1. Atmospheric Boundary Layer
1.1. Introduction
The atmospheric boundary layer (ABL) is the portion of the atmosphere that is affected by the turbulence generated by heating of the earth’s surface by solar radiation (Stull 65). Understanding of the ABL is important because it strongly influences the distribution of moisture in the lower troposphere and the ground level concentration of atmospheric particulates.
1.2. ABL Formation and Structure
The ABL forms within the context of the standard atmosphere (Stull 66).
Recalling the discussion of density above, consider a parcel of air near the surface which is warmed by the sun. Warming the air decreases its density which causes it to buoyantly rise. This tendency of the air to move from its original altitude is referred to as instability (Stull 66). As the air rises, it will encounter an altitude at which it is stable. Surface air will continue to be heated by solar radiation, resulting in a continuous circulation from the surface to this stable level. This constant churning results in homogeneous atmospheric properties in the first one to four kilometers of the troposphere (Stull 66). That is, if air from various altitudes is normalized to the same pressure it will have the same virtual temperature. Above this mixed layer the troposphere has the same temperature, on average, as predicted by the standard atmosphere model (Stull
67).
Clearly, this will result in a sharp temperature shift at the top of the mixed layer.
This creates a stable layer which is often referred to as the capping layer. The area above is called the free troposphere (Stull 67). The capping layer prevents air exchange from the lower troposphere to the free troposphere.
The extent to which the boundary layer follows this pattern with vary with the degree of solar heating. In the case that the surface temperature is similar to air temperature, often associated with overcast daytime conditions, there will be minimal circulation, and a weak capping layer. Under these conditions the atmosphere will closely resemble the standard atmosphere with little evidence of a boundary layer.
Finally, the surface may be cooler than the atmosphere. A thin, 20-500m (Stull,
66) , layer of stable air will form. As circulation ceases, the air between the stable layer and capping inversion will become stable. This area becomes known as the residual layer (Stull, 69). It continues to contains humidity and pollutant contents similar to the boundary layer that existed previously (Stull 69).
1.3. Consequences of Mixing

The mixing within the ABL results in a uniform mixing ratio up to the capping inversion.
Stull Figure 4.9
Reevaluating Figure 5 to account for a uniform mixing ratio below a Mixing
Inversion at 4 km results in an even closer match to figure 6.
2. Inversions
2.1. Importance of Inversions
Though it is desirable to understand the thermal and humidity profiles resulting from the ABL, especially for aircraft that operate primarily below 2km, the most important aspect of the ABL is its capping inversion.
Strong inversions prevent the exchange of air and its contents, such as particulates and humidity, between the boundary layer and the free atmosphere.
This can result in high concentrations of particulates at ground level. Thus it is important to understand when inversions will be present, and at what altitudes.

2.2. Synoptic Weather
Inversion strength is influenced by both the conditions in the ABL, and the interactions of air masses at frontal boundaries, synoptic weather (Stull 68).
Strong inversions will occur at boundaries between warm and cold air masses
(Stull, 263). These tend to be low altitude inversions which result in shallow mixing regions (Zhang, 5529).
The nature of the air mass also affects the strength of the inversion. Within a high pressure mass the inversion is strong (Stull, 68) as is the tendency for mixing resulting in highly homogeneous properties up to a relatively high altitude inversion (Zhang, 5530). Within a low pressure system there will be a weak, if any, capping inversion (Stull, 68). Under these conditions there will be constant gradients from the surface up to the tropopause, as in the standard atmosphere.
2.3. Boundary Layer Thickness Measurements
NASA GEOS data is widely used as source data for Boundary Layer Thickness in atmospheric modeling (Lin, 1726). GEOS data is available from NASA through the MERRA data set. Planetary Boundary Layer Thickness is contained in the 2 dimensional turbulence data set: tavg1_2d_flx_Nx. The Giovanni tool is available for plotting desired parameters over any time scale from 1974 through

approximately 2 months preceding the current date. An example of Giovanni output is shown below.
This information will be used to obtain the crucial boundary layer height parameter when modeling atmospheric structure.
3. Atmospheric Particulates
3.1. Introduction
Particulates of a variety of sizes are suspended in the atmosphere. These particles come from a variety of natural and anthropogenic sources. These particulates are of importance because they can result in fouling of aircraft gas turbine aerodynamic surfaces (Stalder 364), and can cause wear to those same surfaces (Warren, B1). This study will examine the sources of contamination with the aim of understanding variation by region. Vertical concentration profiles will also be examined.
This study will not include events which result in extreme conditions typically avoided by aircraft, such as volcanic eruption and catastrophic forest fires.
Conditions resulting from operations on unimproved surfaces, e.g. helicopter landings on loose soil, will also not be considered.
3.2. Particulate Size Distributions

