atmospheric deposition

7/24/2019 Atmospheric Deposition

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I. INTRODUCTION

Many of the substances in the atmosphere are completely natural, such as the

oxygen we breathe or dust particles that have been resuspended by wind from the

Earths crust. However, human activities have resulted in the presence of othersubstances that we consider pollutants because they are potentially harmful to human or

ecosystem health. Air pollutants posing a risk to human health include gases such as

ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide particle!associated

components of combustion exhaust including carcinogenic polycyclic aromatic

hydrocarbons heavy metals and particles smaller than ".# microns $%M".#&. %revious

'outhern (alifornia )eport (ard articles have dealt with the human health aspects of

regional air *uality $)( +- and "/&, particulates $)( "+&, and personal exposure

$)( "#&.

II. DEFINITION OF TERMS

+. Atmosphere! the whole mass of air surrounding the earth". 0epositon! an act or process of depositing/. Eutrophication! the process by which a body of water becomes enriched in dissolved nutrients $as

phosphates& that stimulate the growth of a*uatic plant life usually resulting in the

depletion of dissolved oxygen1. %oint 'ources

! an identi2able con2ned source $as a smokestack or wastewater treatment plant& fromwhich a pollutant is discharged or emitted

#. 3on!%oint 'ources! a source of pollution that issues from widely distributed or pervasive environmental

elements.4. 'e*uestration! to set apart5. )un 67 ! the portion of precipitation on land that ultimately reaches streams often with

dissolved or suspended material-. %ollutants! means any matter found in the atmosphere other than oxygen, nitrogen, water

vapor, carbon dioxide, and the inert gases in their natural or normal concentrations,

that isdetrimental to health or the environment.

. (ontaminants! biological, chemical, physical, or radiological substance which, in su8cient

concentration,can adversely a7ect living organisms through air, water, soil, and9or food.

+. Aerosols! a suspension of 2ne solid or li*uid particles in gas



III. ATMOSPHERIC DEPOSITION

%ollutants in the atmosphere can deposit on all of the solid surfaces of a watershed

and then be washed o7 by rain, becoming part of the storm water runo7 that reaches

rivers, lakes, and coastal waters. %ollutants may also be deposited directly from the

atmosphere onto the surface of a water body. :he reason to be concerned about

atmospheric deposition is that pollutants that are not washed o7 may accumulate on

surfaces such as soil, forming a reservoir of toxic substances that may later be

resuspended back into the air, causing a threat to human and ecosystem health even

after the original sources of the pollutant have been removed.

'ubstances exist in the atmosphere either as molecules of gases or as solid or li*uid

particles, called aerosols, that range in size from .+ to + microns $it takes a

thousand microns to make a millimeter&. ;oth gases and particles are deposited onsurfaces by one of two general mechanisms. <et deposition occurs when raindrops drag

molecules of gases and particles down with them as they fall. 0ry deposition results from

the combination of molecular di7usion, impaction, and gravitational settling. <et

deposition is the most important deposition mode in regions with appreciable annual

rainfall, but in semi!arid regions such as 'outhern (alifornia atmospheric deposition is

likely to be dominated by dry deposition processes. :he most rapid dry deposition rate is

the gravitational settling of particles in the + to + micron size range.Atmospheric

deposition can be a ma=or environmental problem> acid rain is the most well known

problem of atmospheric deposition and some of the countrys most important water

bodies



Wet deposition

! refers to acidic rain, fog, and snow. As this acidic water ?ows over and through the

ground, it a7ects a variety of plants and animals. :he strength of the e7ects depend

on many factors, including how acidic the water is, the chemistry and bu7ering

capacity of the soils involved, and the types of 2sh, trees, and other living things that

rely on the water.

@Acid rain $acidic deposition& is measured using a scale called pH. :he lower a substanceBs

pH, the more acidic it is. %ure water has a pH of 5.. 3ormal rain is slightly acidic because

carbon dioxide dissolves into it, so it has a pH of about #.#. 'ulfur dioxide $'6"& and nitrogen

oxides $36x& are the main pollutants of acidic deposition. :he sources are emissions from

transportation, electric power plants, industry, and fuel burning. :he emissions are diluted inthe atmosphere and stored in rain or snow.

Dry deposition

! refers to acidic gases and particles. About half of the acidity in the atmosphere falls

back to earth through dry deposition. :he wind blows these acidic particles and gases

onto buildings, cars, homes, and trees. 0ry deposited gases and particles can also be

washed from trees and other surfaces by rainstorms. <hen that happens, the runo7

water adds those acids to the acid rain, making the combination more acidic than the

falling rain alone.

