Abstract
Extracting globally representative trend information from lower tropospheric ozone observations is extremely difficult due to the highly variable distribution and interannual variability of ozone, and the ongoing shift of ozone precursor emissions from high latitudes to low latitudes. Here we report surface ozone trends at 27 globally distributed remote locations (20 in the Northern Hemisphere, 7 in the Southern Hemisphere), focusing on continuous time series that extend from the present back to at least 1995. While these sites are only representative of less than 25% of the global surface area, this analysis provides a range of regional long-term ozone trends for the evaluation of global chemistry-climate models. Trends are based on monthly mean ozone anomalies, and 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. Two high elevation Alpine sites, discussed by previous assessments, exhibit decreasing ozone trends in contrast to the positive trend observed by IAGOS commercial aircraft in the European lower free-troposphere. The Alpine sites frequently sample polluted European boundary layer air, especially in summer, and can only be representative of lower free tropospheric ozone if the data are carefully filtered to avoid boundary layer air. The highly variable ozone trends at these 27 surface sites are not necessarily indicative of free tropospheric trends, which have been overwhelmingly positive since the mid-1990s, as shown by recent studies of ozonesonde and aircraft observations.
Highlights
Of the greenhouse gases (CO2, CH4, O3, N2O, H2O, synthetic greenhouse gases, e.g. HFCs, SF6) tropospheric ozone is perhaps the most difficult to observe and quantify on the global scale, due to its acute spatial variability resulting from its variable lifetime and its range of sources and sinks [Monks et al, 2009; 2015; Lin et al, 2019]
Trends since the early 1970s Before focusing on ozone trends based on monthly anomalies, we first illustrate the range of ozone values that can be found at remote locations around the world, in terms of the seasonal cycle, interannual variability, multi-year fluctuations and long-term trends
We examine ambient monthly mean ozone values at the four sites that provide the longest continuous ozone time series at remote locations (40+ years), beginning in either 1973 or 1975: Barrow Atmospheric Baseline Observatory, Mauna Loa Observatory (MLO), American Samoa Observatory and South Pole Observatory
Summary
Of the greenhouse gases (CO2, CH4, O3, N2O, H2O, synthetic greenhouse gases, e.g. HFCs, SF6) tropospheric ozone is perhaps the most difficult to observe and quantify on the global scale, due to its acute spatial variability resulting from its variable lifetime (minutes in the polluted boundary layer, to roughly three weeks in the free troposphere [Young et al, 2013]) and its range of sources (injection from the stratosphere, or photochemical production from natural and anthropogenic precursor gases) and sinks (surface deposition and chemical destruction) [Monks et al, 2009; 2015; Lin et al, 2019]. Logan 1985, 1999a, b, 2012; Oltmans and Levy, 1994; Oltmans et al, 1998, 2006, 2013; Parrish et al, 2012; Gaudel et al, 2018] These data analyses have been complemented by a series of assessments that summarize the current state of knowledge regarding tropospheric ozone’s global distribution and trends [e.g. IPCC, 2013; Cooper et al, 2014; Gaudel et al, 2018; Blunden et al, 2018]. These studies and assessments have been extremely useful for keeping the research community and policy makers informed on tropospheric ozone’s trends and variability. Most of the previous global trend analyses focused on very few sites, some of which were impacted by local and regional pollution sources, as will be shown later
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