Ozone production efficiency

An important measure of atmospheric ozone-forming oxidation cycles is the ozone production efficiency (OPE). Because NO gets cycled back and forth between NO and N02 in O3 generation, NO can be viewed as the catalyst in 03 formation. One can define the OPE as the number of molecules of 03 formed per each NO removed from the system. The rate of production of 03 is given by (6.9)  

Equation (6.23) can be combined with (6.10) to eliminate [OH] and yield a quadratic equation for [H02] 

Dependence of P0, on NO, Abundance in CO Oxidation One of the key aspects of tropospheric chemistry is the dependence of ozone production on the NO, abundance. We have derived relationships for Po, for the CO system in the limits of low and high NO,. Here we examine how Po, depends on the NO, abundance over the complete range of NO, levels. To do this, we will fix the rate of HO, production, Pho,. and vary the NO concentration at a fixed N02/NO ratio. Under conditions of high H02 radical abundance relative to NO, the primary chain-terminating reaction is the HO, + HO, reaction, HO2 + H02. This condition is referred to as NOx-limited. At sufficiently high NO, levels, chain termination results from the HO, + NO, reaction, OH 4- N02. This condition is called NOx-saturated. By varying the NO, concentration, we can explore the point at which the system crosses over from NO,-limited to NO,-saturated conditions. The crossover point occurs at the NO concentration where 3Poj/3[NO] = 0. The actual value of the NO concentration at this crossover point depends on the values of Pho, and the NO2/NO ratio. 

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Because ozone formation occurs only within these microdischarge channels, ozone-production efficiency for the most part depends on the strength of the microdischarges, which is influenced by a number of factors such as the gap width, pressure, properties of the dielectric and metal electrode, power... 

Folkins, I R. Chatfield, H. Singh, Y. Chen, and B. Heikes, Ozone Production Efficiencies of Acetone and Peroxides in the Upper Troposphere, Geophys. Res. Lett., 25, 1305-1308 (1998). 

A slightly modified approach has also been used in a number of studies in which the sum of (03 + N02) is plotted against NOz (e.g., St. John et al., 1998). This minimizes the effects of short-term variations in 03 caused by its rapid reaction with NO. Thus, when 03 is titrated by the NO reaction, the measured 03 concentrations will be small however, the N02 generated is a source of 03 through its subsequent photolysis. Flence the sum of (03 + N02) is sometimes chosen as a measure of the ultimate formation of ozone. The ozone production efficiency determined from slopes of plots of (03 + N02) against NOz in the Nashville, Tennessee, area was measured to be typically 5-6 if it was assumed that NOr is not removed by other processes. The production efficiency appeared to be about the same for the general urban plume and for an air mass in which a plume from a power plant was also embedded. Including other losses for NOy such as deposition lowers the estimated production efficiency by about a factor of two (St. John et al., 1998 Nunnermacker et al., 1998). 

The central concepts of ozone photochemistry are the split into NOjc-sensitive and NO -saturated photochemical production regimes, the role of the OH radical, and the concept of ozone production efficiency per NOj. ... 

Figure 6 Ozone production efficiency, expressed as the rate of production of odd oxygen (O3 -h NO2) divided by the loss of NO . from steady state calculations. The calculations assume (i) CO and CH4 only (solid line) (ii) anthropogenic VOC with VOC/ NO = 10 (dashed line) (iii) anthropogenic VOC with VOC/NO c = 20 (short dashed line) (iv) CH, CH4 and 1 ppb isoprene (circles) and (v) anthropogenic VOC/NO = lOand 1 ppb isoprene (asterisks) (source Sillman, 1999 (reproduced by permission of Elsevier from Atmos. Environ. 1999, 33, 1821-1845) based on similar unpublished analyses by Greg Frost, NOAA Aeronomy lab).

Relation of 03 to NO, From the definition of the ozone production efficiency, the signature of an NO molecule lost is the appearance of a number of 03 molecules, the specific number depending on atmospheric conditions and the HC and NO levels. Thus, the 03 concentration attained in an airmass should be correlated with the quantity [NO,]-[NO ], which is the total concentration of products of NO oxidation (HNO3, PAN, etc.). That this correlation should exist in airmasses was first pointed out by Trainer (1991, 1993), and it has been subsequently pursued in numerous studies [e.g., Kleinman (1994, 1997), CarpenteT et al. (2000)]. To obtain a good correlation between [03] and [NO,]-[NO ], O3 production must have occurred within a day or so in the airmass, before significant removal of NOv can take place, for example by wet and dry deposition of HN03. 

FIGURE 6.11 Comparison of ozone production efficiencies on high O3 days in Phoenix, Arizona and Houston, Texas. (Courtesy Lany i. Kleinman. Brookhaven National Laboratory.)... 

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