Photochemistry, stratospheric

Stratospheric Photochemistry, Aerosols, and Dynamics Expedition (SPADE), 77 785 Straw... 

Understand how photolysis produces radicals by bond cleavage and account for the importance of radical species in photochemical chain reactions, stratospheric ozone chemistry and the photochemistry of the polluted troposphere. 

Photochemically-generated radicals are encountered as reactive intermediates in many important systems, being a major driving force in the photochemistry of ozone in the upper atmosphere (stratosphere) and the polluted lower atmosphere (troposphere). The photochemistry of organic carbonyl compounds is dominated by radical chemistry (Chapter 9). Photoinitiators are used to form radicals used as intermediates in the chain growth and cross-linking of polymers involved in the production of electronic circuitry and in dental treatment. 

In short, although the history of anthropogenic perturbations to the stratosphere is much shorter, it is clear that these are also important. Indeed, such perturbations are expected to affect the chemistry of the troposphere as well for example, increased UV radiation will alter photochemistry at the earth s surface. 

The reader will encounter numerous other examples of photodissociation throughout this text, so it will not be treated further here. However, as will become obvious in examining the chemistry of both the troposphere and stratosphere in later chapters, it is photochemistry that indeed drives the chemistry of the atmosphere. 

Absorption of sunlight induces photochemistry and generates a variety of free radicals that drive the chemistry of the troposphere as well as the stratosphere. This chapter focuses on the absorption spectra and photochemistry of important atmospheric species. These data can be used in conjunction with the actinic fluxes described in the preceding chapter to estimate rates of photolysis of various molecules as well as the rate of generation of photolysis products, including free radicals, from these photochemical processes. 

There are several highly useful sources of data on the absorption spectra and photochemistry of atmospheric species. NASA publishes on a regular basis a summary of kinetics and photochemical data directed to stratospheric chemistry (DeMore et al., 1997). However, much of the data is also relevant to the troposphere. This document can be obtained from the Jet Propulsion Laboratory in Pasadena, California. Alternatively, the data are available through the Internet (see Appendix IV). IUPAC also publishes regularly in The Journal of Physical Chemical Reference Data a summary directed more toward tropospheric chemistry (Atkinson et al., 1997a, 1997b). Finally, Nolle et al. (1999) have made available a CD-ROM containing the UV-visible spectra of species of atmospheric interest. 

In short, the overall features of the chemistry involved with the massive destruction of ozone and formation of the ozone hole are now reasonably well understood and include as a key component heterogeneous reactions on the surfaces of polar stratospheric clouds and aerosols. However, there remain a number of questions relating to the details of the chemistry, including the microphysics of dehydration and denitrification, the kinetics and photochemistry of some of the C10x and BrOx species, and the nature of PSCs under various conditions. PSCs and aerosols, and their role in halogen and NOx chemistry, are discussed in more detail in the following section. 

Because of these rapid removal processes in the troposphere, the contribution of iodine to stratospheric photochemistry has not received much attention. However, Solomon et al. (1994) suggested that rapid transport from the lower troposphere into the upper troposphere and lower stratosphere via convective clouds could provide a mechanism for injecting such compounds into the stratosphere. While the relevant chemistry of iodine is not well known, it would be expected to interact with the CIO cycles in much the same way as BrO, e.g.,... 

The strategy for research in the stratosphere has been to develop computer simulations to predict trends in photochemistry and ozone change. Incorporated in these simulations are laboratory data on chemical kinetics and photolytic processes and a theoretical understanding of atmospheric motions. An important aspect of this approach is knowing if the computer models represent the conditions of the stratosphere accurately enough that their predictions are valid. These models are made credible by comparisons with stratospheric observations. 

Measurements either from the ground or from satellites have been a major contribution to this effort, and satellite instruments such as LIMS (Limb Infrared Monitor of the Stratosphere) on the Nimbus 7 satellite (I) in 1979 and ATMOS (Atmospheric Trace Molecular Spectroscopy instrument), a Fourier transform infrared spectrometer aboard Spacelab 3 (2) in 1987, have produced valuable data sets that still challenge our models. But these remote techniques are not always adequate for resolving photochemistry on the small scale, particularly in the lower stratosphere. In some cases, the altitude resolution provided by remote techniques has been insufficient to provide unambiguous concentrations of trace gas species at specific altitudes. Insufficient altitude resolution is a handicap particularly for those trace species with large gradients in either altitude or latitude. Often only the most abundant species can be measured. Many of the reactive trace gases, the key species in most chemical transformations, have small abundances that are difficult to detect accurately from remote platforms. 

One of the success stories of in situ measurements in the stratosphere is the confirmation that the rapid loss of ozone over Antarctica each October is indeed caused by photochemistry related to the release of chlorofluoro-carbons at the surface of the earth. Ground-based measurements of the primary chlorine culprit, CIO, and 03 have given a similar picture (4), but not with the fine detail possible from the in situ techniques, as shown in... 

There is already one excellent example of our failure to make such a predictive leap—the Antarctic ozone hole. The reason for the failure to anticipate the rapid loss of ozone in the lower stratosphere was a failure to appreciate the potential role of the subtle photochemistry, in particular, the heterogeneous chemistry. Nor did researchers have a full appreciation for the consequences of the air parcels inside the polar vortices being relatively isolated from midlatitude air. Some of these same issues are important in the Arctic region in wintertime, but researchers lack the predictive capability to determine how ozone will ultimately be affected. 

However, even if such measurements were possible, would the uncertainty of the result be small enough to establish that production does indeed balance observed loss of ozone The calculation of ozone loss in the Antarctic ozone hole was shown to have an uncertainty of 35 to 50%. The uncertainty for analyzing whether production balances loss in the midlatitude stratosphere is similarly 35 to 50%. About half of the uncertainty is in the measurements of stratospheric abundances, which are typically 5 to 35%, and half is in the kinetic rate constants, which are typically 10 to 20% for the rate constants near room temperature but are even larger for rate constants with temperature dependencies that must be extrapolated for stratospheric conditions below the range of laboratory measurements. In addition to uncertainties in the photochemical rate constants, there are those associated with possible missing chemistry, such as excited-state chemistry, and the effects of transport processes that operate on the same time scales as the photochemistry. Thus, simultaneous measurements, even with relatively large uncertainties, can be useful tests of our basic understanding but perhaps not of the details of photochemical processes. 

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