Global Ozone Distribution relevant to Human Health : Metrics and present day levels from the Tropospheric Ozone Assessment Report (TOAR)
Résumé
Using stations from the TOAR surface ozone database, this study quantifies present-day global and regional distributions of five ozone metrics relevant for both short-term and long-term human exposure. These metrics were explored at ozone monitoring sites globally, and re-classified for this project as urban or non-urban using population densities and night-time lights. National surface ozone limit values are usually related to an annual number of exceedances of daily maximum 8-hour running mean (MDA8), with many countries not even having any ozone limit values. A discussion and comparison of exceedances in the different ozone metrics, their locations and the seasonality of exceedances provides clues as to the regions that potentially have more serious ozone health implications. Present day ozone levels (2010-2014) have been compared globally and show definite geographical differences (see Figure showing the annual 4th highest MDA8 for present day ozone for all non-urban stations). Higher ozone levels are seen in western compared to eastern US, and between southern and northern Europe, and generally higher levels in east Asia. The metrics reflective of peak concentrations show highest values in western North America, southern Europe and East Asia. A number of the metrics show similar distributions of North-South gradients, most prominent across Europe and Japan. The interquartile range of the regional ozone metrics was largest in East Asia, higher for urban stations in Asia but higher for non-urban stations in Europe and North America. With over 3000 monitoring stations included in this analysis and despite the higher densities of monitoring stations in Europe, north America and East Asia, this study provides the most comprehensive global picture to date of surface ozone levels in terms of health-relevant metrics. Tropospheric ozone (O3) is an air pollutant that is not directly emitted, but rather produced from emissions of nitrogen oxides and volatile organic compounds in the presence of sunlight, contributing to smog. It is found in the lowest layer of the atmosphere at the surface, where it has adverse effects on humans, crops and ecosystems. This study focusses on the global distribution of tropospheric ozone and how the ozone levels are quantified (as metrics) to provide information on the human health implications. A global database of ozone metrics was produced by the Tropospheric Ozone Assessment Report (TOAR) using data from all available monitoring stations. The 5 chosen ozone metrics detailed in this study have captured both the potential short and long-term effects of ozone on human health, using either the whole year of data or only the warm season, which is particularly favourable for ozone formation. Present day ozone levels (2010-2014) have been compared globally and show definite geographical differences, such as higher levels in western compared to eastern US, and between southern and northern Europe, and generally higher levels in east Asia. Differences between urban and non-urban stations are also discussed.