Formation of Aerosols:
Sea Salt Nuclei & Precipitation Chemistry
01/27/04
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Motivations
for study of chemistry of rain and snow.
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Impacts
of acid precipitation on ecosystems such as lakes and forests;
during past 4 decades,
the loss of fish and many other organisms from fresh water aquatic
systems has been widely observed, initially in Scandinavia, and
later in other regions including the NE USA and eastern Canada.
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widespread
monitoring of the chemistry of rain and snow has become an important
feature
of attempts to reduce the effects of acid precipitation in the
USA for more than 2 decades.
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chemistry
of ice cores from Greenland and the Antarctic has been critical
in deducing
important features of Earth's climate over the past 0.5 million
years.
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(Figure
1) aerosols
are added to atm by a number of processes; general categories
of source types include (1) ocean surface, (2) continental dust,
(3) gas phase reactions, both natural and anthropogenic (especially
combustion), and (4) emission of volatile organics from vegetation
and also from human activities.
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examples
of ions or reactions that can be associated with each of these
process include (1) chloride, (2) calcium (3) hydrogen sulfide
to sulfate (by oxidation), sulfur dioxide to sulfate, and
atmospheric nitrogen to nitrate (primarily by high temperature
combustion) (4) terpenes (polycylic natural carbon compounds
emitted from the leaves of some types of trees), hydrocarbons
and chlorinated hydrocarbons (anthropogenic).
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source
for chloride in aerosols and precipitation is among the
most
simple for any of the major ions, being derived almost
entirely from the sea surface; very little chloride is
found in
continental
dust, except in arid & semi-arid climates where halite
(NaCl) can sometimes be found in the soils.
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most
chloride in rain, even in a continental interior area such
as Kansas, was derived from the sea surface within the previous
1 to 2 weeks prior to a precipitation episode.
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[Cl-]
of precipitation often used as indication of likely contribution
of marine aerosols to other ions.
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(Figure
2) mass
per volume units most often used for precip compositions (i.e.
mg/l), which is also approximately equal to parts per million
(ppm) in mass of ion per mass of water units -molar units (i.e.
milli moles/liter) more useful for comparing amounts of different
ions & chemical reactions.
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equivalents
units (i.e. milli equivalents /liter) = molar units x ion
charge are essential for all charge balance considerations,
and for assessing the portion of acidity associated with a
given anion (i.e. sulfate and nitrate).
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all
ionic aqueous solutions are neutral: if total anions do not
exactly = total cations, which generally is the case, the
following probably contribute: analytical error, lack of inclusions
of some ions (especially organic anions), or both may occur.
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hydrogen
ion MUST be included in charge balances for precipitation
samples,
and often is the most abundant cation in precipitation
in continental regions with appreciable industrial emissions.
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(Figure
3) the
sea surface, in addition to supplying most of the water vapor
to the atmosphere, also is the source of the largest input of
aerosol mass to the atmosphere.
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mechanism
is linked to breaking waves, driven by the wind, which
cause
formation of air bubbles that float back to the surface and
break.
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when
air bubbles break at sea surface, a spherical cavity is
formed
and collapses, causing small droplets of water to be propelled
into the atmosphere immediately above the sea.
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two
general classes of water drops form: (1) those from the
jet
drops which then generate a class of aerosols designated
as "giant" sea salt nuclei, and (2) smaller drops that are fragments
of the very thin upper boundary of the breaking bubble, known
as the film cap, which generate another class of aerosols
designated as "large" sea salt nuclei.
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the
small water droplets produced from breaking air bubbles evaporate
quickly, and leave behind small mixed salt particles with
a bulk chemistry quite similar to that sea water.
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giant
sea salt nuclei have chemistry very similar to the ocean,
while the
large sea salt nuclei appear to have some moderate differences
in bulk chemistry relative to the source ocean water.
