Concentration units mixing ratios

Units for chemical abundance Concentrations and mixing ratios... 

Free radicals such as OH are present in such small concentrations that their concentrations are frequently given in units of molecules cm-3 rather than ppm, ppb, etc. A typical OH concentration in the lower troposphere is 5 X 105 radicals cm-3. What is this concentration in terms of the mixing ratio unit ppt, assuming 298 K and 1 atm pressure ... 

Other common ways of expressing abundances, particularly of solid or liquid particles, is to express them as concentrations in units of micrograms per cubic meter or nanomoles per cubic meter. For purposes of consistency, concentrations expressed in these units should be normalized to standard conditions of temperature and pressure. Because there is some confusion as to what constitutes standard conditions in atmospheric chemistry (273 K and 1.013 bar are commonly used in chemistry and physics and 293 K and 1.013 bar are used in engineering), it is important to define the standard conditions that are assumed when reporting data. This explicit definition is frequently not done. Concentrations expressed in these units can be easily converted to mixing ratios by use of the ideal gas law ... 

This reaction chain requires the presence of sufficient concentrations of NO. At low NO volume mixing ratios, below about 10 pmol/mol (p = pico = 1 O 12 pptv in US units), oxidation of CO leads to ozone destruction since the HO2 radical then reacts mostly with O3 ... 

Since we are dealing in all these equations with dimensionless ratios of mole fractions, we may replace mole fractions by molarities (designated by square brackets) or, indeed, any convenient molecular concentration units. The distribution of H and D among isotopically mixed species is then generally given by a Poisson distribution, i.e. we have for the fractional abundance F of a species XDJ)Hni i, (Cadogan et al., 1955 Kresge, 1964) ... 

It is obvious that a consistent system for the description of concentrations in the gas phase is necessary. Usually, fractional or percentage concentrations are used. Mixing ratios of low concentrated volatile compounds and gases are based on the parts per... unit system. This unit is obsolete but is still used in the current literature. It cannot be utihzed for particle concentrations. Therefore, the WHO (1999) has adopted a mass per volume system with concentrations [C] in mgm . Other units frequently employed to express the concentration of gases include moles per volume or molecules per volume (vanLoon and Duffy, 2000, Kurzweil, 1999). 

The odorous air sample is diluted automatically with neutral air by use of an olfactometer. This procedure leads to the definition of an odor concentration according to Eq. 7-9 that is applied for the determination of mixing ratios of known flow rates V (dynamic dilution method). The abbreviation ou stands for odor unit (Treitinger and Meyer-Pittroff 1997). The odor concentration is inversely proportional to the dilution. Actually, odor concentrations are dimensionless (see Eq. 7-9) and they simply reflect a dilution to threshold ratio. However, it is common to express odor concentration in odor units per cubic meter ([odor]/ou m ). 

Salt in crude oil must be removed before the oil undergoes processing in a refinery. The crude oil is fed to a washing unit where freshwater feed to the unit mixes with the oil and dissolves a portion of the salt contained in the oil. The oil (containing some salt but no water), being less dense than the water, can be removed at the top of the washer. The spent wash water (containing salt but no oil) is removed at the bottom of the washer. If the spent wash water contains 15% salt and the crude contains 5% salt, determine the concentration of salt in the washed oil product if the ratio of crude oil (with salt) to water used is 4 1. 

In the literature, the expressions mixing ratio and concentration are frequently used indiscriminately. This may give rise to confusion. As long as one talks loosely about the abundance of a gaseous constituent in surface air both terms are justifiable, but when units are given the two terms must be distinguished. 

Fig. 10-8. Flux diagram for the disposal of anthropogenic sulfur emissions in the continental troposphere of the northern hemisphere. Fluxes are given in units of Tg S/yr. Numbers on boxes indicate column densities in units of mg S/m2. They were derived from the adopted ground-level concentrations and scale heights below 5.5 km. Above this level, mixing ratios are assumed constant with m(S02) = 65 ng S/m3 and mfSOj-) = 70 ng S/m3. Chemical conversion of S02 to SC>4 occurs only in the lower troposphere. Dry deposition velocities are 8 mm/s for S02) 0.15 mm/s for SO2- on flat terrain, and 5 mm/s for SO2- interception by forests, which are assumed to occupy 50% of the continental area.

Air is ma.de up primarily of N2, O2, and Ar, which comprise 99.9% of dry air. There is a variable amount of water vapor, and many minor and trace gaseous components, as well as aerosol and particulate species. Table 26.1 lists some atmospheric gaseous components of environmental interest, along with representative concentrations in the troposphere. Typically, gaseous concentrations are expressed as mixing ratios, that is, volume/volume concentrations. A 1-ppm concentration represents 1 volume in 10 volumes of air. Such mixing ratios are independent of temperature and pressure. Environmental effects, though, may be quantitatively related to mass concentrations, and concentrations may be reported as mass per unit volume, usually mg/m of air, under specific conditions of temperature and pressure. Aerosols and particulates are reported in this way. 

At ambient conditions the second-order rate constant for the OH + N02 reaction is, in mixing ratio units, approximately 1.7 x 104 ppm-1 min-1. Considering an average urban mix of VOCs, an average VOC-OH rate constant, expressed on a per carbon atom basis, is about 3.1 x 103 ppmC-1 min-1. Using this value for an average VOC-OH rate constant, the ratio of the OH-N02 to OH-VOC rate constants is about 5.5. Thus, when the VOC N02 concentration ratio is approximately 5.5 1, with the VOC concentration expressed on a carbon atom basis, the rates of reaction of VOC and N02 with OH are equal. If the VOC NO2 ratio is less than 5.5 1, reaction of OH with N02 predominates over reaction of OH with VOCs. The OH-N02 reaction removes OH radicals from the... 

Atmospheric trace gas levels are frequently expressed in terms of mixing ratios. The volume mixing ratio of a species i (EJ is identical to its mole fraction. We have developed forms of the atmospheric diffusion equation using the concentration c, as the dependent variable. Let us take c, as the molar concentration, expressed in units of mol i m-3. Since the mass concentration m, and the molar concentration c, are related by m, = c, Af where Mi is the molecular weight of species i, the atmospheric diffusion equation applies equally well to the mass concentration. 

FIGURE 5.21 Ranges of VOC, NO, and ozone concentrations in the atmospheric boundary layer for four regions of the atmosphere (National Research Council, 1991). VOC is shown as propene equivalent mixing ratio in units of ppbC. 

Atmospheric concentrations are normally expressed as mixing ratios, the preferred unit being dry air mole fraction, normally parts per trillion (10 "), ppt, or pmol mol. For standards prepared volumetrically, corrections for nonideal behavior should be applied. 

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