Troposphere formaldehyde

Similar to several other trace species, formaldehyde (HCHO) occurs in elevated concentrations as a result of pollution in the lower troposphere. Formaldehyde is generated during the oxidation of hydrocabons in copious amounts. However, as it is photolysed and reacts with OH, it has a relatively short tropospheric lifetime. The observation of formaldehyde from GOME over industrial and biomass burning regions has been discussed by Pemer et al. (1998) and Burrows et al. (1999). Thomas et al. (1998) also observed formaldehyde over Borneo using GOME data. 

Lowe DC, Schmidt U, Ehhalt DH. 1980. A new technique for measuring tropospheric formaldehyde [CH2O]. Geophys Res Lett 7 825-828. 

Dufour, G, S. Szopa, M.P. Barkley, C.D. Boone, A. Perrin, PI. Palmer, and P.F. Bernath (2009), Global upper-tropospheric formaldehyde seasonal cycles observed by the ACE-FTS satellite instrument, Atmos. Chem. Phys. Disc., 9, 1051-1095... 

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. 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

Photolytic. Irradiation of vinyl chloride in the presence of nitrogen dioxide for 160 min produced formic acid, HCl, carbon monoxide, formaldehyde, ozone, and trace amounts of formyl chloride and nitric acid. In the presence of ozone, however, vinyl chloride photooxidized to carbon monoxide, formaldehyde, formic acid, and small amounts of HCl (Gay et al, 1976). Reported photooxidation products in the troposphere include hydrogen chloride and/or formyl chloride (U.S. EPA, 1985). In the presence of moisture, formyl chloride will decompose to carbon monoxide and HCl (Morrison and Boyd, 1971). Vinyl chloride reacts rapidly with OH radicals in the atmosphere. Based on a reaction rate of 6.6 x lO" cmVmolecule-sec, the estimated half-life for this reaction at 299 K is 1.5 d (Perry et al., 1977). Vinyl chloride reacts also with ozone and NO3 in the gas-phase. Sanhueza et al. (1976) reported a rate constant of 6.5 x 10 cmVmolecule-sec for the reaction with OH radicals in air at 295 K. Atkinson et al. (1988) reported a rate constant of 4.45 X 10cmVmolecule-sec for the reaction with NO3 radicals in air at 298 K. 

Iraci, L. T and M. A. Tolbert, Heterogeneous Interaction of Formaldehyde with Cold Sulfuric Acid Implications for the Upper Troposphere and Lower Stratosphere, . /. Geophys. Res., 102, 16099-16107 (1997). 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO" are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO", particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO" in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

Formaldehyde has been detected recently in the interstellar medium by microwave spectroscopy (593), It is a combustion product of hydrocarbons. The photolysis of H2CO by sunlight in the troposphere may produce H02 radicals by reactions such as... 

Tracers of photochemical reactions include low-molecular-weight compounds, such as formaldehyde, pyruvate, and acetylaldehyde. The rates of these photochemical reactions are important to measure so that natural degradation of DOM can be quantified. Also, their variability due to increased ultraviolet radiation (from decreases in tropospheric ozone levels) should be studied. The ChemRawn IV conference had a major focus on photochemical reactions (Goldberg, 1988). 

Formaldehyde, HCHO, photolysis [64-76] provides a major source of free radicals in troposphere [2] (Fig. 7). It has a highly structured UV-vis ab-... 

The temperature and density structure of the troposphere, along with the concentrations of major constituents, are well documented and altitude profiles have been measured over a wide range of seasons and latitudes for the minor species water, carbon dioxide, and ozone. A few profiles are available for carbon monoxide, nitrous oxide, methane, and molecular hydrogen, while only surface or low-altitude measurements have been made for nitric oxide, nitrogen dioxide, ammonia, sulfur dioxide, hydrogen sulfide, and nonmethane hydrocarbons. No direct measurements of nitric acid and formaldehyde are available, though indirect information does exist. The concentrations of a number of other important species, such as peroxides and oxy and peroxy radicals, have never been determined. Therefore, while considerable information concerning trace constituent concentrations is available, the picture is far from complete. 

On the basis of ratios of C and C present in carbon dioxide, Weinstock (250) estimated a carbon monoxide lifetime of 0.1 year. This was more than an order of magnitude less than previous estimates of Bates and Witherspoon (12) and Robinson and Robbins (214), which were based on calculations of the anthropogenic source of carbon monoxide. Weinstock (250) suggested that if a sufficient concentration of hydroxyl radical were available, the oxidation of carbon monoxide by hydroxyl radical, first proposed by Bates and Witherspoon (12) for the stratosphere, would provide the rapid loss mechanism for carbon monoxide that appeared necessary. By extension of previous stratospheric models of Hunt (104), Leovy (150), Nicolet (180), and others, Levy (152) demonstrated that a large source of hydroxyl radical, the oxidation of water by metastable atomic oxygen, which was itself produced by the photolysis of ozone, existed in the troposphere and that a chain reaction involving the hydroxyl and hydroperoxyl radicals would rapidly oxidize both carbon monoxide and methane. It was then pointed out that all the loss paths for the formaldehyde produced in the methane oxidation led to the production of carbon monoxide [McConnell, McElroy, and Wofsy (171) and Levy (153)1-Similar chain mechanisms were shown to provide tropospheric... 