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(Figure
4) conventional
terminology for aerosol size:
giant
= > 1 micron
large = 0.1 to 1 micron
Aitken nuclei = < 0.1 micron
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note that the "large" aerosol
category includes the wave lengths of visible light (0.4
to 0.7 microns).
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main EPA regulations
for urban air quality previously referred to aerosols with
radii
of <10 microns (referred to as PM 10); recently the category
for regulation of urban air quality has been changed to be <2.5
microns (referred to as PM 2.5), because the smaller aerosols
can be more readily inhaled into lungs and cause health effects.
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Two larger size
categories ("large" & "giant") intensively studied in marine
ATM using very simple collection devices, consisting of
glass slides cut to form sections of varying width, and
placed
on
a long rod out of the side of a small airplane.
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air stream passing
over the slides has increased velocity and causes aerosols to
be impacted onto the surface of the glass side.
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The smallest particles
(which are much more abundant) tend to accumulate on the glass
slide segment that is the most narrow, and largest particles
collect on the segment with greatest width.
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impaction of
aerosols on leaves of terrestrial vegetation occurs by similar
mechanism and substantially increases the amounts of ions added
to terrestrial ecosystems above that delivered by precipitation
(wet deposition).
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after particles
are collected, the exposed glass slide is placed in a controlled
humidity (about 70%) and the sea salt aerosols, which are hygroscopic
sorb water vapor and become small water droplets with a radius
of 2 to 3 times the original particle (size is function of humidity).
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droplet
size is measured with an optical microscope and the size
spectrum
of aerosols in marine air can then be derived.
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(Figure
5) similar
technique for sample collection using a pump to pass air through
a series of orifices that are progressively smaller.
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immediately opposite
to each orifice is a very clean glass slide, against which aerosols
impact and stick while the air moves around the slide and onto
the next stage in the impactor.
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after
a number of hours of pumping, there are aerosols segregated
by size
on the slides, that can be removed and measured by eye under
an optical microscope or leached with distilled water to
permit
chemical analysis.
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(Figure
6) technique
for measuring very small aerosols was developed about 80 years
ago by J. Aitken: air is first evacuated with a small pump (like
a bicycle pump) from a long cylinder that has two compartments.
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then the "sample"
half of the chamber is closed off from the other "expansion" portion
of the chamber.
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a valve is opened
which slowly lets external air into the sample chamber, which
has a water saturated liner (made of felt?).
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air sample at ambient
air pressure is allowed to equilibrate with moist liner & become
saturated in water vapor (a few minutes, at most).
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then a valve is opened
which permits the sample chamber air to expand very rapidly (adiabatically)
into the full volume of the cylinder, which causes the air temperature
to drop rapidly, and the resultant lower pressure air sample to
become supersaturated in water vapor (by about 400%).
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essentially all
of the aerosol particles then serve as condensation sites
for
water
vapor, forming a diffuse "cloud", with the size of water drops
being nearly independent of the initial size of an aerosol
particle.
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this cloud in the
cylinder absorbs part of a light beam, reducing light arriving
at a photo cell, proportionally to the total number of aerosol
particles in the initial air sample.
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range of sensitivity
is very large (5-6 orders of magnitude).
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typical numbers of
total particles in air sample are:
| 105
to 106 |
cities |
| 104 |
clean
continental sites |
| 102
to 103 |
clean
oceanic sites |
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(Figure
7)
distribution of number of aerosol particles as function of radius
has been synthesized by using a range of measurement techniques,
each of which is sensitive to only a portion of the entire size
spectrum: Aitken nuclei counter, cascade impactor, diffusion experiments,
etc.
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maximum
in particle number occurs at similar size (a bit less than
0.1
micron) over both land & sea, but total number of particles
over land is about a factor to 100 greater, suggesting source
of small particles probably mostly over land.
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maximum
size is about 10 to 20 microns, with larger particles falling
to the surface quite rapidly.
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note
very large range of particle number as a function of radius
of particles (about 8 orders of magnitude).
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decline
in numbers at very small radii is due to aggregation of very
small particles by collision with other particles.
