Reaction Kinetics of OH Radicals

When an organic pollutant enters the atmosphere a number of factors determine whether it stays in the area where it was released or is transported long distances. For example, if the chemical is volatile it may readily disperse in the air; if it is polar (water soluble) it may be washed out of the air with a rain or snow shower. Some chemicals react with atmospheric oxidants such as hydroxyl radicals (OH), ozone (O3), or are photolyzed by UV radiation from the sun. When this happens a contaminant may either break down into simple molecules (e.g., CO2 and H2O) or is transformed into a different chemical that may be more harmful than its precursor.

Presently, we are trying to determine how fast some chemicals react with OH radicals (the most important atmospheric oxidant) and what products may be formed when they do. To do this we have developed a technique that uses mass spectrometry to measure both relative rate constant and the products of the reaction of OH radicals with chemicals in the gas phase. This system consists of a quartz reaction chamber mounted inside a GC oven (for temperature control) and coupled to a mass spectrometer via a capillary (see Figure 1). The advantage of this system is that it allows increased sensitivity toward semi-volatile chemicals and the ability to easily control the temperature of a reaction over a wide range of temperatures (e.g., -20 to 200 °C). We have recently used this instrument to measure the kinetics and products of the reactions of OH radicals with oxygenated hydrocarbons such as acetone and anthropogenic contaminants such as polybrominated diphenyl ethers (PBDEs).

engine used for kinetics studies

Fig.1 The instrument used for the kinetics studies.

We are currently investigating the kinetics of terpenes such as myrcene, ocimene, and farnesene with OH radical. It has been suggested that the discrepancies between regional air pollution models and observed data could be the result of unknown factors such as terpenes. Thus, determining the kinetics of these compounds should produce better models for regional air pollution.





				Reaction Kinetics of OH Radicals When an organic pollutant enters the atmosphere a number of factors determine whether it stays in the area where it was released or is transported long distances. For example, if the chemical is volatile it may readily disperse in the air; if it is polar (water soluble) it may be washed out of the air with a rain or snow shower. Some chemicals react with atmospheric oxidants such as hydroxyl radicals (OH), ozone (O3), or are photolyzed by UV radiation from the sun. When this happens a contaminant may either break down into simple molecules (e.g., CO2 and H2O) or is transformed into a different chemical that may be more harmful than its precursor. Presently, we are trying to determine how fast some chemicals react with OH radicals (the most important atmospheric oxidant) and what products may be formed when they do. To do this we have developed a technique that uses mass spectrometry to measure both relative rate constant and the products of the reaction of OH radicals with chemicals in the gas phase. This system consists of a quartz reaction chamber mounted inside a GC oven (for temperature control) and coupled to a mass spectrometer via a capillary (see Figure 1). The advantage of this system is that it allows increased sensitivity toward semi-volatile chemicals and the ability to easily control the temperature of a reaction over a wide range of temperatures (e.g., -20 to 200 °C). We have recently used this instrument to measure the kinetics and products of the reactions of OH radicals with oxygenated hydrocarbons such as acetone and anthropogenic contaminants such as polybrominated diphenyl ethers (PBDEs). Fig.1 The instrument used for the kinetics studies. We are currently investigating the kinetics of terpenes such as myrcene, ocimene, and farnesene with OH radical. It has been suggested that the discrepancies between regional air pollution models and observed data could be the result of unknown factors such as terpenes. Thus, determining the kinetics of these compounds should produce better models for regional air pollution.