Deposition of Ozone for Stomatal Exchange

Ozone (O3) is one of the components of the atmosphere and mostly found in the stratosphere and troposphere. Tropospheric ozone, a secondary air pollutant, is recognized as one of the most important greenhouse gases (IPCC, 2007; Stevenson et al., 2013) that possess a threat to human health (WHO, 2003; Lim et al., 2012) and have adverse effects on vegetation (Fowler et al., 2009). Phototoxic ground-level ozone, as the name implies, is also known to harm plants, which eventually causes a decrease in the carbon uptake into the biosphere (Sitch et al., 2007), and impact building and transport structures, as well (Kumar and Imam, 2013). 

It has been established that direct exposure of ozone above 50 ppb is dangerous to human health (WHO, 2006); notwithstanding that, it is uncertain as to the threshold to cause harm in human health since impacts usually occur at 35 ppb though susceptibility to harm differs from person to person. Health-related issues from ozone are mostly respiratory system problems, such as lung irritation, reduced lung function and sometimes, mortality. It has been estimated that 21,400 premature deaths are recorded annually in Europe as a result of ozone pollution (EEA, 2007). 

Effects of O3 on plants is species-specific and depends on environmental conditions, too. Plants show high sensitivity to O3, by reducing tree growth and carbon sequestration and modify species composition of plants (Ashmore, 2005; US EPA, 2006). O3 is known to reduce crop yields, thereby compromising future food security. An Increase of ozone concentrations in the atmosphere exerts much pressure on already stressed agricultural systems from climate change, land degradation and pests and diseases (Royal Society, 2008).

The concentration of O3 in the atmosphere is dependent on the rates of O3 formation and destruction. Ozone is destructed in the stratosphere immediately it is produced, and this accounts for the natural balance between the production and destruction of O3 in the stratosphere. The amount of atmospheric ozone has been estimated 4500 Tg y–1, and the net flux of O3 from the stratosphere down to the troposphere is about 540 Tg y–1 (Royal society, 2008). The lifespan of O3 after its formation is solely determined by the sinks, which remove ozone from the atmosphere. 

On average, the lifetime of tropospheric ozone has been estimated at 22 (±2) days (Stevenson et al., 2006); however, these days differ at different altitudes and ranges between 1-2 days in the boundary layer, where massive O3 removal happens through dry deposition, for a couple of weeks in the upper troposphere (Stevenson et al., 2006). Upper tropospheric O3 is recognized as the third-most potent greenhouse gas responsible for the global greenhouse effect (IPCC, 2007). In order to decipher the various features of tropospheric ozone, it is very crucial to get in-depth knowledge on the vertical distribution of O3 in all the sublayers, namely lowermost troposphere, middle troposphere, and upper troposphere on a global scale.

The O3 budget is regulated by three major processes: chemical destruction/production, atmospheric transport and loss to surfaces via dry deposition. O3 is transported to surfaces at the ground level by atmospheric turbulence, which can be measured easily or modelled using well-known techniques such as Generalized Additive Mixed Model (GAMM), DO3SE model, European Monitoring and Evaluation Programme (EMEP) model etc. 

The rate of O3 removal from ground level surfaces determines the level of exposure to vegetation and humans, and accounts for most of the nocturnal decrease in ozone in rural places since deposition onto surfaces takes O3 below a night temperature inversion. During daytime, the vertical movement of ozone to the surface layers is usually enough to balance the mixing ratios within 10% of the boundary layer average values. On the contrary, in cities, especially near major roads, local nitric oxide sources remove O3 through titration (Colette et al., 2011). Chemical destruction of O3 molecules in the atmosphere accounts for 4100 Tg y–1, and dry deposition to the Earth surface is estimated 1000 Tg y–1 (Haines, 2003; Stevenson et al., 2006; Gettelman et al., 1997; Wild, 2007; Wu et al., 2007). 

The interaction between dry deposition of O3 in the Earth’s surface layers and the continuous mixing from the upper layers in the atmosphere have a significant effect on the amount of ozone-exposed to ecosystem and humans. During the mixing process between the surface layers, wind and its interaction with frictional force pull at the surface. Usually, deposition rates are calculated as a deposition velocity (vg) expressed as a vertical flux with a velocity (mms-1). Deposition velocity varies with height as ambient concentrations decrease towards the surface. Dry deposition rates, coupled with resistance analogy, are used to determine the various constituents in the deposition trajectory from the boundary layer to sinks (Monks et al., 2005).