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. 

One can then ask under what conditions the reaction of ROz with NO will be equivalent in rate to that with H02. To make this calculation, one can estimate the concentration of NO at which k23[NO] = k24[H02]. Since k23 k24 1 X 10-11 cm3 molecule-1 s-1, this suggests that the removal of R02 by H02 will compete with that by NO when the concentrations of these two, H02 and NO, are equal. Typical peak H02 concentrations in polluted areas are believed to be of the order of 101J cm-3, corresponding to 40 ppt. Average H02 concentrations may more typically be (1-2) X 108 cm"3, corresponding to concentrations of 4-8 ppt. Such equivalent small concentrations of NO are found in the remote troposphere and, under such conditions, reactions of R02 with H02 or other R02 can become quite important (see Section J.2b). 

Chatfield, R. B., Anomalous HN03/NOr Ratio of Remote Tropospheric Air Conversion of Nitric Acid to Formic Acid and NO, Geophys. Res. Lett., 21, 2705-2708 (1994). 

S. Madronich, HN03/N0, Ratio in the Remote Troposphere during MLOPEX 2 Evidence for Nitric Acid Reduction on Carbonaceous Aerosols Geophys. Res. Lett., 23, 2609-2612... 

To treat the chemistry of oxides of nitrogen, which play such a central role in the chemistry of both the polluted and remote troposphere, in a consistent manner, we have discussed the formation and fates of... 

In summary, there are a variety of methods of measuring NOz that are reasonably accurate for higher concentrations, particularly those found in polluted areas. However, at smaller concentrations found in the remote troposphere, there are significant discrepancies between the various methods. 

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 OH intercomparison results were not of sufficient quantity to allow one to conclude that OH measurements in the clean, remote troposphere can be made with sufficient accuracy or reliability. The near-zero nighttime results from the two LIF techniques, however, do indicate that there are no major problems that can be attributed to artifacts or interference effects for clean, remote tropospheric measurements. Finally, it is of some significance that within the accuracy of the ensemble of OH measurements reported during... 

Atmospheric Oxidation of DMS. DMS reacts fairly rapidly in the atmosphere to produce either SO% which is further oxidized to sulfate, or methane sulfonate (CH3SO3 ) (Figure 1). The relative abundances of the products in the remote troposphere suggest that sulfate is the major atmospheric end product (4 ), although methane sulfonate has been identified under laboratory conditions as a dominant end product (46.47). 

Air estimated tropospheric chemical lifetimes, x = 5 h, 3 d and > 150 d for reactions with OH, N03 and 03, respectively, under typical remote tropospheric conditions (Falbe-Hansen et al. 2000)... 

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

An order of magnitude estimate of the OH concentration in the remote troposphere may be obtained by considering OH to be in a photochemical equilibrium established by reactions (R3), (R4), (R5), (R6), (R7) and that no regeneration of OH from HO2 occurs. Under these assumptions. 

While Eqn. (1) predicts OH levels in the remote troposphere in reasonably good agreement with the predictions of more elaborate photochemical models which properly treat the HO2/OH coupling, for conditions appropriate for less remote regions where enhanced NOx levels are commonly encountered Eqn. (1) does not accurately calculate the OH concentrations. This is because as NOx levels increase, a greater fraction of the HO2 radicals produced from the methane oxidation reaction sequence react with NO via (R8) to regenerate OH. Thus as illustrated in Figure 4, the levels of OH calculated in a complete photochemical model increase substantially as NOx levels increase from the pptv level (typical of remote marine conditions) to the more polluted ppbv level. For NOx levels in... 

Table 2 - Parameters Used for Order-of-Magnitude Estimate of OH Levels in Remote Troposphere.

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. 

Formation of ammonium nitrate aerosols also affects the global troposphere by transporting NO from polluted regions to remote locations (Horowitz et al., 1998). Gas-phase organic nitrates such as PAN, formed in polluted regions and exported to the remote troposphere, are often a significant source of in remote locations. Because ammonium nitrate is relatively long-lived (with a lifetime of days to weeks, similar to other fine particulates) it can also transport NO to the remote troposphere. 

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). 

Weber RJ, McMurry PH, Mauldin RL, Farmer DJ, Eisele FL, Clarke AD, Kapustin VN (1999) New particle formation in the remote troposphere A comparison of observations at various sites. Geophys Res Lett 26 307-310... 

In fact, a detailed calculation of the CF3O2NO2 half life and its dependence with altitude and latitude shows a maximum half life at aroxmd 15 km. This maximum reaches values as long as 220 days for mid-latitudes allowing the compound to be transported though the atmosphere. Therefore, CF3O2NO2 could be transported either through convective stream to the stratosphere, or to remote tropospheric places where it can dissociate again. 

Thus the net effect of the sequence of Eqs. 22 and 24 would be one of apparent catalysis. Chain reactions are less likely in aquatic systems than they are in atmospheric systems, but there is strong evidence that free chain reactions involving S(IV) may be taking place in cloudwater droplets, especially in the remote troposphere where H202 is in short supply. 

Oxidation of methane is one of the sources of atmospheric CO. Another internal source of importance is the oxidation of terpenes and isoprenes emitted by forests (Crutzen, 1983). The carbon monoxide concentration in the atmosphere ranges from 0.05 to 0.20 ppmv in the remote troposphere (with considerable differences between the northern and southern hemispheres), which means that about 0.2 Pg of carbon is present as CO in the atmosphere. 

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