Biochemistry & Physiology: Open Access
Open Access

Tropospheric O₃ formation

O₃ in the stratosphere filters UV radiation, but in the troposphere O₃ is a damaging air pollutant to human and plant health [Environmental Protection Agency EPA. Tropospheric O₃ (trioxygen) is an allotrope of oxygen that forms through chemical reactions with two chemically distinct precursors: nitrogen oxides (NOx = NO + NO₂) and reactive carbon molecules including carbon monoxide (CO), methane (CH4), and VOCs Rates of O₃ formation depend on sunlight and the relative concentrations of NOx and reactive carbon molecules; namely, methane and VOCs [ ]. The reaction of nitric oxide (NO) with the peroxy radical (RȮ₂) is the central reaction for the formation of O₃ in the troposphere. In this reaction, NO is converted to NO₂ which is rapidly photolyzed to form O₃ and recycle NO. The efficiency with which O₃ is produced from NOx pollution varies with the location and time of emissions. For example, in the polluted regions at the Earth’s surface, NOx rapidly reacts to form HNO₃, which serves as a reservoir for NOx. In less polluted areas, NO₂ photolysis competes more effectively with HNO₃ production and more molecules of NOx react with peroxy radicals to form O₃. In regions where NOx is propelled into the free troposphere, like the tropics, O₃ production is especially efficient. Additionally, the VOC:NOx ratio determines the O₃ concentration. In urban areas with elevated NOx due to high emissions, O₃ formation is limited by VOCs, leading to locally suppressed O₃ concentrations. NOx transported away from urban centers can mix with VOCs, resulting in greater O₃ concentrations in suburban areas [1].

VOCs

Plants produce a vast diversity of biogenic VOCs, including isoprene, monoterpenes, and higher terpenoids .Terrestrial vegetation emits isoprene at high levels (~400–600 Tg C year⁻ ) and isoprene has high chemical reactivity in the troposphere .The tropospheric lifetime of isoprene is only ~ –2 h and it is rapidly oxidized by hydroxyl radicals, O₃, and nitrate radicals (NO₃). The degradation of VOCs leads to the formation of peroxyl radicals. Those react with NO to form NO₂, which then photolyzes to form O₃ In areas with very low NOx, peroxy radicals formed from isoprene oxidation react with each other or O₃, resulting in net O₃ destruction. Globally, modeling studies estimate that forestemitted isoprene increases the tropospheric O₃ concentration by 5–8% .Isoprene oxidation can also produce peroxyacylnitrates (PANs), which can be transported long distances under cool, high-altitude conditions. The long-distance transport of PANs can contribute to O₃ formation far from the pollutant source. Thus, globally, biogenic VOCs contribute to O₃ formation in the troposphere, although there is significant variation in isoprene emissions among ecosystems and species .For example, broadleaf forests have average isoprene emissions of 2.6 mg m^-2 h^- , needle-leaf evergreen trees emit 2.0 mg m^-2 h^- , and crops emit very little, only 0.09 mg m^-2 h^-; . This variation in emission led to concerns that increasing the planting of isoprene-emitting bioenergy species will increase O₃ stress, leading to crop yield loss and increased human mortality [2].

Stomatal control of O₃ deposition

Deposition of O₃ to terrestrial ecosystems is a significant sink for O₃, and understanding variation among ecosystems and species in O₃ uptake is needed for accurate prediction of tropospheric O₃ concentrations .Dry deposition occurs when atmospheric turbulence transports O₃ close to a surface and then O₃ moves through a boundary layer around a surface. O₃ dry deposition occurs through stomata as well as other, non-stomatal pathways including uptake by leaf cuticles, soil, water, snow, and manmade surfaces. A synthesis of observation studies found that stomatal uptake accounts for 45% of O₃ deposition on average across ecosystems. This percentage varies with season and ecosystem, but given a prominent role of stomata in O₃ deposition, understanding O₃ flux through stomata is a major research focus. To estimate O₃ diffusion through stomata, the resistance of stomata to water vapor is multiplied by the ratio of the diffusivity of water vapor to that of O₃ (.6 ), with the assumption that the water leaving a leaf is proportional to the O₃ entering and that O₃ reactions in the leaf do not limit stomatal uptake. Both of these assumptions, that water loss is proportional to O₃ uptake and that there is negligible resistance to O₃ destruction inside the leaf, have been questioned and remain active research areas. Furthermore, long-term exposure to elevated O₃ pollution often reduces plant biomass and stomatal conductance, which limits subsequent O₃ deposition and can feed forward to increase atmospheric O₃ concentration [3].

The rapid response of stomata to O₃

Greater stomatal conductance tends to lead to more sensitivity to O₃, often attributable to greater O₃ uptake and subsequent oxidative damage. In the model species arabidopsis, natural variation in O₃ sensitivity, measured as ion leakage, was correlated with wholerosette conductance. Additionally, the greater O₃ sensitivity of the Cape Verde island accession has been linked to constitutively high stomatal conductance caused by impaired function of mitogenactivated protein kinase [4]. Thus, stomatal closure is a direct way to reduce O₃ uptake by leaves and alleviate oxidative damage. Stomatal pores close rapidly in response to acute O₃ exposure, followed by reopening, which depends on the O₃ treatment concentration and duration . Some low-level O₃ exposure may also allow a faster response to higher doses of O₃ and therefore provide protection against greater O₃-induced injury, a process known as priming. In an experiment with common bean, exposure of leaves to 30 min of 200-ppb O₃ before a greater, 600-ppb treatment resulted in greater stomatal closure and lower VOC emissions compared with the 600-ppb treatment alone. The correlation of sensitivity to O₃ stress with stomatal conductance, and the fact that stomata close in response to O₃, suggest that greater O₃ tolerance could be engineered by altering stomatal conductance. Stomata are the entry points for the CO₂ used for photosynthesis and so reducing stomatal conductance might also reduce CO₂ entry into the leaf and compromise productivity. However, recent work has demonstrated that genetic manipulations to reduce stomatal density only moderately reduced stomatal conductance and did not change photosynthesis, suggesting that there is room to optimize stomatal density to atmospheric conditions [5].

Identifying the extent to which plant biochemistry and physiology contribute to tropospheric O₃ formation, destruction, and deposition will help in understanding the mechanisms that underpin plant O₃ sensitivity and improve predictions of global tropospheric O₃ concentration. Plant species release more than 30 000 different biogenic VOCs, including reactive classes of non-methane biogenic VOCs such as isoprene, which are emitted in large enough quantities to impact tropospheric O₃ concentrations. While isoprene can increase O₃ concentrations locally, monoterpenes and higher terpenoid compounds also rapidly react with O₃ in the leaf boundary layer and can protect plants from oxidative stress. Deposition of O₃ into vegetation is related to stomatal conductance and leaf structural traits. While there is evidence that antioxidants quench ROS within leaves, variation in detoxification capacity among different species is significant and the biochemical fate of O₃ once it enters leaves and reacts with aqueous surfaces remains largely unknown New techniques for the tracking and identification of initial products have the potential to shed light on that question and could improve the identification of targets to increase O₃ tolerance.

Acknowledgement

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Conflict of Interest

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