Regional trend analysis of surface ozone observations from monitoring networks in eastern North America, Europe and East Asia

Abstract

Surface ozone is a greenhouse gas and pollutant detrimental to human health and crop and ecosystem productivity. The Tropospheric Ozone Assessment Report (TOAR) is designed to provide the research community with an up-to-date observation-based overview of tropospheric ozone’s global distribution and trends. The TOAR Surface Ozone Database contains ozone metrics at thousands of monitoring sites around the world, densely clustered across mid-latitude North America, western Europe and East Asia. Calculating regional ozone trends across these locations is challenging due to the uneven spacing of the monitoring sites across urban and rural areas. To meet this challenge we conducted a spatial and temporal trend analysis of several TOAR ozone metrics across these three regions for summertime (April–September) 2000–2014, using the generalized additive mixed model (GAMM). Our analysis indicates that East Asia has the greatest human and plant exposure to ozone pollution among investigating regions, with increasing ozone levels through 2014. The results also show that ozone mixing ratios continue to decline significantly over eastern North America and Europe, however, there is less evidence for decreases of daytime average ozone at urban sites. The present-day spatial coverage of ozone monitors in East Asia (South Korea and Japan) and eastern North America is adequate for estimating regional trends by simply taking the average of the individual trends at each site. However the European network is more sparsely populated across its northern and eastern regions and therefore a simple average of the individual trends at each site does not yield an accurate regional trend. This analysis demonstrates that the GAMM technique can be used to assess the regional representativeness of existing monitoring networks, indicating those networks for which a regional trend can be obtained by simply averaging the trends of all individual sites and those networks that require a more sophisticated statistical approach.

Tropospheric Ozone Assessment Report: Present-day tropospheric ozone distribution and trends relevant to vegetation

“…These bounds on p-values are based on analysis of the regional average daytime ozone trend across eastern North America by Chang et al (2017) using a generalized additive mixed model (GAMM). They found that trends at individual sites with p-values up to 0.34 consistently displayed cohesive regional relationships with the pattern of trends where p < 0.05.…”

“…TOAR-Vegetation also provides a unique opportunity to examine trends in ozone metrics over 15-and 20-year periods (see Chang et al, 2017 and TOAR-Metrics for sources of uncertainty in the trends analysis). A full analysis of regional trends in the TOAR data in NAM, EUR and EAS is provided in Chang et al (2017).…”

“…A full analysis of regional trends in the TOAR data in NAM, EUR and EAS is provided in Chang et al (2017). From a vegetation impacts perspective, it is most striking from all of the vector maps and bar charts included here, that the dominant trends are region-specific over the period 1995-2014.…”

“…AOT40 and particularly W126 are determined to a greater extent by high ozone during pollution episodes compared with the M12 metric, and therefore their reductions reflect the decreases in regional precursor emissions (see TOAR-Metrics for further details). Changes in M12, on the other hand, are more indicative of changes across the full ozone distribution, including increases in lower ozone levels linked to regional decreases of NO x emissions across EUR and NAM resulting from successful abatement strategies (Chang et al, 2017;CLRTAP, 2016;Paoletti et al, 2014;Simon et al, 2015;TOAR-Climate;TOAR-Metrics). The consequences of this shift in ozone profile for vegetation require further study as almost all published experimental studies have exposed plants to simulated high peak, low background diurnal profiles.…”

“…Local controls on precursor emissions have partially reduced peak ozone levels in regions such as the USA and Europe in the last two decades (CLRTAP, 2016;Cooper et al, 2012;Derwent et al, 2010;Simon et al, 2015). However, trends in daytime average ozone values are more varied with general decreases across North America during the growing season, weaker decreases across Europe and increases across Asia (see Gaudel et al, 2017, hereinafter referred to as TOAR-Climate for more details, and Chang et al, 2017).…”