Chemical amplification, which has been investigated as a method for detecting
atmospheric peroxy radicals is based on the following chain reaction, where RH is
an organic species:
Previous work has used carbon monoxide (CO) as a chain carrier, although larger
chain lengths, and thus greater sensitivity, can be a expected for certain organic
species. This is predicted by faster rates for reaction (3) that would reduce the
influence of chain termination reactions, and more complex secondary chemistry,
that could yield more than one peroxy radical for each cycle of the chain.
The experimental work used dimethyl ether (DME) as the chain carrier, as well as
CO for comparison, while radicals were generated by the photolysis of ethanal in the
presence of oxygen. Under identical conditions DME gave a chain length (1400)
almost three times that of CO (475). The responses of the system to the initial
concentration of NO (0.6-10pmm), and to the duration of the photolysis (l-25ms)
were also investigated. It was found that, over the range of concentrations
investigated, greater concentrations of NO gave longer chain lengths although longer
photolysis periods produced shorter chain lengths.
Computer models used to simulate the chemistry were found to closely reproduce the
experimental data. In addition, models were constructed for several other organic
species; ethene, propene, ethanol and ethyne. Using these models, all six compounds
were examined for their responses to reactor conditions and interference from
pollutants, such as ozone, peroxyacetyl nitrate (PAN) and peroxynitric acid (PNA).
While the alkenes gave consistently higher chain lengths under pollutant-free
conditions, they were also the most susceptible to interferences. The best
combination of high chain length, and low interference was shown by DME.
In addition, the photolysis of N 02 in the presence of 2-butyne (CH3OhCH3) was
investigated, as a means of simplifying the measurement of the ambient N 02
photolysis rate. If the concentration of 2-butyne is chosen correctly, all the 0 (3P)
produced by the photolysis reacts with it, rather than with more N 02