Abstract
This study quantifies the present-day global and regional distributions (2010–2014) and trends (2000–2014) for five ozone metrics relevant for short-term and long-term human exposure. These metrics, calculated by the Tropospheric Ozone Assessment Report, are: 4th highest daily maximum 8-hour ozone (4MDA8); number of days with MDA8 > 70 ppb (NDGT70), SOMO35 (annual Sum of Ozone Means Over 35 ppb) and two seasonally averaged metrics (3MMDA1; AVGMDA8). These metrics were explored at ozone monitoring sites worldwide, which were classified as urban or non-urban based on population and nighttime lights data. Present-day distributions of 4MDA8 and NDGT70, determined predominantly by peak values, are similar with highest levels in western North America, southern Europe and East Asia. For the other three metrics, distributions are similar with North–South gradients more prominent across Europe and Japan. Between 2000 and 2014, significant negative trends in 4MDA8 and NDGT70 occur at most US and some European sites. In contrast, significant positive trends are found at many sites in South Korea and Hong Kong, with mixed trends across Japan. The other three metrics have similar, negative trends for many non-urban North American and some European and Japanese sites, and positive trends across much of East Asia. Globally, metrics at many sites exhibit non-significant trends. At 59% of all sites there is a common direction and significance in the trend across all five metrics, whilst 4MDA8 and NDGT70 have a common trend at ~80% of all sites. Sensitivity analysis shows AVGMDA8 trends differ with averaging period (warm season or annual). Trends are unchanged at many sites when a 1995–2014 period is used; although fewer sites exhibit non-significant trends. Over the longer period 1970–2014, most Japanese sites exhibit positive 4MDA8/SOMO35 trends. Insufficient data exist to characterize ozone trends for the rest of Asia and other world regions.
Highlights
Introduction to the Tropospheric Ozone AssessmentReport (TOAR) and human health metrics Tropospheric ozone is a secondary air pollutant that is detrimental to human health (LRTAP Convention, 2015; WHO, 2013a; US Environmental Protection Agency (EPA), 2013), and crop and ecosystem productivity (Ainsworth et al, 2012; Mills et al 2017: Tropospheric Ozone Assessment Report (TOAR)-Vegetation)
The goal of this paper, TOAR-Health, is to present the global distribution and trends of ozone using all available surface ozone observations and relying on ozone metrics that are relevant to human health
Using the TOAR-surface ozone database, global and regional present-day distributions and trends for the period 2000–2014 are analyzed for five health relevant ozone metrics
Summary
The number of days with MDA8 ozone greater than 70 ppb represents peak shortterm exposure. Standards in Europe for short-term exposure to ozone are based on limit values of 60 ppb (Figure 1) The sensitivity of this metric to a lower benchmark level of 60 ppb (i.e. NDGT60) is discussed, and this metric forms the basis of section 5.2 which estimates the population exposed to NDGT60 > 25 days. 4. 3MMDA1: The annual maximum of the 3-month running mean of the daily maximum 1-hour ozone value This metric has been used to quantify mortality attributable to long-term ozone exposure used by the GBD project (see section 2.1). AVGMDA8: The 6-month or warm season often termed the “ozone season” (April to September in the Northern Hemisphere and October to March in the Southern Hemisphere) mean of MDA8 ozone These five ozone metrics are calculated for all urban and non-urban ozone monitoring stations (section 4.2) available in the TOAR database, as presentday averages for 2010–2014 (section 5), as well as trends between 2000–2014 (section 6)
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