For air quality measurements particle sizes are usually reported in terms of PM-
10 and PM-2.5. PM-10 refers to the quantity of particles 10 micrometers in diameter or less. Likewise, PM-2.5 refers the particles 2.5 micrometers or less.
Thus, PM-2.5 is a subset of PM-10. PM-2.5 is often referred to a fine particles, while PM-10 may be referred to as a coarse particle measurement (EPA 2011,
20)
PM10 was the dominant measurement until the mid 1990’s when evidence increasingly showed that PM-2.5 was a more important measure of the human health impact of particles (EPA 1995, 26). In recent literature PM-2.5 is more commonly reported.
Neither PM-2.5 nor PM-10 provides a complete profile of particles in the air.
Average atmospheric particulate profiles have been reported for the purposes of aircraft contamination exposure estimation (SAE 2007, 5).
This figure agrees well with the recommended particulate distribution given in
SAE ARP986 which is shown below. This particle distribution will be used for scaling reported PM-2.5 and PM-10 particulate to obtain a complete ground level particle distribution.

The mean particle size, and thus the particle distribution, remains remarkably constant with respect to altitude (Zhang, 5530), so the same distribution will be used for all altitudes.
3.3. Sources
3.3.1. Natural
Naturally occurring atmospheric particulates originate from regions with loose soil and sufficient winds to lift those particulates from the surface
(Engelstaedter, 76). The largest emitter is Saharan Africa, which accounts

for the majority of global dust emissions (Engelstaeder, 75). The below figure shows measurement of dust concentration in terms of an aerosol index (Engelstaeder, 73) and highlights the importance of Saharan Africa as an emitter of natural particulates.
It should be noted that the data source used tends to exclude results less than 1 km from the surface which means anthropogenic high concentration events will not be shown (Englestaeder, 77). This is because, as discussed above, high concentration anthropogenic events are associated with a shallow boundary layer, usually much less than 1 km.
3.3.2. Anthropogenic
Anthropogenic sources of atmospheric particulates are primarily related to combustion. These include burning of fossil fuels and biomass for energy and transportation, as well as burning of biomass. Human activities can also exacerbate the natural emissions discussed above through improper land use.
Anthropogenic sources can account for nearly 75% of measureable particulates in urban areas (Baek, 211), and about 65% in suburban areas
(Baek, 210) . The anthropogenic portion of measure particulates represents an increase over the natural background quantity of particulate (Jones,
4467). The amount of particulate in the atmosphere due to anthropogenic sources is sensitive to the level of human activity, even varying noticeably between weekday and weekend automotive traffic levels (Jones, 4467).
Given the variability and significance of anthropogenic sources it is clear that

this study will require local average particulate levels to develop an understanding of particulate impacts at any given location.
3.4. Concentration
Particulate concentration at ground level is heavily dependent upon boundary layer thickness (Zhang, 5529). Conditions within the boundary layer also strongly affect the variation in particulate distribution with respect to altitude due to the variations in mixing discussed above. This variation will strongly impact the dust conditions at any given moment, however, average conditions will give a reasonable representation of long term operation within a specific area.
Average concentrations will be considered in two regions: the boundary layer and the free troposphere. Within an average boundary layer the air may be considered to be well mixed. Likewise, the residual layer under night time conditions will remain well mixed. For this reason, concentration within the boundary layer will be considered constant up to the top of the boundary layer.
Since the boundary layer is topped by a capping inversion particulates have difficulty ascending above the boundary layer, and therefore generally have low concentrations above that height. This atmospheric structure is clearly shown for several inversion heights (Type 1~ 3 km, Type 2~ 2 km, Type 3~ 1 km) in the figure below. The different colors in the plot represent different dates and slightly different inversion heights.
3.5. Transport in Free Troposphere
Atmospheric particulates are known to be transported between regions. Dust from Asian sources contributes to 41% of major dust events in the Western

United States (Fairlie, 1251). Dust originating in Saharan Africa can be transported across the Atlantic Ocean to North and South America, north to
Europe, and east to the Middle East Region and Asia (Engelstaedter, 80).
While seasonably variable, these particulate transport paths follow a predictable annual cycle. Years with El Nino events show increased dust transport, and there may be an increasing in the trend due to climate change. The cycle is sufficiently consistent, however, that the effect of transport can be considered to be accounted for in local averages (Engelstaeder, 91).
It must be considered whether aircraft operating within the particle transport paths will be exposed to higher concentrations of particles. The particles involved in transport are at the level of the top of the boundary layer.
Concentrations closely resemble the concentration in the originating region
(Collette, 393). Therefore the model discussed above will be appropriate for an aircraft encountering an transport layer on departure. Air aircraft at cruise will be operating well above the top of the boundary layer so this contamination is not of concern. Therefore the model developed above is adequate.
3.6. Global Distribution
The global distribution of particulate contaminants is available from satellite measurements (van Donkelaar, 850). The figure below shows global averages for 2001 through 2006. These reference values will be used in combination with the model developed above to estimate particulate profiles for cities of interest.
Combined with the boundary layer thickness measurements discussed above, this information will be used to determine the amount of exposure experienced by aircraft operating in the region.