<ater pollutants of concern that may deposit from the atmosphere include compounds that

increase the acidity of rainfall or fog, nutrients that may cause excess algal growth

$eutrophication&, and toxic organic and inorganic $metals& compounds. Acid rain, primarily

caused by the emission of nitrogen and sulfur from motor vehicles, industries and power

plants, harms vegetation and impairs water *uality.



Eutrophication of water bodies by excess nutrients results in lowered, often zero, dissolved

oxygen levels and conse*uent death of 2sh and other organisms in addition to dramatic

changes in taste and odor of the water

3utrient additions by atmospheric deposition are thought to be a primary cause of the

decrease in the clarity of Cake :ahoe. Among the organic compounds of interest in a*uatic

systems are pesticides such polycyclic aromatic hydrocarbons $%AHs& and polychlorinated

biphenols $%(;s&, all of which are internationally recognized as important persistent organic

pollutants $%6%s&. Metals identi2ed as important water pollutants are copper, cadmium,

chromium, lead, mercury, nickel, and zinc. :hese organic compounds and metals are present

in the sediments of many water bodies and are of concern because of their e7ects on

a*uatic organisms, and, in the case of lead and mercury, on human health.



Polycyclic aroatic !ydrocar"ons #PAHs$

! are a group of more than + di7erent chemicals that are released from burning coal,

oil, gasoline, trash, tobacco, wood, or other organic substances such as charcoal!

broiled meat.

Persistent Or%anic Poll&tants #POPs$

! are organic compounds that are resistant to environmental degradation through

chemical, biological, and photolytic processes.

I'. SOURCES OF META(S TO THE ATMOSPHERE

%oint sources are 2xed sources associated with speci2c large industrial facilities

mobile sources are moving vehicles and area sources include construction vehicles,

distributed smaller industrial sources, and resuspended dust.

:he most signi2cant source of metals to the atmosphere is resuspension of dust,often called fugitive dust, from roads by moving vehicles and from other paved and

unpaved surfaces by wind.

FUGITIVE! di8cult to grasp or retain

(hemical studies of the dust indicate it is primarily composed of natural material

typical of the earths crust but also contains signi2cant amounts of the metals we are

concerned with here with regard to water pollution. :hese metals have become

intimately mixed with the crustal material, making identi2cation of their real sources

di8cult.



'. PATTERNS OF ATMOSPHERIC DEPOSITION

Most of the deposited material falls on land or urban surfaces rather than directly on

a water surface, but there is some deposition on coastal waters because of night!time

breezes from the land and winds. :he pattern of dust and metal concentrations in the

atmosphere and the associated deposition on land is relatively uniform spatially although

deposition near ma=or sources, such as freeways, is higher than the regional background

rate within about + meters of the road. Dn the urban areas, daytime concentration anddeposition of metals is greater than nighttime because of the in?uence of tra8c on

resuspension. :hese patterns have been documented by direct measurements of

deposition using specially designed deposition surfaces.

:he modeled and observed patterns of atmospheric concentrations and deposition of

heavy metals, combined with the measured properties of regional dust, has led scientists

to hypothesize that dust!associated substances including metals deposit relatively closeto the original source of the material but then are resuspended and redeposited

numerous times before being carried out of the region by winds, se*uestered on the land

surface, or washed o7 by rainfall. :hus deposition from the atmosphere is only one

component of a complex system of pollutant transport operating at the land!air

boundary.



'I. IMPORTANCE OF ATMOSPHERIC DEPOSITION

:he relationship between atmospheric deposition of metals and water *uality

has been documented by a combination of model simulations and water

sampling. :he 2ndings are that nearly all the metals deposited on impervious

urban surfaces wash o7 with the next rainfall, but that on more natural landsurfaces between " and / of the metals are se*uestered from immediate

runo7

(omparison of the mass of metals reaching the land surface by atmospheric

deposition with the mass found in runo7 and with known mass inputs from other

sources clearly shows atmospheric deposition is a potentially signi2cant source of

metals to water bodies $Figure -&. :he contribution of atmospheric deposition can

be as high as in the case of lead, for which other contemporary sources are

negligible.



!!End G))E;IJD6 "+4

Tec!nolo%ical Instit&te o) t!e P!ilippinesKuezon (ity

Colle%e o) En%ineerin%*ac!elor o) Science in En+ironental and Sanitary En%ineerin%

EnSE ,-

Air Poll&tion F&ndaentals

Cecture L +

METHODS OF DEPOSITION

RON /U(IENNE C. RE*U0IO



February +", "+4

atmospheric deposition

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