NMHC, Nonmethane hydrocarbons

NMHC (nonmethane hydrocarbons) Oxidation capacity Photochemistry 2.1 Tg 0.2... 

NMHC = Nonmethane hydrocarbons, i.c., all hydrocarbons excluding methane. 

Fig. 7.1 Concentrations of CO2, CO, and various other gases in the smoke from an experimental fire of Trachypogon grass from Venezuela as a function of time and the stack gas temperature. The dotted line separates the flaming phase from the smoldering phase. The flaming stage in this fire lasted for about 96 s. Concentrations are in percent by volume for CO2, in volume mixing ratios (ppm) for the other species (1 % = 10,000 ppm). Note that CO2, NOx, SO2, and N2O are mainly emitted in the flaming phase and the other gases in the smoldering phase NMHC, nonmethane hydrocarbons...

Many GTL-derived fuels are being considered for blending with gasoline and diesel to achieve emission reductions of particulate matter (PM), carbon monoxide (CO), nitrogen compounds (NOx) and nonmethane hydrocarbons (NMHC). The most promising fuels converted from natural gas are methanol and ethers such as dimethyl ether (DME) and mcthyl-t-bntyl ether (MTBE). 

Methane and the Nonmethane Hydrocarbons. It is traditional to distinguish CH4 from all other atmospheric hydrocarbons. Methane is by far the most abundant atmospheric hydrocarbon and has very large natural emissions. Its abundance in auto exhaust but low atmospheric reactivity has led air pollution scientists to enact controls on nonmethane hydrocarbons NMHC (also called VOC for volatile organic compounds, which include oxygenated hydrocarbons). 

A definition based upon the NMHC concentration might require nonmethane hydrocarbons to contribute insignificantly to atmospheric reactivity. Such a region might still contain very low concentrations of nonmethane hydrocarbons, so long as their sum total reactivity does not significantly contribute to that of methane and carbon monoxide. This is to say, in clean air... 

At levels of nonmethane hydrocarbons where their reaction rates with HO are a significant fraction of the reaction rate of HO with CO and methane, it may happen that the clean-air HO concentration remains unchanged. This would result if the increase in HO removal by NMHC s is compensated by increasing HO sources such as aldehyde or ketone photolysis and reactions such as R21. These considerations are examined below. 

Explicit mechanisms attempt to include all nonmethane hydrocarbons believed present in the system with an explicit representation of their known chemical reactions. Atmospheric simulation experiments with controlled NMHC concentrations can be used to develop explicit mechanisms. Examples of these are Leone and Seinfeld (164), Hough (165) and Atkinson et al (169). Rate constants for homogeneous (gas-phase) reactions and photolytic processes are fairly well established for many NMHC. Most of the lower alkanes and alkenes have been extensively studied, and the reactions of the higher family members, although little studied, should be comparable to the lower members of the family. Terpenes and aromatic hydrocarbons, on the other hand, are still inadequately understood, in spite of considerable experimental effort. Parameterization of NMHC chemistry results when NMHC s known to be present in the atmosphere are not explicitly incorporated into the mechanism, but rather are assigned to augment the concentration of NMHC s of similar chemical nature which the... 

S02 and nonmethane hydrocarbons (NMHC) were removed, 67-78% and 17-20%, respectively, by the windowed panels. For NMHC, it is assumed that the fractions of lower aldehydes and olefins are removed, since Ti02 degrades such fractions much more rapidly than saturated and aromatic hydrocarbons. 

The useful life of the emissions control system is expected to be five years or 80,000 km, unless otherwise noted, k NMHC = nonmethane-reactive hydrocarbons. 

In reality, the situation is somewhat more complicated owing to the presence at high concentrations of NO , of increased levels of nonmethane hydrocarbons (NMHCs), especially in places such as the urban... 

FIGURE 8 Changes in nonmethane hydrocarbon (isoprene) emissions predicted to accompany a shift from savanna grassland to a savanna woodland at the La Copita site in southern Texas (based on Guenther et al, 1999). Predictions from a coupled succession-NMHC emission model are compared with values measured from flux towers. The measured values shown for the historic landscape are from a tower located in a savanna grassland landscape with low woody cover. 

In addition to these compounds, the photochemical production of small amounts of nonmethane hydrocarbons (NMHC) such as ethene, propene, ethane, and propane has also been reported. Production of these compounds appears to result from the photolysis of the CDOM, with O values of the order of 10 -10 . The overall... 

Figure 2 Simplified scheme of marine nonmethane hydrocarbon (NMHC) production. In the marine troposphere NMHC acts as a sink for hydroxyl (OH) radicals and thereby plays a key role in ozone chemistry.

Chan LY, Chu KW, Zou SC, Chan CY. Wang XM, Barletta B, Blake DR. Guo H. Tsai WY (2006) Characteristics of nonmethane hydrocarbons (NMHCs) in industrial, industrial-urban, and industrial-suburban atmospheres of the Pearl River Delta (PRD) region of south China. J Geophys Res Atmos 111 011304... 

For the purpose of EKMA modeling, hydrocarbons include only nonmethane hydrocarbons (NMHC), or nonmeihane organic carbon (NMOC) methane is excluded due to its comparatively low reactivity. The concentration of NMHC in an air sample is usually expressed in terms of the moles of carbon per mole of air, denoted ppmC or ppbC (parts per million carbon or parts per billion carbon). For any hydrocarbon, its air concentration in ppm(v) or ppb(v) can be converted to ppmC or ppbC by... 

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