Abstract
The Tropospheric Ozone Assessment Report (TOAR) is an activity of the International Global Atmospheric Chemistry Project. This paper is a component of the report, focusing on the present-day distribution and trends of tropospheric ozone relevant to climate and global atmospheric chemistry model evaluation. Utilizing the TOAR surface ozone database, several figures present the global distribution and trends of daytime average ozone at 2702 non-urban monitoring sites, highlighting the regions and seasons of the world with the greatest ozone levels. Similarly, ozonesonde and commercial aircraft observations reveal ozone’s distribution throughout the depth of the free troposphere. Long-term surface observations are limited in their global spatial coverage, but data from remote locations indicate that ozone in the 21st century is greater than during the 1970s and 1980s. While some remote sites and many sites in the heavily polluted regions of East Asia show ozone increases since 2000, many others show decreases and there is no clear global pattern for surface ozone changes since 2000. Two new satellite products provide detailed views of ozone in the lower troposphere across East Asia and Europe, revealing the full spatial extent of the spring and summer ozone enhancements across eastern China that cannot be assessed from limited surface observations. Sufficient data are now available (ozonesondes, satellite, aircraft) across the tropics from South America eastwards to the western Pacific Ocean, to indicate a likely tropospheric column ozone increase since the 1990s. The 2014–2016 mean tropospheric ozone burden (TOB) between 60°N–60°S from five satellite products is 300 Tg ± 4%. While this agreement is excellent, the products differ in their quantification of TOB trends and further work is required to reconcile the differences. Satellites can now estimate ozone’s global long-wave radiative effect, but evaluation is difficult due to limited in situ observations where the radiative effect is greatest.
Highlights
This rapid shift, coupled with limited monitoring in developing nations, has left scientists unable to answer the most basic questions: Which regions of the world have the greatest human and plant exposure to ozone pollution? Is ozone continuing to decline in nations with strong ozone precursor emissions controls? To what extent is ozone increasing in the devel oping world? Are natural sources of tropospheric ozone and its precursors changing? How can the atmospheric sciences community facilitate access to ozone metrics nec essary for quantifying ozone’s impact on climate, human health and crop/ecosystem productivity?
IASI+Global Ozone Monitoring Experiment (GOME) 2 (LISA): In order to better characterize the vertical distribution of tropospheric ozone down to the lowermost troposphere (LMT, surface to 3 km a.s.l.), a new multispectral approach called IASI+GOME 2 combines the information provided by thermal IR radiances measured by the IASI instrument and Earth reflectance UV spectra from GOME 2 (Cuesta et al, 2013)
Both anthropogenic and natural, influence these ozone values (Monks et al, 2015; Neu et al, 2014), and a consideration of all of these processes is beyond the scope of this paper
Summary
This paper (hereinafter referred to as TOAR Climate) presents seasonal surface ozone metrics that are designed to understand mean changes of ozone around the world, and that are appropriate for evaluating the global atmospheric chemistry models that calculate ozone’s radiative forcing (Stevenson et al, 2013). Improvement to the estimate of ozone’s radiative forc ing requires greater confidence in global atmospheric chemistry model estimates of the tropospheric ozone burden (TOB: the total mass of ozone in the troposphere, Tg) in pre industrial times, plus an accurate observation based quantification of the present day TOB and its hori zontal and vertical distribution. TOAR Climate assesses ozone’s global distribution and trends from the mid 1970s to 2016, with the goal of providing a wide range of in situ and remotely sensed ozone observa tions for quantifying the present day TOB and to evaluate the global atmospheric chemistry models that estimate pre industrial and future scenario tropospheric ozone.
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