The temperature profile strongly influences those reactions whose rate coefficients have large activation energies. As will be shown in Sections IV, V, and VI, a number of reaction paths, while dominant in the lower troposphere, lose their importance with increasing altitude as the temperature drops sharply. Particularly affected are the altitude profiles of the hydroxyl radical, formaldehyde, and nitric oxide number densities. 

Dhar and Ram (50) found formaldehyde in rain water and estimated a tropospheric mixing ratio of 0.7 ppb, while Cauer (36) measured a mean value of 0.4 ppb. Lodge and Pate (160) obtained an average value of 1.1 ppb for the total aliphatic aldehydes in surface air in the tropics. Levy (152) proposed the formation of formaldehyde via the tropospheric oxidation of methane and calculated (155) an upper limit of 1 ppb for the mixing ratio, with an altitude profile for a summer midlatitude decreasing from 0.6 ppb at the ground to less than 0.01 ppb in the upper troposphere, where methane oxidation is very slow (154). 

Molecular hydrogen is assumed to be well mixed in the troposphere, with a mixing ratio of 0.4 to 0.6 ppm [Junge (128) and Scholz, Ehhalt, Heidt, and Martell (219)]. Koyama (142) found that swamps and paddies are very small natural sources. Levy (153) proposed both an atmospheric source (photodissociation of formaldehyde) and an atmospheric sink (oxidation by hydroxyl radical). From daily average number densities for the hydroxyl radical and a daily average hydrogen production rate,... 

Formaldehyde is formed in the atmospheric degradation of virtually all hydrocarbons. Its photolysis has an important effect on the atmosphere s oxidation capacity since it is a significant source of HOx radicals in the middle and upper troposphere, and in polluted regions [135] ... 

A. Stickler, et ah. Influence of summertime deep convection on formaldehyde in the middle and upper troposphere over Europe, /. Geophys. Res.-Atmos. Ill (D14) (2006) D14308. 

Sumner A. L. and Shepson P. B. (1999) Snowpack production of formaldehyde and its effect on the Arctic troposphere. Nature 398, 230-233. 

Methane is oxidized primarily in the troposphere by reactions involving the hydroxyl radical (OH). Methane is the most abundant hydrocarbon species in the atmosphere, and its oxidation affects atmospheric levels of other important reactive species, including formaldehyde (CH2O), carbon monoxide (CO), and ozone (O3) (Wuebbles and Hayhoe, 2002). The chemistry of these reactions is well known, and the rate of atmospheric CH4 oxidation can be calculated from the temperature and concentrations of the reactants, primarily CH4 and OH (Prinn et al., 1987). Tropospheric OH concentrations are difficult to measure directly, but they are reasonably well constrained by observations of other reactive trace gases (Thompson, 1992 Martinerie et al., 1995 Prinn et al., 1995 Prinn et al., 2001). Thus, rates of tropospheric CH4 oxidation can be estimated from knowledge of atmospheric CH4 concentrations. And because tropospheric oxidation is the primary process by which CH4 is removed from the atmosphere, the estimated rate of CH4 oxidation provides a basis for approximating the total rate of supply of CH4 to the atmosphere from aU sources at steady state (see Section 8.09.2.2) (Cicerone and Oremland, 1988). 

Formaldehyde concentrations in jet engine exhaust have been found to range from 0.761 to 1.14 ppm (Miyamoto 1986). Formaldehyde is formed in large quantities in the troposphere by the oxidation of hydrocarbons (Calvert et al. 1972 WHO 1989) leading to elevated formaldehyde levels shortly after periods of high vehicular traffic (Grosjean et al. 1996). 

Air (tropospheric) Drawing of fdtered air through a nebulization/rcflux concentrator (scrubber) at rate of 7-8 L/min where formaldehyde is reacted to form DNPH derivative. HPLC/UV 0. 12 g/m (0.1 ppb) 90-96 Cofer and Edahl 1986... 

Regardless of the chemical path taken (with the exception of the precipitation process in the troposphere), the carbon atom of the methane molecule must eventually end up as formaldehyde. It should... 

In the troposphere and in the lower stratosphere, the chemical production of H2 is due to the photolysis of formaldehyde produced by methane oxidation. About 13 Tg H2/yr are produced by this mechanism (Schmidt et al., 1980). Oxidation of isoprene and other terpenes in the lower troposphere leads to an additional production of about 10-35 Tg H2/yr. A precise determination of these quantities requires a... 

Aldehydes are emitted by combustion processes and also are formed in the atmosphere from the photochemical degradation of other organic compounds. Aldehydes undergo photolysis, reaction with OH radicals, and reaction with N03 radicals in the troposphere. Reaction with N03 radicals is of relatively minor importance as a loss process for these compounds, but can be a minor contributor to the H02 (from formaldehyde) and peroxyacetyl nitrate (PAN) formation during nighttime hours (Stockwell and Calvert, 1983 Cantrell et al., 1985). Thus, the major loss processes involve photolysis and reaction with OH radicals. 

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