Ozone formation, pollution

Gulf of Maine Oxidant Study (GOMOS) a study to investigate the sources and transport of pollutants contributing to ozone formation. 

Existence of the PSS was predicted theoretically by Leighton (61), and experimental studies of this relationship date back almost 20 years. These experiments have been accomplished in smog chambers (62), polluted urban air (63,64,65), rural environments (66), and in the free troposphere (67). The goal of these experiments has been to verify that our understanding of NOjj chemistry is fundamentally correct, and to ver the role of H02 and R02 in ozone formation. Studies in polluted air seem to confirm the dominance... 

The European Commission has adopted a Proposal for a Directive on national emissions ceilings for certain atmospheric pollutants and a Proposal for a Directive relating to ozone in ambient air. The national emissions ceilings Directive will set individual limits for each Member State s total emissions in 2010 of the four pollutants responsible for acidification, eutrophication and ozone formation in the lower atmosphere sulphur dioxide, nitrogen oxides, VOCs and ammonia. The EU Solvents Directive has been formally adopted by the Commission. 

The different greenhouse gases can have complicated interactions. Carbon dioxide may cool the stratosphere which slows the process that destroys ozone. Stratospheric cooling can also create high altitude clouds which interact with chlorofluorocarbons to destroy ozone. Methane may be produced or destroyed in the lower atmosphere at various rates, which depend on the pollutants that are present. Methane can also affect chemicals that control ozone formation. 

No one pollutant can be blamed as the major cause of ozone formation. Replacing the more reactive hydrocarbons with less reactive ones would delay the formation of ozone, but would not prevent it. Reducing the NO, concentration seems to reduce the maximal oxidant concentrations observed, but the effect is nonlinear. Heavy injections of nitric oxide into the air can temporarily reduce the local ozone concentration, as often happens in urban centers, but additional oxidant formation can be expected later downwind. Although these effects can be understood qualitatively, it is not yet possible to make accurate predictions of oxidant formation, even in lalx)ratoty experiments. 

This is a major chain propagation step in the overall reaction mechanism for ozone formation in photochemical air pollution. Because H02 is intimately tied to OH through reaction (17) and cycles such as that in Fig. 1.4, when NO is present the sources and sinks of H02 are, in effect, sources or sinks of the OH radical. 

Haagen-Smit, A. J., and M. M. Fox, Photochemical Ozone Formation with Hydrocarbons and Automobile Exhaust, J. Air Pollut. Control Assoc., 4, 105-108, 136 (1954). 

As discussed earlier, the N02 then photolyzes to 0(3P), which adds to 02 to form 03. Under these conditions, O, will be formed. The concentration of NO at which this crossover from ozone destruction to ozone formation occurs is central to the chemistry of both remote and polluted regions. 

Walcek, C. J., H.-H. Yuan, and W. R. Stockwell, The Influence of Aqueous-Phase Chemical Reactions on Ozone Formation in Polluted and Nonpolluted Clouds, Atmos. Environ., 31, 1221-1237 (1997). 

These layers containing higher concentrations of pollutants provide an important mechanism for transport of ozone, particles, and their precursors to the free troposphere. In addition, in the morning when solar heating causes turbulent mixing (Fig. 2.20), these pollutants are mixed down to the surface. This not only increases the surface concentrations but also provides species that can initiate the VOC-NO chemistry that leads to more ozone formation. As a result, there is a carryover from one day to the next, leading to smog episodes in which the pollutant concentrations increase from day to day. 

There is a vaiiety of problems associated with air pollution, starting from photochemical smog, ozone formation, and acid rain at a regional level, to the greenhouse effect and ozone-layer depletion at a global level. These problems have an adverse impact on both environment and public health (Table 1.1) the last two problems are a threat to life on Earth generally. 

Keywords Hemispheric background, Long-term trends, NOx and VOCs, Ozone, Photochemical ozone formation, Regional pollution controls... 

The examples above illustrate the difficulties and dangers of a myopic approach to air pollution control. What is required is a complete systems approach which takes into account all of the known direct and indirect interactions. Even this approach will have to be modified as new knowledge becomes available. It cannot be claimed, for example, that knowledge of all atmospheric interactions is complete nor that all known interactions have been assessed. Thus our attention has been focused largely on the most obvious and dramatic atmospheric interactions. Other less dramatic reactions also require further assessment. For example, aldehydes absorb sunlight energy and are capable of reactions leading to ozone formation... 

