Ozone and Other Photochemically Derived Species

Isopleths for Ozone and Other Photochemically Derived Species 

Low VOC/NO, ratios, e.g., point D, have been found to be typical of polluted air masses found in many major urban centers, e.g., downtown Los Angeles (DTLA). Here, reducing VOC at constant NO, along the line DE results in rolling down the ozone hill. However, reducing NO, at constant VOC, along the 

FIGURE 16.14 Typical peak ozone isopleths generated front initial mixtures of VOC and NO, in air. (a) Two-dimensional depiction generated from the EKMA model (Dodge, 1977a) (b) three-dimensional depiction prepared by B. Dickerson. The VOC-liniited region (e.g., at point D) is found in some highly polluted urban centers while the NOA (e.g., at point A) is typical of downwind suburban and rural areas (adapted from Finlayson-Pitts and Pitts, 1993). 

This complex behavior is due to several effects. First, the reaction of 03 with NO is rapid, so that 03 is effectively titrated by the high NO concentrations. For example, measurements made in Nashville, Tennessee, show that during the early to mid-morning hours, CO and NO increased due to emissions, while 03 decreased to almost zero due to the 03-N0 reaction (Nunnermacker et al., 1998). Decreased 03 concentrations in the Vancouver, Canada, region have also been attributed to this reaction (Pryor, 1998). 

A second reason for the response of 03 to NOx at low VOC/NOx ratios is that at these high NOx concentrations, N02 competes with VOC for the OH radical by forming HN03 (reaction (13)). This terminates the chain oxidation of VOC and removes N02 from the system without forming 03. This chemistry has been confirmed by direct OH radical measurements thus, Eisele et al. (1997) report that OH concentrations increase with the NO concentration up to l-2 ppb but decrease thereafter due to the OH + N02 reaction. This is consistent with model calculations of isopleths of OH concentrations as a function of the VOC/NOx ratio, similar to the ozone isopleths in Fig. 16.14 (e.g., Kley, 1997) in the high NOx-low VOC 

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