Oxidation, troposphere aqueous phase

Herrmann, H., H.-W. Jacobi, G. Raabe, A. Reese, Th. Umschlag and R. Zellner Free radical reactions in the tropospheric aqueous phase, in B. Larsen, B. Versino, G. Angeletti, (eds). The Oxidizing Capacity of the... 

When NMHC are significant in concentration, differences in their oxidation mechanisms such as how the NMHC chemistry was parameterized, details of R02-/R02 recombination (95), and heterogenous chemistry also contribute to differences in computed [HO ]. Recently, the sensitivity of [HO ] to non-methane hydrocarbon oxidation was studied in the context of the remote marine boundary-layer (156). It was concluded that differences in radical-radical recombination mechanisms (R02 /R02 ) can cause significant differences in computed [HO ] in regions of low NO and NMHC levels. The effect of cloud chemistry in the troposphere has also recently been studied (151,180). The rapid aqueous-phase breakdown of formaldehyde in the presence of clouds reduces the source of HOj due to RIO. In addition, the dissolution in clouds of a NO reservoir (N2O5) at night reduces the formation of HO and CH2O due to R6-RIO and R13. Predictions for HO and HO2 concentrations with cloud chemistry considered compared to predictions without cloud chemistry are 10-40% lower for HO and 10-45% lower for HO2. 

Numerous field studies of the rate of S02 oxidation in the troposphere have shown that the oxidation rate depends on a number of parameters. These include the presence of aqueous phase in the form of clouds and fogs, the concentration of oxidants such as H202 and... 

As we shall see in the following sections, these observations are readily understood in terms of the kinetics and mechanisms of oxidation of S02. The oxidation of S02 occurs in solution and on the surfaces of solids as well as in the gas phase. Indeed, under many conditions typical of the troposphere, oxidation in the aqueous phase provided by clouds and fogs predominates, consistent with the observed dependence on these factors. The presence of oxidizers to react with the S02 is, of course, also a requirement hence the dependence on 03 (which is a useful surrogate for other oxidants as well) and sunlight, which is needed to generate significant oxidant concentrations. 

As discussed in detail in Sections C.3.d and C.3.e, the fastest atmospheric reactions of S02 are believed to be with H202 and perhaps with Os at higher pH values. Under extreme conditions of large fog droplets (—10 yu,m) and very high oxidant concentrations, the chemical reaction times may approach those of diffusion, particularly in the aqueous phase. In this case, mass transport may become limiting. However, it is believed that under most conditions typical of the troposphere, this will not be the case and the chemical reaction rate will be rate determining in the S(IV) aqueous-phase oxidation. 

Much more relevant to the aqueous phase in clouds and fogs in the atmosphere is the catalyzed oxidation of S(IV) by 02. Both Fe3+ and Mn2+ catalyze the oxidation and as described in Chapter 9, both are common constituents of tropospheric aerosols even in remote... 

Although the gas phase provides major pathway for hydroxyl radical and hydrogen peroxide production in the atmosphere, there is overwhelming evidence [158-168] that aqueous phases in the troposphere also provides a significant medium for the photolytic production of these important oxidants. 

The net effect of these two links between sulfur and halogen chemistry is to decrease the gas phase concentration of SO2 via a reduced yield of SO2 from the oxidation of DMS and the stronger aqueous phase sink for S(IV) which results in enhanced uptake of SO2 by droplets and aerosols. A critical prerequisite for new particle formation in the marine troposphere is the reaction chain ... 

Extrapolation to the K/T boundary requires consideration of the time scales of acid deposition. Nitric acid formation occurs rapidly by aqueous phase reaction of NO and NO2 with liquid water produced by tlie incident K/T bolide on both impact and infall of ejecta. For tlie quantities of NO produced by the K/T impact ( 10 5 moles), conversion to HNO3 occurred wiUiin days, assuming sufficient liquid water was available in the posl-K/T atmosphere. The nitric acid will form an acid rain of pH 0 for a liquid water content of 1 g/m (typical of tropospheric clouds) but will contain enough protons to weather only 3 x 10 moles of Sr, for Sr/(Ta -0.003 in soil and bedrock minerals. Sulfuric acid formation occurred on a time scale of years [7] due to the slow rate of gas phase SO2 oxidation. Spread evenly over 10 years, 10 moles of SO2 produced a global acid rain of pH —4, and released —3 x 10 moles of Sr. 

Aqueous phase tropospheric simulation chamber, On-line mass spectrometry, N-methylpyrrolidone, Kinetic rate constants, OH-oxidation, Reaction products. Chemical mechanisms... 

