Precursor emissions

Blumenthal and co-workers used the earlier data and a case study of Denver, Colorado, to develop arguments as to the source of high concentrations of ozone or ozone precursors that are found in some nonurban areas. Th believe that downwind areas as long as 260 km can exceed the standard because of precursor emission from an urban source. Although th do not present any ozone production estimates related to photochemistry, their data analyses confirm hypotheses of transport from urban to nonurban areas. Th point out that rural areas have had concentrations as high as 0.3 ppm wi no local source of reactant. 

Developing control strategies for ozone is very different than for relatively unreactive species such as CO. In the latter case, the concentrations in air are a direct result of the emissions, and all things being equal, a reduction in emissions is expected to bring about an approximately proportional reduction in concentrations in ambient air. However, because O, is formed by chemical reactions in air, it does not necessarily respond in a proportional manner to reductions in the precursor emissions. Indeed, as we shall see, one can predict, using urban airshed or simple box models, that under some conditions, ozone levels at a particular... 

Linear rollback As the name implies, linear rollback is based on the assumption that pollutant concentrations will decrease proportionally to a decrease in the precursor emissions. For a pollutant such as CO, for example, the percentage reduction in emissions required to meet air quality standard for CO (Aco) in a region that currently has observed concentrations as high as Cco is given by linear rollback as ... 

The composition of fine particles varies from region to region, depending on the precursor emissions. In the northeastern USA, central Europe, and southeastern Asia, more than half of the composition is made up of sulphate particles, due to the combustion of high-sulphur coal and oil. The rest is made up of nitrate particles, carbonaceous material (elemental and organic carbon), and crustal matter (fugitive particles from soil, clay, and rock erosion). 

The origin of these episodic peak ozone levels is unquestionably photochemical ozone formation from man-made NOx and VOC precursors. Efficient long-range transboundary transport ensures that these episodic peak levels are observed at even the most remote monitoring stations in the EMEP monitoring network. Equally well, efficient transboundary transport ensures that responses to reductions in man-made NOx and VOC precursor emissions should be felt across Europe in terms of reductions in episodic peak ozone levels. 

Derwent RG, Witham CS, Utembe SR, Jenkin ME, Passant NR (2010) Ozone in central England the impact of 20 years of precursor emission controls in Europe. Environ Sci Policy 13 195-2004... 

Drechsler S, Uhmer U, Lumpp R (2006) Sensitivity of urban and rural ammonium nitrate particulate matter to precursor emissions in Southern Germany. Workshop on Contribution of Natural Sources to PM Levels in Europe, JRC Ispra, 12-13 October 2006... 

A Strategy for Controlling Acid Rain Precursor Emissions... 

Mexico City suffers from severe air quality problems with maximum ozone values up to 250 ppb. In order to study the effect of different precursor emissions and possible mitigation measures on the ozone levels in Mexico City the MCCM was applied in the Greater Area of the city (Jazcilevich et al. 2003 Forkel et al. 2004). Figure 7.4a shows that, in agreement with observations, the simulated O3 maxima occur in the southwest of the city (measurement station FED), which is down-wind of the city centre, as an uphill flow is prevailing during the afternoon. The minimum ozone concentrations are found in the centre of the city (station MER), where the NO emitted by traffic titrates the ozone, and in the northern part of the city at places where the NO emissions from industry and power plants locally reduce the ozone concentrations (near station XAL). 

An important aspect for emission reduction policies is the study of the regional sensitivity to precursor emission reduction. For this purpose indicator species are computed to asses reduction in NOx and VOC over the whole Italy and on subdomains centered on specific spots the Milan and Rome areas and some of the major industrial areas. Different periods were selected, and runs with both chemical schemes were performed over Italy comparing indicator species computed for different reduction scenarios. For all the periods investigated, it was found that Italy, including the large islands Sicily and Sardinia, is mostly dominated by NOx chemical regimes, independently of the photochemical mechanism used. However, the effect of the NOx reduction predicted with the CB-IV mechanism is lower than that predicted with S APRC90 mechanism. In addition, the urban areas... 

Processes needed aerosol thermodynamics/d3mamics, aqueous-phase chemistry, gaseous precursor emissions, primary aerosol emissions, and water uptake... 

The above results make it very clear that forecasts of the future oxidation capacity of the atmosphere depend critically on the assumed emissions. The IPCC did not assign probabilities to its emission scenarios but it is apparent that some of these scenarios are highly improbable for oxidant precursor emissions. Integrated assessment models, which couple global economic and technological development models with natural Earth system models provide an alternative approach to the IPCC scenario approach with the added advantage that objective estimates of individual model uncertainties can be combined with Monte Carlo approaches to provide more objective ways of defining means and errors in... 

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 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. 

Significant downward trends were found for SO2 (Fenger et al., 1999). Kuebler et al. (2001) reported a significant decrease in precursor emissions in Switzerland but little change in ozone, possibly due to transport from elsewhere in Europe or to an increase in global background ozone. 

The first two goals would have the effect of evaluating the accuracy of emission inventories, while the third goal would evaluate the accuracy of photochemical production rates. Model applications that met these criteria would be very likely to predict the relation between ozone and precursor emissions correctly. Conversely, major errors in photochemistry or in emission inventories would be quickly apparent in this type of analysis. Analyses of these species should also facihtate the identification of long-term trends in both ozone and its precursors. 

The relation between ambient levels and precursor emissions is somewhat easier to identify for particulates than for ozone, because the chemical composition of individual particulates provides evidence for their origin sulfate particulates are associated with sulfur dioxide emissions organic particulates with specific VOCs, and so on. A variety of statistical methods have been used to identify source types for particulates based on chemical composition, especially in terms of trace metal components (e.g., Henry, 1992 Seinfeld and Pandis, 1998). This type of analysis requires sophisticated measurements of the chemical composition of individual particulates, rather than the more common measurement of summed concentrations. Statistical methods have also been used to gain information about ozone and ozone precursors (e.g., Buhr et al., 1995 Stehr et al., 2000). 

In polluted areas, oxidant production reaches a maximum in the afternoon, and the highest values are often observed some distance away from the main precursor emissions. During night there is no production of oxidants, and in the polluted urban air the oxidants will be rapidly consumed in various oxidation reactions. The final products are nitric acid, various organic nitrates, aldehydes and organic acids. Oxidants, moved out of one polluted area, may the following day enhance the problems in downwind areas more than 500 km away (12,1J). 

Kley, D, H. Geiss and V. A. Mohnen (1994) Tropospheric ozone at elevated sites and precursor emissions in the United States and Europe. Atmospheric Environment 28, 149-158 Klingenberg, H. (1996) Automobile exhaust emission testing. Measurements of regulated and unregulateed exhaust gas components, exhaust emission tests. Springer-Verlag, Berlin,... 

This is qualitatively in accordance with the few available surface ozone measurements of rural stations (see Section VII) and with the strong increase in European ozone precursor emissions which were particularly large in the 1970s. However, the three long-term European stations show different trends in the period after 1980 and only Payeme shows a significant increase in the 1980s. 

Whilst there is some certainty about the rates at which these transformations take place/ there is considerable uncertainty about the detailed mechanisms involved. This is particularly true of the products of wet precipitation. It is not difficult to see why this uncertainty exists. The parent compounds of rainwater can only be indirectly specified because rainwater consists of dissolved ions which are not directly associated with their precursor emission compounds. It is generally thought that NH4/ Na/ Ca, SO4/ NO3 and C1 form the parent compounds in rainwater. 

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