Surface Ozone During the Second Half of the Nineteenth Century

During the last few years, increases in tropospheric ozone concentration have been detected and the need for more study has been recognized. There is very little knowledge about surface ozone background concentrations prior to the advent of widespread gasoline combustion. Therefore, the aim of this study was to uncover any useful information from the nineteenth century, and to explore the feasibility of converting it to obtain an approximate level of ozone concentration. After discovering ozone, Schönbein promulgated a simple method using iodized starch paper for qualitatively assessing its amount present in the air. This method was implemented at a few hundred sites, and although vulnerable to influence by humidity and oxidants in the air, continued to be used into the early years of this century. In the search for the best method of converting the Schönbein data, the most useful data sources turned out to be the 31-year series of regular quantitative ozone concentration measurements at Montsouris Observatory and, in particular, one year of simultaneous measurements by two methods carried out there. The nonlinear dependence of the coloration of Schönbein test-paper on the humidity was also considered. The corrections needed to convert the Schönbein data are discussed. The results indicate that during the last 30 years of the 19th century in the Great Lakes area of North America the average daily maximum of the surface ozone partial pressure was approximately 1.9 ± 0.2 mPa. European measurements between the 1850s and 1900s indicate little scattering outside of the range 1.7 to 2.3 mPa. All of these values are only about half of the mean of the daily maximum values of precise surface ozone measurements taken in the same geographical regions during the last 10–15 years. The annual cycle of surface ozone had an April–June maximum and an October–December minimum, and it was similar to the cycle now registered at stations generally free from local contaminations in midlatitudes. In general, the differences in the ozone partial pressure between old converted data and present-day measurements is greater for the summer than for the winter months. A statistically significant tendency for a surface ozone increase is noted in the data for Montsouris; however, at some other stations the trend would not be statistically significant. Further verification of the conversion technique may allow information on the background surface ozone concentration to be expanded.