Ozone remote troposphere

R is hydrogen, alkenyl, or alkyne. In remote tropospheric air where NO concentrations ate sometimes quite low, HO2 radicals can react with ozone (HO2 + O3 — HO + 2 O2) and result in net ozone destmction rather than formation. The ambient ozone concentration depends on cloud cover, time of day and year, and geographical location. 

In remote tropospheric air, where NO concentrations can be quite low (17), the HO + CO oxidation mechanism can follow other pathways, leading to net ozone destruction rather than formation (18, 19). Reactions 1 through 5 typify the more complex catalytic reactivity of HO with hydrocarbons, which produce a complex array of oxidation products while generating ozone pho-tochemically (11-13). 

The net result of methane oxidation in the remote troposphere by hydroxyl radical produces 3 molecules of ozone for each molecule oxidized. 

In fact, evidence gathered by Seiler and Fishman (1981), appears to support the hypothesis that ozone can be produced photochemicaUy in the remote troposphere. They found, for instance, that in several cases measurements in the free troposphere (the region above 1 km) show concentrations of ozone that are correlated positively with levels of carbon monoxide, a photochemical precursor of ozone. 

NO the critical precursor for ozone formation, typically has daytime concentrations of 5-20 ppb in urban areas, 0.5-1 ppb in polluted rural areas during region-wide events, and lO-lOOppt in the remote troposphere. An NO t concentration of 1 ppb is associated with ozone formation at rates of 2-5 ppb h which is fast enough to allow ozone concentrations to increase to 90 ppb when air stagnates in a polluted region for two days or more. Ozone production rates as high as 100 ppb h have been observed in urban locations (e.g., in the recently completed Texas Air Quality Study in Houston (Kleinman et al, 2002)). 

Production rates are much slower in the free troposphere, and loss usually exceeds production. However, NO concentrations of 100 ppt, which are much too small to allow the formation of episodic high ozone levels, would still allow ozone to remain at a steady-state concentration of —80 ppb. As of early 2000s, the level of background ozone in the lower troposphere (20-40 ppb) is closely related to the photochemical steady state, achieved over several months, based on concentrations of NO t and organics in the remote troposphere. 

An analogous split between NOj -sensitive and NOjc-saturated chemistry occurs in the remote troposphere, but the implications are somewhat different. Increased CO and VOCs always contribute to increased ozone in the remote troposphere, even under NO -sensitive conditions (Jaegle et at, 1998, 2001), whereas ozone in polluted regions with NOj -sensitive chemistry is largely insensitive to CO and VOCs. 

The ambient ratio NO2/NO is controlled by a combination of the interconversion reactions (Equations (11) and (5)) and the ozone-producing reactions (3) and (4). Because the ozone-producing reactions involve conversion of NO to NO2 and affect the ratio NO2/NO, measured values of this ratio can be used (especially in the remote troposphere) to identify the process of ozone formation. When the ratio NO2/NO is higher than it would be if determined solely by reactions (5) and (11), it provides evidence for ozone formation (e.g., Ridley et al., 1992). Reactions (3)-(5), and (11) can be combined to derive the summed concentration of HO2 and RO2 radicals from measured O3,... 

NO c-saturated photochemical regimes in the remote troposphere. However, most analyses show that ozone in the remote troposphere would increase in response to increases in either NO , CO, methane (CH4), or various VOCs. Ozone at the global scale is also affected by the complex coupling between these species and OH (e.g.. Wild and Prather, 2000) and shows a very different sensitivity to precursors than in polluted regions. 

The largest sink for alkanes in the atmosphere is reaction with OH and NO3 radicals. The formation of photochemical smog is described in detail in (Chapter 9.11, Sillman). Mono-aromatic hydrocarbons react only slowly with O3 and NO3 radicals in the troposphere. The only important atmospheric processes for mono-aromatic hydrocarbons, and naphthalene and dinaphthalenes are reactions with OH radicals (Atkinson, 1990). The products of these reactions include aldehydes, cresols, and, in the presence of NO, benzylnitrates. Methane can be an important contributor to ozone formation, especially in the remote troposphere, as described in (Chapter 9.11, Sillman). 

Chatfield, R., and H. Harrison (1976). Ozone in the remote troposphere, mixing versus photochemistry. J. Geophys. Res. 81, 421-423. 

Chameides and co-workers (1992) examined the observed concentrations of ozone and its precursors, NOx and VOC, in a variety of tropospheric locations, from remote marine areas to polluted urban regions. Figure 16.38 shows ranges of observed NOx and OH-reactivity adjusted VOC (expressed relative to propene) in four... 

Although only 10% of atmospheric ozone resides in the troposphere (0-15 km altitude) it has a profound impact on tropospheric chemistry. Ozone concentrations in the troposphere vary from typically 20-40 ppb for a remote pristine site to 100-200 ppb in a highly polluted urban environment. Ozone is a reactive molecule, which readily adds to carbon-carbon double bonds [8]. Reaction with ozone provides an important removal mechanism for many unsaturated reactive organic compounds. 

Case Study II — Photochemical control of ozone in the remote marine boundary layer (MBL) - An elegant piece of experimental evidence for the photochemical destruction of ozone comes from studies in the remote MBL over the southern ocean at Cape Grim, Tasmania (41 In the MBL, the photochemical processes are coupled to physical processes that affect the observed ozone concentrations, namely deposition to the available surfaces and entrainment from the free troposphere. The sum of these processes can be represented in the form of an ozone continuity equation (a simplified version of Equation 2.6), viz... 

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