Transformations in the atmosphere

Air pollution can be considered to have three components sources, transport and transformations in the atmosphere, and receptors. The source emits airborne substances that, when released, are transported through the atmosphere. Some of the substances interact with sunlight or chemical species in the atmosphere and are transformed. Pollutants that are emitted directiy to the atmosphere are called primary pollutants pollutants that are formed in the atmosphere as a result of transformations are called secondary pollutants. The reactants that undergo transformation are referred to as precursors. An example of a secondary pollutant is O, and its precursors are NMHC and nitrogen oxides, NO, a combination of nitric oxide [10102-43-9] NO, and NO2. The receptor is the person, animal, plant, material, or ecosystem affected by the emissions. 

Air pollutants reach receptors by being transported and perhaps transformed in the atmosphere (Fig. 18-1). The location of receptors relative to sources and atmospheric influences affect pollutant concentrations, and the sensitivity of receptors to these concentrations determines the effects. The location, height, and duration of release, as well as the amount of pollutant released, are also of importance. Some of the influences of the atmosphere on the behavior of pollutants, primarily the large-scale effects, are discussed here, as well as several effects of pollutants on the atmosphere. 

Acid deposition occurs when sulfur dioxide and nitrogen oxide emissions are transformed in the atmosphere and return to the earth in rain, fog or snow. Approximately 20 million tons of SOj are emitted annually in the United States, mostly from the burning of fossil fuels by electric utilities. Acid rain damages lakes, harms forests and buildings, contributes to reduced visibility, and is suspected of damaging health. 

Although chemical transformations in the atmosphere may seem peripheral to this discussion, these reactions should be considered since their products may subsequently enter the aquatic and terrestrial environments the persistence and the toxicity of these secondary products are therefore relevant to this discussion. 

When analyzing air contamination by pesticides, it is important to consider not only pesticides in their unchanged form, but also how they transform in the atmosphere, particularly through photochemical oxidation (photolysis). In many cases, photolysis creates products that remain in the environment for a... 

After release from fires, organic and some inorganic components undergo rapid or more delayed chemical transformation in the atmosphere. The physical properties as well as chemical composition of smoke particles may alter on the way from the source areas (biomass burning areas) to the measurement sites in Northern Europe. There are several reasons why particle properties change. Chemical components can, e.g., become oxidized or substituted in particles, but also the condensation of secondary material onto the LRT particles during the transport changes the particle properties. 

Many of these processes are analogous to processes that deposit aquatic chemicals to the sediments of a lake or a river. Chemical transformation in the atmosphere typically results in the production of more oxidized species, and the complete mineralization of many organic chemicals is possible. In this section, physical processes that result in the removal of chemicals from the atmosphere without changes to their chemical structure are discussed. In Section 4.6, chemical transformation processes are presented. 

Water droplets and particulate matter often influence the rates of chemical transformations in the atmosphere. Whereas homogeneous reactions involve only gaseous chemical species in the atmosphere, reactions involving a liquid phase or a solid surface in conjunction with the gas phase are called heterogeneous reactions. Reactions that occur much more rapidly in water than in air may occur primarily in droplets, even though the droplets constitute only a small fraction of the total atmospheric volume. Solid surfaces also can catalyze reactions that would otherwise occur at negligible rates specific examples are discussed in the following sections on acid deposition and stratospheric ozone chemistry. 

Direct experimental studies on catalytic reactions using real atmospheric aerosols are just beginning. Therefore, the anticipated role of such reactions in the Earth s atmosphere is based mainly on estimates from experiments made with model catalysts, together with known data that characterizes the atmosphere as a sort of global catalytic reactor. For example, a drastic acceleration of chemical transformations in the atmosphere after volcanos eruptions has been observed [1]. Also, the possible... 

The aim of this book is, first of all, to present the atmospheric cycle of the trace constituents. We will discuss in more detail the trace substances (Chapter 3) with relatively short residence time (<10 yr). The study of these compounds is particularly interesting since their sources and sinks as well as their concentrations are very variable in space and time. They undergo several physical and chemical transformations in the atmosphere. Among these transformations the processes leading to the formation of aerosol particles have unique importance. The aerosol particles control the optical properties of the air, the formation of clouds and precipitation and, together with some gases, the radiation and heat balance of the Earth-atmosphere system. Because of their importance the physical and chemical characteristics of aerosol particles will be summarized in a separate chapter (see Chapter 4). 

Due to the thermal instability of PAN, reaction R7 is highly reversible. Thus, PAN is a temporary sink for NO2 in cycle of photochemical reactions. NOjr eventually undergoes further oxidation to nitric acid, which is one of direct causes of acid rain. Some portions of VOCs arc transformed in the atmosphere into aerosols, which are contributors to the decrease in visibility. For the effective control of photochemical smog, reduction of both precursors (VOCs and NOx) should be considered at the same time. 

It has been proven that the acidification of precipitation is caused principally by the presence in the atmosphere of the gaseous components sulfur dioxide (SO2) and nitrogen oxides (NO and NO2, the mixture indicated as NOx). These gases are emitted in large quantities in the air by human activities. SO2 is formed by oxidation of the sulfur present in fossil fuels, while NO is formed in every combustion process in which high temperatures arise. SO2 and NOx can be transformed in the atmosphere into compounds that are soluble in water, such as cloud water and raindrops, forming at the same time sulfuric acid and nitric acid (Table 1). 

A. Kallend, Chemical Transformation in the Atmosphere Conference at the National Society for Clean Air, Brighton, 1981. 

Of a total flux of non-methane reduced organic compounds into the atmosphere of about 1,350 Tg year [35, 36], rally 10% or so leads to organic aerosol [25, 37], However, less than 1 % of the primary organic emissions into the atmosphere have a sufficiently low volatility to remain in the condensed phase under ambient conditions, so SOA formation must be a huge part (90% or more) of the OA story [38]. The straightforward fact is that only a small fraction of all organic compoimds (by mass) in the atmosphere have what it takes to stay on or in a particle. That special property is low volatility, and most compounds acquire that low volatility via chemical transformation in the atmosphere. 

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