Formaldehyde mixing ratios

Lowe and Schmidt (1983) have measured formaldehyde over the Atlantic Ocean as a function of latitude. Their results are shown in Fig. 4-6. In the region 33°S-40°N the data scatter around a mean mixing ratio of wn(HCHO) = 0.22 ppbv. Further northward they decline. The lifetime of formaldehyde due to photodecomposition by sunlight is about 5 h at midlatitudes, and 15 h when averaged over a full day (Warneck et al., 1978). In addition, formaldehyde reacts with OH. Where the oxidation of methane is the sole source, formaldehyde mixing ratios are expected to be in steady state with methane [dm(HCHO)/dr = 0] ... 

Inomata, S.,Tanimoto, H., Kameyama, S., Tsunogai, U., Irie, H., Kanaya, Y., Wang, Z. (2008) Technical note determination of formaldehyde mixing ratios in air with PTR-MS laboratory experiments and field measurements. Atmospheric Chemistry and Physics, 8,273-284. 

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). 

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,... 

Wet-chemical analysis and optical methods have been used to determine the abundance of formaldehyde in ambient air. Table 4-9 shows mixing ratios observed at various locations. The highest values are associated with urban air, especially under conditions of photochemical smog. Maximum values may then reach 70-100 ppbv. Mixing ratios in rural areas are of the order of a few ppbv, and still lower values are found in marine air masses. 

Table 4-9. Mixing Ratios of Formaldehyde Observed at Urban, Rural Continental, and Maritime Locations...

At night the thermal decomposition of PAN in an atmosphere with a relatively high mixing ratio of NO (e.g.,10 ppb) can initiate reaction 5.96 that converts NO to NO2 and produces a methylperoxy radical, which itself may convert another NO to NO2 and produce a molecule of formaldehyde, reaction 5.45. This sequence could lead to a reservoir of photo-chemically active species when the Sun rises. 

However, it has to be considered that it is neither the content of free formaldehyde itself nor the molar ratio which eventually should be taken as the decisive and the only criterion for the classification of a resin concerning the subsequent formaldehyde emission from the finished board. In reality, the composition of the glue mix as well as the various process parameters during the board production also determine both performance and formaldehyde emission. Depending on the type of board and the manufacturing process, it is sometimes recommended to use a UF-resin with a low molar ratio F/U (e.g. F/U = 1.03), hence low content of free formaldehyde, while sometimes the use of a resin with a higher molar ratio (e.g. F/U = 1.10) and the addition of a formaldehyde catcher/depressant will give better results [17]. Which of these two, or other possible approaches, is the better one in practice can only be decided in each case by trial and error. 

Addition of melamine in various forms (pure melamine, MF/MUF-powder resin, melamine acetates) to an UF-resin during the application of the glue mix. In the case of the addition of pure melamine the UF-resin must have a rather high molar ratio, otherwise there is not enough formaldehyde available to react with the melamine in order to incorporate it into the resin. 

The fixative system generally used is a two-vial technique with one vial containing 5 to 10% buffered Formalin and the other vial containing polyvinyl alcohol (PVA) fixative. A portion of the specimen is added to the fixative in a ratio of approximately 3 parts fixative to 1 part specimen and thoroughly mixed to ensure adequate fixation. An alternative to Formalin is Merthiolate-iodine-formaldehyde (MIF), which fixes and stains at the same time. If unfixed specimens are processed in the laboratory, fecal films may be prepared and immediately fixed in Schaudinn fixative. 

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