Deposition to surfaces

Contaminant concentrations Dispersal of airborne contaminants such as odors, fumes, smoke, VOCs, etc. transported by these airflows and transformed by a variety of processes including chemical and radiochemical transformation, adsorption, desorption to building materials, filtration, and deposition to surfaces evolution of contaminant concentrations in the individual zones air quality checks in terms of CO2 levels cross-contamination evaluation of zones air quality evaluations in relation to perception as well as health. Methods ate also applicable to smoke control design. 

In COMIS, source or sink strength can be defined as time dependent but not dependent on actual concentrations or temperatures. COMTAM96 includes more sophisticated models such as chemical reactions, adsorption and desorption to building materials, filtration, and deposition to surfaces. 

An obvious correlation between polar and alpine environments is the decrease in temperature with increasing latitude or elevation. This temperature change leads to a shift in environmental phase distribution equilibria - i.e. a chemical moves from the atmosphere to terrestrial surfaces, including direct deposition to surface waters, but also to snowpack and soils from which movement into surface and groundwater is possible. This process has been termed cold condensation but should more correctly be called cold-trapping because the contaminants are not actually condensing. 

Substances can be removed from the atmosphere by dry deposition to surfaces. A method for obtaining the parameters of dry deposition uses eddy correlation flux measurements that require chemical sensors with very fast... 

The primary sources that are responsible for the presence of this family of compounds in the atmosphere emit NH3, N20, and NO to the troposphere, the lowest level of the atmosphere, which extends to approximately 10 km from the earth s surface. NH3 seems to undergo very little chemistry in the atmosphere except for the formation of aerosols, including ammonium nitrate and sulfates. NH3 and the aerosols are highly soluble and are thus rapidly removed by precipitation and deposition to surfaces. N20 is unreactive in the troposphere. On a time scale of decades it is transported to the stratosphere, the next higher atmospheric layer, which extends to about 50 km. Here N20 either is photodissociated or reacts with excited oxygen atoms, O (lD). The final products from these processes are primarily unreactive N2 and 02, but about 10% NO is also produced. The product NO is the principal source of reactive oxidized nitrogen species in the stratosphere. 

When aerosols are in a flow configuration, diffusion by Brownian motion can take place, causing deposition to surfaces, independent of inertial forces. The rate of deposition depends on the flow rate, the particle diffusivity, the gradient in particle concentration, and the geometry of the collecting obstacle. The diffusion processes are the key to the effectiveness of gas filters, as we shall see later. 

A chemical may be removed from the atmosphere either by physical processes or by chemical transformation. If a chemical exists in the atmosphere in the form of sufficiently large particles or liquid droplets, physical deposition of the chemical to soil, vegetation, or water bodies may occur by gravitational settling. Smaller particles with negligible settling velocities also may be deposited to surfaces by impaction or diffusion. Chemicals in gaseous form may either sorb onto surfaces directly or sorb onto airborne particles that are subsequently removed. 

The reactions of amines with the OH radial are rapid, with lifetimes during daylight hours being on the order of hours. Ammonia, however, being highly soluble and reactive with atmospheric acids, will be removed preferentially by those routes rather than through reaction with the OH radical. Particulate-nitrate salts will undergo dry and/or wet deposition to surfaces. 

Some of the nitrogen inputs from atmospheric deposition is nitrogen deposited directly onto the surface of coastal waters. This direct deposition to surface waters often contributes between 1% to 40% of the total nitrogen inputs to coastal ecosystems (Nixon et al. 1996 Paerl 1997 Howarth 1998 Paerl and Whitall 1999 Valigura et al. 2000). The direct deposition is most significant in very large systems, such as the Baltic Sea (Nixon et al. 1996) or in coastal systems which have relatively small watersheds in comparison to the area of the surface waters, such as Tampa Bay (Zarbock et al. 1996). 

Deposited POPs remain for the longest time in soil [10, 18]. Nevertheless, their deposition to surface water [73,92] cannot be neglected. Of course, the content in the bottom sediments depends on the water level, base flow, and the POP migration in the river. The UNEP-BTF mission found relatively high levels of PAHs and PCBs in the sediment in the Danube upstream from Novi Sad in August 1999 [75] and indicated them as chronic pollution. The PAH contents in sediments of the Danube 100 m and 1,200 m downstream from of the wastewater canal at Pandevo on April 28,1999 were the order of 10 and 1 mg kg , respectively [74], while the upstream value was 0.1 mg kg". On 1 July, the upstream value was 0.25 mg kg", whereas the values 100 m and 1,200 m downstream were 0.18 and 0.04 mg kg", respectively. Finally, on 22 July, the corresponding values were upstream 0.018, 100 m downstream 0.012, and 1,200 m downstream 0.037 mg kg" . The content in the wastewater canal decreased in this period from 12 to 0.87 mg kg . It is obvious that a meritorious assessment needs more research than that performed by the UNEP-BTF [75]. 

Transport efficiency (TE) Estimated percentage of a substance that is deposited to surface media after transport away from the point of release this metric describes the potential for atmospheric transport and deposition of a compound in a remote region. The benchmark value for TE is 2.248% based on the model results for the reference chemical PCB-28. 

Chemical contaminants in the atmosphere can be deposited to surfaces in association with aerosol particles or falling rain and snow. These are advective transport processes, since the chemical moves in association with aerosol particles, raindrops, or snowflakes. This chapter describes methods for estimating chemical fluxes associated with deposition of aerosol particles and precipitation, and provides recommended values for mass transfer coefficients for a range of environmental conditions. In this chapter we do not consider transport of gaseous species in the atmosphere and adjacent surfaces. These convective transport processes, termed dry deposition of gases, are covered in Chapter 2, Section 2.5.6 and Chapter 7, Section 7.3. exchange between air and plants in Chapter 7, air and water in Chapter 9, and air and snow in Chapter 18. 

In this chapter, we focus on deposition to surfaces with low surface roughness, such as bare soils and water. Dry deposition of fine particles to grassland or forested... 

With respect to PAHs content is sediments, Sprovieri et aL have reported PAHs contents in highly contaminated surface sediments in Naples harbor varying fiem 9 to 318,000 pg kg [258], whereas total content values up to 3766 pg kg have been foimd in the Mersey Estuary (UK) [259]. Slater et al. have reported significant temporal variability in PAH deposition to surface sediments both proximate and distal to the industrialized shoreline [253]. 

---