Multi-Decadal Surface Ozone Trends at Globally Distributed Remote Locations

Tropospheric ozone is extremely difficult to monitor on the global scale due to its highly variable distribution and interannual variability, and the ongoing shift of ozone precursor emissions from high latitudes to low latitudes. This paper provides a necessary update on surface ozone trends at 27 globally distributed remote locations (20 in the Northern Hemisphere, 7 in the Southern Hemisphere), focusing on continuous ozone time series that extend from the present back to at least 1995. Trends are based on monthly mean ozone anomalies, and importantly, all sites have at least 20 years of data, which improves the likelihood that a robust trend value is due to changes in ozone precursor emissions and/or forced climate change rather than naturally occurring climate variability. Since 1995, the Northern Hemisphere sites are nearly evenly split between positive and negative ozone trends, while 5 of 7 Southern Hemisphere sites have positive trends. Positive trends are in the range of 0.5-2 ppbv decade^-1, with ozone increasing at Mauna Loa by roughly 50% since the late 1950s. Decreasing ozone at two high elevation Alpine sites differed from the trend based on IAGOS aircraft observations in the European lower free-troposphere. The Alpine sites frequently sample polluted European boundary layer air, especially in summer, and only represent lower free tropospheric ozone if the data are carefully filtered to avoid boundary layer air masses. In contrast, the IAGOS observations show that lower free tropospheric ozone increased at the rate of 1.3 ± 0.8 ppbv decade^-1 since 1994. The 27 selected sites are representative of less than 25% of the global surface area, and therefore they cannot provide a global mean surface ozone trend, however, they are useful for examining regional trends and for evaluating chemistry-climate models.