Long-term changes in tropospheric ozone
Introduction
Ozone in the troposphere is a key ingredient in a number of atmospheric physical and chemical processes. These include radiative forcing as ozone is an infrared absorber (greenhouse gas). It is also a precursor to the formation of the hydroxyl radical which affects the oxidizing (cleansing) capacity of the atmosphere. Tropospheric ozone is an effective absorber of solar ultraviolet (UV) radiation that on a per molecule basis is more effective than stratospheric ozone in filtering UV-B (Bruhl and Crutzen, 1989). The larger tropospheric column ozone amounts in the N.H. as compared to the S.H. may be an important contributor to the larger surface UV amounts recorded in the S.H. (McKenzie et al., 2003). In addition, human health, terrestrial ecosystems, and materials degradation are impacted by poor air quality resulting from high ozone levels caused by photochemical ozone production of human-emitted precursors. Tropospheric ozone changes may also come about from climate changes, such as an increase in stagnation episodes or other altered transport patterns. While air quality concerns are focused near ground level, the climatic and oxidizing impacts of tropospheric ozone are significant through the entire depth of the troposphere.
Determination of the long-term changes in tropospheric ozone on a global or hemispheric basis is hampered by the relative scarcity of representative observing locations with observational records of 15 years or more. With a lifetime of several days to several weeks, ozone is not a globally or even hemispherically well-mixed constituent of the troposphere. Representative observations on geographic scales of 500–1000 km (Prinn, 1988) are required for characterizing ozone behavior in background air on a regional basis. While surface ozone is extensively observed at sites in N.H. continental regions as part of air quality monitoring networks, these sites are often only representative of local conditions. Profile measurements that include the troposphere are very limited spatially and are carried out relatively infrequently (in most cases weekly). In the S.H. measurements are limited but with fewer continental regions with large precursor emissions, the tropospheric ozone distribution may present a more homogeneous picture than in the N.H. Even with these notable limitations it is useful to examine the available time series to get as broad a picture as possible of changes that are occurring. It is changes on these larger scales that may play the biggest role in climate forcing and oxidizing capacity by tropospheric ozone. Several earlier studies have addressed changes in tropospheric ozone with the focus on developing as broad a picture as possible of the geographic distribution of these changes (Logan, 1994; Logan et al., 1999; Oltmans et al., 1998; Vingarzan, 2004). This work builds on the analysis of Oltmans et al. (1998) by extending many of the time series, supplementing with additional locations, and using several new statistical tools to do this analysis.
The approach taken in this study is to focus on tropospheric ozone time series with relatively long records (mostly 15 years or longer) and locations (Table 1) that are representative of large geographic regions not immediately influenced by nearby precursor sources. We designate these sites as “background” stations. This designation is somewhat arbitrary but considers distance from precursor sources and the character of ozone variation at the site. In some cases a degree of compromise is made in these criteria because of the longevity of a record at a key location. In discussing the record from a particular site, the potential departure from background characteristics is considered. Data from surface measuring sites are drawn primarily from two sources. Most of the remote or high-altitude sites are part of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) network and data are available from the World Data Centre for Greenhouse Gases (WDCGG). Several North American sites are located in US National Parks and the data come from the US EPA Air Quality System (AQS) data archive. The vertical profile measurements are from balloon-borne ozonesondes. These observations are some of the longest records of tropospheric ozone. Many of these sites are also part of the GAW network and the data are available from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC). Investigators associated with most of the surface and ozonesonde data sets are part of this study. The following sections will discuss the measurement methods and data sets to be used in the analysis, the statistical models used in the treatment of the observations, a presentation of the trend results, and a discussion of the pattern of long-term changes and in some cases possible keys to understanding tropospheric ozone changes. Tropospheric ozone changes are investigated using a selected series of surface and ozonesonde sites to give a broad geographic picture of longer-term variations. The longest time series are nearly 40 years in length with the majority of records extending back about 20 years.
Section snippets
Surface observations
Most of the surface measurements used in this study were obtained using continuously operating analyzers that are based on the absorption of UV radiation at 254 nm in a cell of known length (Meyer et al., 1990). For the most part the ozone analyzers are referenced to a standard instrument also employing the UV absorption technique. These references are traceable to the Standard Reference Photometer (SRP) maintained by the US National Institute of Standards and Technology (NIST). In recent years
Treatment of the data
Several approaches have been used to analyze the time-varying characteristics of the tropospheric ozone time series. The seasonal dependence of the trends has been deduced by separately analyzing calendar monthly averages of the data records. In addition, to highlight the decadal changes in these seasonally dependent trends, the trends have been examined for three decadal periods: 1995–2004, 1985–1994 and 1975–1984, the last of which may have a different start year depending on the data record
Results and discussion
Changes in tropospheric ozone are presented for the N.H. and S.H. In the N.H. because of the greater availability of data and the heterogeneous character of the changes, individual consideration is given to various regions.
Conclusions
The picture of long-term tropospheric ozone changes is a varied one in terms of both the sign and magnitude of trends and in the possible causes for the changes. In several geographical regions the changes in time are broadly consistent with the expected behavior from changes in precursor emissions. In Hawaii a strong link to decadal variability in transport patterns could be identified. Especially for the most recent 10–15-year period, there are a number of locations such as the ozonesonde
Acknowledgments
A large number of people have been responsible for the measurements at the sites used in this study. Their careful work is gratefully acknowledged. The work of Gerry Spain at the Mace Head Station, Alan Yoshinaga and Darryl Kuniyuki at the Mauna Loa Observatory, and Dan Endres at the Barrow Observatory is specifically acknowledged.
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