Abstract
Abstract. Hydroxyl (OH) radicals play a vital role in maintaining the oxidizing capacity of the atmosphere. To understand variations in OH radicals both source and sink terms must be understood. Currently the overall sink term, or the total atmospheric reactivity to OH, is poorly constrained. Here, we present a new on-line method to directly measure the total OH reactivity (i.e.~total loss rate of OH radicals) in a sampled air mass. In this method, a reactive molecule (X), not normally present in air, is passed through a glass reactor and its concentration is monitored with a suitable detector. OH radicals are then introduced in the glass reactor at a constant rate to react with X, first in the presence of zero air and then in the presence of ambient air containing VOCs and other OH reactive species. Comparing the amount of X exiting the reactor with and without the ambient air allows the air reactivity to be determined. In our existing set up, X is pyrrole and the detector used is a proton transfer reaction mass spectrometer. The present dynamic range for ambient air reactivity is about 6 to 300 s−1, with an overall maximum uncertainty of 25% above 8 s−1 and up to 50% between 6–8 s−1. The system has been tested and calibrated with different single and mixed hydrocarbon standards showing excellent linearity and accountability with the reactivity of the standards. Potential interferences such as high NO in ambient air, varying relative humidity and photolysis of pyrrole within the setup have also been investigated. While interferences due changing humidity and photolysis of pyrrole are easily overcome by ensuring that humidity in the set up does not change drastically and the photolytic loss of pyrrole is measured and taken into account, respectively, NO>10 ppb in ambient air remains a significant interference for the current configuration of the instrument. Field tests in the tropical rainforest of Suriname (~53 s
Highlights
Every year, approximately 1.3 billion tonnes of carbon are released into the troposphere due to natural and anthropogenic gaseous emissions (Goldstein et al, 2004)
Some general criteria that the reagent molecule X must satisfy are: 1. it reacts with OH at a suitable rate so as to compete with reactive species in ambient air; 2. the rate coefficient for reaction with OH should be well established; 3. it must be volatile; 4. it must have the necessary physical and chemical properties for easy and accurate detection using a suitable detector; 5. it should not be present in ambient air as this can complicate the analysis
Several tests with single and mixed hydrocarbon standards were performed to ascertain whether the Comparative Reactivity Method can reliably quantify samples of known OH reactivity
Summary
Approximately 1.3 billion tonnes of carbon are released into the troposphere due to natural and anthropogenic gaseous emissions (Goldstein et al, 2004). Photochemical reactions, initiated by the hydroxyl radical (OH), oxidize many of these emitted primary atmospheric pollutants such as carbon monoxide (CO), sulphur dioxide (SO2), nitrogen oxides (NOx=NO and NO2) and VOCs (Volatile Organic Compounds) into forms, which are more readily removed from the atmosphere by deposition or formation of aerosol. In order to ascertain how well we understand these OH initiated photochemical processes, measured ambient OH radical concentrations from field studies are often compared with OH radical concentrations predicted by photochemical models The accuracy of photochemical models depends to a large extent on how well the OH sources, OH sinks and associated chemical mechanisms are represented. If the model predicts significantly higher OH concentrations than the measured OH concentrations, it could be due to an overestimation of the OH sources and/or an underestimation of the OH sinks.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
