Tropospheric HOx Cycle

Tropospheric HOx Cycle

Shown in Figure 2 is a simplified schematic of the tropospheric HOX cycle for high-NOX conditions. It is this set of reactions that is largely responsible for the oxidation of most volatile organic compounds (VOCs) in the troposphere, the formation of tropospheric ozone, and the chemical coupling between the nitrate, sulfate, carbon, and heavy-metal reservoirs linking urban-to-regional toxicity. Thus, a detailed, quantitative understanding of the individual steps of the cycle is important for our understanding of many of the processes discussed in the Introduction.

HOx Reaction Cycle

The rate-limiting step in the cycle is the attack of a hydrocarbon (HC) by the hydroxyl radical, OH. The resulting alkyl radical (R) will quickly combine with molecular oxygen to form an alkylperoxy radical (RO2). Under high-NOX conditions, RO2 will react with NO to yield either an alkoxy radical (RO) or an organic nitrate (RONO2). The alkoxy radical, which is formed chemically activated, may react via a number of pathways: dissociation forming a smaller radical, isomerization, or reaction with oxygen to form a carbonyl and HO2. Finally, hydroxyl is regenerated by reaction of HO2 with NO. The net result of this cycle is to oxidize the hydrocarbon (oxidation state +4), and to form two molecules of ozone (via NO2 photolysis).

Because of the high reactivity and resulting low concentrations of most radical species, studies of this cycle have relied largely on indirect methods of radical detection, measuring rates of loss or formation of molecular species, or on measurements of one radical in the cycle (typically OH or HO2) to obtain information about the others. In general, these approaches do not allow for the direct examination of individual steps of the above cycle; rather, they extract information about individual species from behavior of the system as a whole.

We propose to examine directly many of the individual steps in isolation, by spectroscopic measurement of radical species and reactive intermediates. This allows for the direct determination of rates and product yields of the individual reactions. Studies will be performed under conditions in which secondary chemistry (gas-phase or heterogeneous) is negligible, by using the complementary methods described above: a supersonic jet expansion coupled with CRDS detection for the preparation and identification of complexes and short-lived species, and our HPF systems for the measurement of radical-molecule reaction rates down to temperatures of 150 K.

Many of the steps in the above cycle present considerable theoretical challenges, as few proceed via simple barriers, instead proceeding via pre-reactive complexes, loose transition states, and short-lived intermediates. In many cases the basic potential energy surface remains largely unknown. By combining pressure- and temperature-dependent measurements with results from statistical-dynamical calculations, we may gain insight into the individual reaction mechanisms, as well as the fundamental principles governing the behavior of short-lived organic radicals.

In particular, our focus is on the four factors that directly control the overall rate and efficiency of the tropospheric HOX cycle:

  1. rates of radical initiation and radical termination reactions, which govern total HOX concentration
  2. rate of attack of hydrocarbons by OH
  3. branching between various product channels available to each radical, particularly RO2 and RO
  4. the possible influence of radical-molecule complexes on rates and branching ratios.

Our current experimental setup (high-pressure flow), suite of sensitive spectroscopic techniques (LIF, ARF, FTIR, CRDS), and theoretical tools (time-dependent RRKM/master equation calculations) put us in an excellent position to examine many of the existing areas of uncertainty within these four topics.