Most other major cities in the USA and in Europe also record events with ozone in excess of 125 ppb, but these occur only a few times per year. Severe air pollution events occur less frequently in these cities, because the meteorological conditions that favor rapid formation of ozone (high sunlight, warm temperatures, and low rates of dispersion) occur less frequently. Significant excess ozone is formed only when temperatures are above 20 °C, and smog events are usually associated with temperatures of 30 °C or higher. In the major cities of northeastern USA and northern Europe, ozone levels exceed 80 ppb on —30-60 days per year. At other times, a combination of cool temperatures and/or clouds prevents ozone formation, regardless of the level of precursor emissions. 

The most severe pollution events occur when a combination of light winds and suppressed vertical mixing prevents the dispersion of pollutants from an urban center. The process of ozone formation typically requires several hours and occurs only at times of bright sunlight and warm temperatures. Eor this reason, peak ozone values typically are found downwind of major cities rather than in the urban center. During severe events with light winds, high ozone concentrations are more likely to occur closer to the city center. 

NO the critical precursor for ozone formation, typically has daytime concentrations of 5-20 ppb in urban areas, 0.5-1 ppb in polluted rural areas during region-wide events, and lO-lOOppt in the remote troposphere. An NO t concentration of 1 ppb is associated with ozone formation at rates of 2-5 ppb h which is fast enough to allow ozone concentrations to increase to 90 ppb when air stagnates in a polluted region for two days or more. Ozone production rates as high as 100 ppb h have been observed in urban locations (e.g., in the recently completed Texas Air Quality Study in Houston (Kleinman et al, 2002)). 

The relative impact of NO, . and VOCs on ozone formation during pollution events represents a major source of uncertainty. The chemistry of ozone formation is highly nonlinear, so that the exact relation between ozone and precursor emissions depends on the photochemical state of the system. Under some conditions, ozone is found to increase with increasing NO , emissions and to remain virtually unaffected by changes in VOCs. Under other conditions, ozone increases rapidly with increased emission of VOCs and decreases with increasing NO. This split into NOj -sensitive and NOjc-saturated (or VOC-sensitive) photochemical regimes is a central feature of ozone chemistry and a major source of uncertainty in formulating pollution control policy. 

Among freshly emitted pollutants, the initial rate of ozone formation is often controlled by the amount and chemical composition of VOCs. For this reason, ozone formation in urban centers is often (but not always) controlled by VOCs. As air moves downwind, ozone formation is increasingly controlled by NO rather than VOCs (Milford et al, 1989). Ozone in far downwind and rural locations is often (but not always) controlled by upwind NOjc emissions (Roselle and Schere, 1995). Rural areas also tend to have NO -sensitive conditions due to the impact of biogenic VOCs (see Section 9.11.2.2.5). However, this description represents a general trend only and is not universally vahd. NO c-sensitive conditions can occur even in large urban centers, and VOC-sensitive conditions can occur even in aged plumes. For a more complete discussion, see NARSTO (2000) and Sillman (1999). 

Ozone air pollution events generally have lower concentrations of sulfates than the winter fog events, because the dynamics that lead to ozone production usually have much more rapid vertical dilution than the fog events. However, ozone events can lead to a significant enhancement of sulfates, especially at the regional scale. The same photochemical processes that lead to ozone formation also cause rapid photochemical conversion of sulfur dioxide to sulfates. Conversion of sulfur dioxide to sulfates during air pollution events occurs on a timescale of 1-2 days. This allows for significant accumulation of sulfates during regional air pollution events. 

For NO t > 0-5 ppb (typical of urban and polluted rural sites in the eastern USA and Europe) Equations (3) and (4) represent the dominant reaction pathways for HO2 and RO2 radicals. In this case the rate of ozone formation is controlled largely by the rate of the initial reaction with hydrocarbons or CO (Equations (1) and (2)). Analogous reaction sequences lead to the formation of various other gas-phase components of photochemical smog (e.g., formaldehyde (HCHO) and PAN) and to the formation of organic aerosols. 

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