Ozone (O3) is present throughout the atmosphere although there are concentration peaks at two levels, the stratosphere (15 - 50 km) and troposphere (0-15 km), with the largest fraction and concentrations being in the stratospheric O3 layer ( Royal Society, 2008). Stratospheric O3 is important as it regulates the transmittance of ultraviolet light to the surface of the earth. Hence reductions in stratospheric O3 in polar regions, particularly the Antarctic "ozone hole", are of concern regarding the health effects of exposure to increased levels of UV-B.

In contrast, O3 in the troposphere is regionally important as a toxic air pollutant and greenhouse gas. Concentrations have increased during this century. Mixing with stratospheric air provides a natural global average background of around 10-20 parts per billion (ppb), though there is some debate about the concentration. Additional quantities of tropospheric O3 are produced by photochemical reactions from nitrogen oxides (NOX) and volatile organic compounds (VOCs), which include various hydrocarbons. NOX and VOCs originate from fossil fuel combustion and natural sources. As these emissions have increased since the mid 19th century, tropospheric O3 has increased. The chemical cycles producing and destroying O3 depend on pollutant and light levels so different reactions are established during the day and night. For a description of these reactions see Royal Society, 2008 and ROTAP, 2012. The daytime reactions with VOCs form a catalytic cycle, which builds up O3 concentrations. In warm, summer conditions photochemical events often occur in which O3 concentrations increase successively over several days. These "ozone episodes" provide concentrations of O3 (>40 ppb) which are toxic both to human health, buildings and vegetation.

Prior to the industrial revolution natural sources of NOX and VOCs (see table) would have generated O3 in the troposphere, adding to that transported from the stratosphere. However, the large amounts of NOX and VOCs released by human activities, such as the combustion of fossil fuels, has led to a large increase in the northern hemisphere background concentration. Evaluation of historical O3 measurements indicate that since the 1950s, the background ozone concentration has roughly doubled (Volz and Kley, 1988, Vingarzan, 2004), although there has been some slowing down of this trend in the last decade (Derwent et al., 2013).  ). As a result the episodes of peak O3 concentration combine with the increased mean concentrations to provide even larger concentrations, sometimes >100 ppb.

O3 pollution effects on vegetation are described by cumulative metrics that are based on either atmospheric concentration of O3 or modelled uptake of O3 through the stomatal pores on the leaf surface (O3 flux).  A recent analysis of field evidence for O3 effects confirmed that maps generated using an O3 flux metric more closely matched locations of damage than those based on O3 concentration (Mills et al., 2011a). New O3 flux-based critical levels have been derived for crops, tree species and provisionally for grassland species (Mills et al., 2011b).

Summary of natural and anthropogenic NOX and VOC sources







Soils, natural fires

transport (road, sea and rail), power stations, other industry and combustion processes, agriculture

Vegetation, natural fires

Transport, combustion processes, solvents, oil production


Map of ozone concentrations over the UK

Map of ozone concentrations over the UK including altitude effects.

Effects of increased O3 concentrations on the natural environment may lead to:

  • reduction in growth of agricultural crops and decreased forest production
  • altered species composition of semi-natural plant communities


Derwent, R.G.; Manning, A.J. ; Simmonds, P.G. ; Spain, T.G. ; O'Doherty, S. 2013 Analysis and interpretation of 25 years of ozone observations at the Mace Head Atmospheric Research Station on the Atlantic Ocean coast of Ireland from 1987 to 2012. Atmospheric Environment 80 361-368
Mills, G.; Pleijel, H.; Braun, S.; Buker, P.; Bermejo, V. ; Calvo, E. ; Danielsson, H. ; Emberson, L. ; Fernandez, Gonzalez ; Grunhage, L.; Harmens, H. ; Hayes, F.; Karlsson, P.E.; Simpson, D. 2011 New stomatal flux-based critical levels for ozone effects on vegetation. Atmospheric Environment 45 5064-5068
Mills, G.; Hayes, F.; Simpson, D. ; Emberson, L. ; Norris, D.; Harmens, H. ; Buker, P. 2011 Evidence of widespread effects of ozone on crops and (semi-)natural vegetation in Europe (1990–2006) in relation to AOT40- and flux-based risk maps. Global Change Biology 17 592-613
Vingarzan, R. 2004 A review of surface ozone background levels and trends. Atmospheric Environment 38 3431-3442