Neutral atmospheric stability

There are several factors that contribute to the difficulty of predicting the vertical transport of aerosols. Most obviously, the flow field is not homogeneous in the vertical direction. For neutral atmospheric stability conditions, the mean wind velocity follows an approximately logarithmic velocity profile, given by (Wieringa 1980)... 

The typical values suggested by Sutton for /i and the dispersion coefficients in equation (3.3) only apply under conditions of neutral atmospheric stability. Neutral stability occurs when air temperature decreases with height above ground at a rate close to PC per 100 m, which is known as the adiabatic lapse... 

A correlation based on experimental data is provided by MacKay and Matsuga (1973). This correlation assumes neutral atmospheric stability and applies only for a pure component. 

The quasilaminar sublayer resistance / b describes the excess resistance for the transfer of matter from the atmosphere to the surfaces of the vegetation, that is, the difference between the resistance for matter and the resistance for momentum. It is primarily associated with molecular diffusion through quasi laminar boundary layers. Several parameterizations for Rb have been developed, but that employed by Brook et al. (1999), which like Equations 7.3 and 7.6 is valid for conditions of neutral atmospheric stability, is particularly easy to apply ... 

Comparing the results. Method B yields a value for kg that is 3.5 times higher than the value from Method A. Possible explanations include an underestimation by Method A, for instance because the measured data were based on net, not gross, accumulation in the grass, or an overestimation by Method B, for instance due to the assumption of neutral atmospheric stability, or because the canopy resistance Ro was not negligible in this case. 

Using literature values of a and oz for three typical atmospheric stabilities (Pasquill classes B, D and F or unstable/convective, neutral and stable conditions) and typical wind speeds of 2, 5, and 2ms respectively the points shown in Figure 4 can be calculated. The values of R=10 and S=5 were used. The hnes in Figure 4 are equation (7) with the range... 

This atmospheric stability versus friction velocity plot is capable of representing any atmospheric condition except that when the stability is neutral. The information contained in Figure 1 and equation 2 allows the relationship between the value of the surface flux and the required chemical sensor resolution to be estimated for a full range of atmospheric conditions. 

Intrinsic thermal stability. This could be judged, for example on the basis of determinations of the rate of substrate consumption d[PH]/dt in a neutral atmosphere. According to this criterion, polyethylene, which begins to decompose at about 450° C, would appear more stable than... 

The theoretical concentration in air downwind of an area source as a function of the roughness length and atmospheric stability has been given by Wilson (1982). Shinn et al. found z0 = 0.02 m at the Nevada site. The dimensionless concentration u Xi/Q from (6.25) is 8.3, and this would agree with Wilson s calculations for average (neutral) stability if the fetch of wind over the source of resuspended dust was 10 km. 

Flux is a somewhat abstract entity and does not relate this information directly. It quantifies the chemical movement rate across an interface plane into a receiving media such as the air boundary layer (BL) in the above example. Only when it is coupled with an air dispersion model does it produce concentrations in air. In the case of a large soil surface area source, a simple relationship exists between flux and concentration. For neutral air stability conditions in the atmospheric BL with steady-state wind speed v (m/sec), the concentration in air, c (mg/m ), can be approximated by... 

This profile relation may be viewed as a correction to the logarithmic law. For an almost neutral atmosphere, L is a large positive number, and the relationship between velocity and height is logarithmic. As stability increases, the positive L decreases and the deviation from the logarithmic behavior increases. Finally, for very stable conditions, L approaches zero, the second term in (16.78) dominates, and the velocity profile becomes linear. 

A case study is performed assuming an instantaneous release of the toxic liquid acrylonitrile from a rail tankwagon. After the release of the toxic liquid a pool of 600m is formed from which evaporation occurs, leading to a vapour cloud. This vapour cloud travels with the wind and disperses. The degree of dispersion is determined by the wind speed, the stability of the atmosphere and the surface roughness. The stability of the atmosphere is indicated by the pasquill-stabUity class. By day, the most common atmospheric stability class is class D and a wind speed of 5 m/s is assumed, this weather condition is abbreviated with D5. At night class F is the most common atmospheric stability, associated with a wind speed of 1.5 m/s this weather condition is abbreviated as FI.5. The evaporation and dispersion calculations are performed with EFFECTS 7.6. A neutral gas model is used for the dispersion calculations. 

The Richardson number is a turbulence indicator and also an index of stability. Meteorologists classify atmospheric stability in the surface layer as unstable, neutral, and stable. Strongly negative Richardson numbers indicate that convection predominates, winds are weak, and there is a strong vertical motion characteristic of an unstable atmosphere. Smoke leaving a source spreads rapidly vertically and horizontally. As mechanical turbulence increases, the Richardson number approaches zero, and the... 

The model applies a Gaussian plume model up to a specified transition distance, which is dependent on atmospheric stability 200 m for unstable, 500 m for neutral, and 1000 m for stable stratification, respectively. Outside this nearby zone, the gradient-transfer approach is used for computing the diffusion in the vertical direction. 

In these computations we have assumed a constant dry deposition velocity, although it is actually dependent, among other factors, on wind speed and atmospheric stability. In particular, the dry deposition velocity is typically smaller in stable conditions. However, the loss of pollutant from the plume depends also on the vertical concentration distribution. For stable conditions, the plume is substantially more shallow dry deposition therefore causes an increased decrease of concentration at larger distances, compared with the corresponding case in neutral stability. The apparent convergence of the concentration curves for the two stability classes is fortuitous. 

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