N-methyl-pyrrolidone (NMP) is used in the industry as a solvent. Its atmospheric lifetime is moderate, 13 h, if [OH] = 10 moleeules em (Aschmann and Atkinson, 1999). However, NMP is a highly soluble eompound (Kh = 6.4 10" M atm", Hine and Moorkerjee, 1975), thus it is likely to enter into tropospherie droplets. The present work was aimed at determining the OH-oxidation rate eonstant of NMP and the reaction products formed in the aqueous phase under tropospheric conditions. NMP is a medium sized molecule (Table 1), containing 5 carbon atoms, so its OH-oxidation may give rise to a large number of products. This study presents a new on-line teehnique suitable to identify as many reaetion products as possible, during the reaetion. Two different kinds of experiments were condueted to determine respectively the kineties and the reaetion products formed. 

Based on the results, a possible mechanism is suggested here for the OH oxidation of NMP in the aqueous phase under tropospheric conditions. The reaction probably proceeds via three different pathways (Figure 7). 

Patiiway (c) is more speculative, but it has been previously suggested by Horikoshi et al., 2001, who investigated the OH oxidation of 2P in the aqueous phase, in the presence of Ti02. This pathway may occur for NMP under real tropospheric conditions. It proceeds via a ring-opening mechanism, leading to the formation of N-methyl-4-aminobutanoic acid. 

The reactivity of NMP towards OH radicals was studied in the aqueous phase, under tropospheric conditions. The kinetic results show that the OH oxidation of NMP is fast compared to that of other WSOC, and thus should induce modifications of the composition of water droplets, due to the reaction products formed. A new experimental technique was developed to study the aqueous phase OH oxidation of NMP. A mass spectrometer was coupled to an aqueous phase simulation chamber, thus providing an on-line analysis of the solution. The mass spectrometer was equipped with an electrospray ionisation (ESI) unit and a triple quadrupole, which allowed ESI-MS, ESI-MS, and ESI-MS-MS analysis. The results proved that this experimental technique is highly promising, as it allowed us to detect the formation of 66 reaction products, of which 24 were positively identified. Based on the results obtained, a chemical mechanism has been suggested for the OH oxidation of NMP in the aqueous phase. The developed equipment can be used to study other molecules and other reactions of atmospheric interest. 

In continental clouds, hydrogen peroxide is the most important oxidant of suhur dioxide dissolved in the aqueous phase, contributing about 80% to the total oxidation rate. Ozone and peroxynitric acid oxidize up to 10% each, and the gas-phase reaction of SO2 with OH radicals adds about 3%. Clouds are estimated to occupy about 15% of the airspace in the lower troposphere. In-cloud reactions thus oxidize 70-80% of SO2 in the troposphere, the remaining 20-30% of SO2 is oxidized in the gas-phase by reaction with OH radicals in cloud-free air. 

Iron complex photolysis is mie of the processes that produce reduced iron (Fe(n)) in a highly oxidizing enviromnent like the atmospheric aqueous phase. There are numerous other processes such as reactions with HO species or Cu(I)/ Cu(n) which can reduce or oxidize iron in the troposphere. These reactions can take place simultaneously and cause iron to undergo a so-caUed redox-cycling [167]. Because of the large number of complex interactions in the atmospheric chemistry of the transition metal iron, it is useful to utilize models to assess the impact of the complex iron photochemistry. 

The main fate in the aqueous phase is likely to be oxidation following reaction with radicals, rather than photolysis, because the hydrated form of this compound contains no light-absorbing carbonyl group, HOCH2CH(OH)2 and is not subject to photolysis within the troposphere. Reaction in the aqueous phase leads to multifunctional compounds, such as glycolic and oxalic acids. 

Oxidation rate constant k, for gas-phase second order rate constants, kOH for reaction with OH radical, kNG3 with N03 radical and kG3 with 03 or as indicated, data at other temperatures see reference k0H(calc) = 1.8 x to 12 cm3 molecule-1 s 1 at room temp. (SAR, Atkinson 1987) tropospheric lifetime x(calc) = 8-17 d, based on kOH(calc.) = (1.4 - 2.9) x io 12 cm3 molecule-1 s 1 for dichlorobiphcnyls at room temp. (Atkinson 1987) koH.(aq.) = 7.9 x 109 dm3 mol-1 s-1, PCB in Aroclor 1242 mixture, oxidized by hydroxyl radicals generated with Fenton s reagent in aqueous solutions at 25°C, half-lives range from 4-l 1 d in freshwater systems, 0.1-10 d in cloud water, > 1000 d in oceans for PCBs with as many as 8 chlorines (relative rate method, Sedlak Andren 1991) tropospheric lifetime x(calc) = 3.4-7.2 d, based on the experimentally determined gas-phase reaction kOH(exptl) = (2.0-4.2) x io-12 cm3 molecule-1 s-1, and the calculated kOH(calc) = (1.4 - 3.1) x io-12 cm3 molecule-1 s 1 at room temp. (Kwok et al. 1995)... 

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