncertainty due to SO₂ interference during the pre-1976 period could have a significant effect on the apparent upward trend, and possibly lead to its overestimation by a factor of 3. There is evidence to show that the NO₂ interference at the Observatory may not be entirely insignificant under certain meteorological conditions. The effect of the uncertainty due to this interperence on the long-term surface ozone trend also needs to be evaluated. In view of the importance of the conclusions concerning increasing surface ozone concentrations, which have stemmed from previous analyses of the Hohenpeissenberg Observatory data set, it is essential to discuss these uncertainties in the open literature so that a consensus of opinion may be formed on the data quality of the early years' observations.