Nonmethane hydrocarbons

Fig. 8. isopleth diagram showing the response of concentrations to changes in initial NO and nonmethane hydrocarbon concentrations expressed as parts per million of carbon atoms (ppm C). The response to NO reductions is dependent on the particular initial concentrations. At point A, the graph indicates that decreasing NO would increase formation, and at point C, a decrease in the NO concentration results in a much larger decrease in... 

Fenner (11) has pointed out that short-lifetime constituents of the atmosphere such as nitrogen oxides, carbon monoxide, and nonmethane hydrocarbons may also play roles related to global warming because of their chemical relations to the longer-lived greenhouse gases. Also, SO, with a very short life interacts with ozone and other constituents to be converted to particulate sulfate, which has effects on cloud droplet formation. 

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). 

HO oxidation of CO is much faster than the reaction with methane, resulting in a mean CO lifetime of about two months, but considerably slower than reaction with the majority of the nonmethane hydrocarbons. Table I gives representative removal rates for a number of atmospheric organic compounds their atmospheric lifetimes are the reciprocals of these removal rates (see Equation E4, below). The reaction sequence R31, R13, R14, R15 constitutes one of many tropospheric chain reactions that use CO or hydrocarbons as fuel in the production of tropospheric ozone. These four reactions (if not diverted through other pathways) produce the net reaction... 

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... 

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

It appears from these data that the O standard (i.e., 0.08 ppm for 1 h] could not be met if the aldehydes remained high, ICH,0] 0.10, (CH,CH01 0.06 ppm, even if nearly all of the olefrnic hydrocarbon were removed.. .. We should learn from these data that the true relationship between nonmethane hydrocarbons and maximum 1-h oxidant at low hydrocarbon levels could be a critical function of a variable which is not routinely measured now, namely the concentration of the impurity aldehydes. 

Hydrocarbon impurities y y < 0.6 ppm as hexane Produce ozone at < 3 ppb Nonmethane hydrocarbon at 1 ppb Ambient (high)... 

Tunney s Pasture in Ottawa in 1973, in which oxidant concentrations approached 0.14 ppm. This episode took place on a July weekend, and the nitrogen dioxide and NOx concentrations appear to be very small. Nonmethane hydrocarbons, however, were relatively high, about 0.6 ppm. 

FIGURE 4-7 Ozone, nitrogen dioxide, NO, and nonmethane hydrocarbon concentrations in pollution episode at Tunney s Pasture, Ontario. Redrawn from Quickert et al. ... 

Ozone and ozone precursor concentrations at nonurban locations in the eastern United States were studied extensively. The three parts of the study were field measurements, a quality assurance program, and an airborne monitoring program. The main objective of the study was to establish a data base for nonurban ozone and precursor concentrations. Simultaneous statistical summaries of the concentrations of nitrogen dioxide and nonmethane hydrocarbons were also provided. Another objective was to search for relationships between ozone concentrations and nitrogen dioxide and nonmethane hydrocarbon concentrations. 

For the measurement of the hydrocarbon precursors of photochemical oxidants, the naturally occurring methane must be separated from the other so-called nonmethane hydrocarbons. Only one procedure, gas chromatography coupled with flame ionization detection, is available for this separation and measurement. Although instrumentation for routinely accomplishing this process is commercially available, its maintenance (continued operation) requires a degree of operational know-how that may be too costly for most public agencies in the United States to support. Consequently, the data currently are insufficient to relate the occurrence of photochemical oxidants and ozone accurately to some of their most important precursors, the nonmethane hydrocarbons. 

Thus, despite the remarkable progress in the monitoring for ozone, nitrc en oxides, and nonmethane hydrocarbons, which has strengthened the implementation and evaluation of control programs, substantial research and development are still required to help resolve the uncertainties that are inhibiting the actual achievement of desired air quality standards. 

Nonmethane hydrocarbons and both oxides of nitrogen should be monitored concurrently whenever photochemical oxidant or ozone is monitored. 

Inherently low exhaust emission levels of nonmethane hydrocarbons and air pollutants such as benzene... 

Bonsang, B D. Martin, G. Lambert, M. Kanakidou, L. C. Le Roulley, and G. Sennequier, Vertical Distribution of Nonmethane Hydrocarbons in the Remote Marine Boundary Layer, J. Geophys. Res., 96, 7313-7324 (1991). 

T. Smith, Jr., R. V. Arrieta, R. Rodriquez, and J. W. Birks, Vertical Profiling and Determination of Landscape Fluxes of Biogenic Nonmethane Hydrocarbons within the Planetary Boundary Layer in the Peruvian Amazon, J. Geophys. Res., 103, 25519-25532 (1998a). 

Jaffe, D. A., T. K. Berntsen, and I. S. A. Isaksen, A Global Three-Dimensional Chemical Transport Model. 2. Nitrogen Oxides and Nonmethane Hydrocarbon Results, J. Geophys. Res., 102, 21281-21296(1997). 

Plass-Diilmer, C A. Khedim, R. Koppmann, F. J. Johnen, J. Rudolph, and H. Kuosa, Emissions of Light Nonmethane Hydrocarbons... 

Rockmann, T C. A. M. Brenninkmeijer, P. Neeb, and P. J. Crutzen, Ozonolysis of Nonmethane Hydrocarbons as a Source of the Observed Mass Independent Oxygen Isotope Enrichment in Tropospheric CO, J. Geophys. Res., 103, 1463-1470 (1998). 

Bernardo-Bricker, A., C. Farmer, P. Milne, D. Riemer, R. Zika, and C. Stoneking, Validation of Speciated Nonmethane Hydrocarbon Compound Data Collected during the 1992 Atlanta Intensive as Part of the Southern Oxidants Study (SOS), J. Air Waste Manage. Assoc., 45, 591-603 (1995). 

Boudries, H G. Toupance, and A. L. Dutot, Seasonal Variation of Atmospheric Nonmethane Hydrocarbons on the Western Coast of Brittany, France, Atmos. Environ., 28, 1095-1112 (1994). 

Greenberg, J. P D. Helmig, and P. R. Zimmerman, Seasonal Measurements of Nonmethane Hydrocarbons and Carbon Monoxide at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 2, J. Geophys. Res., 101, 14581-14598 (1996). 

Penkett, S. A, N. J. Blake, P. Lightman, A. R. W. Marsh, P. Anwyl, and G. Butcher, The Seasonal Variation of Nonmethane Hydrocarbons in the Free Troposphere over the North Atlantic Ocean Possible Evidence for Extensive Reaction of Hydrocarbons with the Nitrate Radical, J. Geophys. Res., 98, 2865-2885 (1993). 

K. Olszyna, T. Kleindienst, W. Lonneman, S. Bertman, P. Shep-son, and T. Starn, Observations of Nonmethane Hydrocarbons and Oxygenated Volatile Organic Compounds at a Rural Site in the Southeastern United States, J. Geophys. Res., 103, 28111-28128 (1998). 

Rudolph, J., A. Khedim, and D. Wagenbach, The Seasonal Variation of Light Nonmethane Hydrocarbons in the Antarctic Troposphere, J. Geophys Res., 94, 13039-13044 (1989). 

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