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Most
particles generated by breaking bubbles at sea surface are
towards the
large end of the size spectrum.
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(Figure
8) aerosol
population sizes and chemical compositions have broad
similarities over different areas of the oceans & differ
substantially compared with over continents.
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vertical
mixing over land is much more vigorous because much less solar
energy goes evaporating water and more into heating the land
surface.
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depth
of vertical mixing over much of the ocean, especially in the
subtropics, is less than half of the height to the tropopause.
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Air
above mixed layers over ocean and
land contains less aerosols than closer
to the surface, but still a substantial
number of particles that are sometimes referred to as
tropospheric "background",
since they represent the minimum
particle population found throughout a large volume of
the troposphere.
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(Figure
9)
rain drops are about 0.5 to 2 mm in radius, while cloud droplets
are only 5 to 20 microns in radius: mass of rain drop is greater
by factor of about a million.
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number
of cloud droplets is typically about 200/cc or only about 1%
of the total aerosol population over continental areas; thus
only a small fraction of continental aerosols are
classified as "condensation nuclei", the remainder are
less hygroscopic and thus more inert in formation of
rain.
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collisions
of cloud drops in turbulent air could cause coalescence, given
sufficient time, but model calculations indicate this could
be several days, whereas many clouds can begin generation of
precipitation within a few hours or less after formation.
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key
process appears to be to have a small population of much larger
cloud drops which can fall through the other droplets and grow
by collisions with smaller drops (very high surface tension
of liquid water is a key factor in causing coalescence to occur
when two spherical cloud drops collide.
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rain
forms from many clouds in the marine environment that have
cloud
top temperature > 0°C, & hence cannot involve
any ice phases; giant sea salt aerosols appear to be very
effective
in initiating precipitation in such clouds.
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Rain,
snow, etc where cloud tops are < 0°C appear to involve
ice nuclei that can grow at the expense of the liquid drops
due to lower vapor pressure at the same temperature.
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The
number of these ice nuclei is quite small compared to the
total
aerosol population & their origins are still obscure.
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cloud
seeding in relatively dry areas such as eastern Colorado has
been attempted by a number of processes; initial attempts were
with dry ice, to stimulate formation of ice crystals that would
then seed formation of rain drops; later attempts used AgI,
which has a crystal structure similar to ice.
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some
evidence exists that PbI (generated especially from auto
exhaust PbBr2
which later reacts with atmospheric iodide) can serve as ice
nuclei.
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(Figure
10) data derived from high altitude aircraft flights between
several cities in the central USA ( Sioux Falls, Omaha, St. Cloud,
Bismark, Rapid City), conducted on June 24, 1960.
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total
number of aerosol particles (dominated by the Aitken nuclei)
are reasonably constant (200 to 400 /cc) over mid continent
USA from 5 km to the tropopause (11-13 km) and then decline
substantially (two orders of magnitude) as elevation increases
to about 18 km, and then remain about 1 cc for the next
10 km
of elevation; pattern strongly suggests that main source
of Aitken nuclei is below the mid troposphere & that most
particles in the stratosphere are derived from the troposphere
and transported
upward, while aerosol removal processes (formation of precipitation)
are relatively unimportant above about 5 km.
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if
we consider the vertical distribution of only large particles
(about 0.2 microns), the trend is quite different.
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near
surface concentrations are high (about 100/cc), and the decline
with elevation is very rapid, reaching about 0.01/cc by the
base of the stratosphere (12 km).
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in
lower stratosphere (18 to 23 km), the population of large aerosol
particles increases to about 0.1/cc, indicating there is a source
of particles in this part of atmosphere, several kilometers
above the tropopause.
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composition
of these particles indicates that S is the most abundant element.
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probable
mode of formation: diffusion of sulfur dioxide from the troposphere
into the stratosphere, followed by oxidation to sulfur trioxide
(probably by ozone ?), which then hydrolyzes to sulfuric
acid.
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(Figure
11) most abundant S gases in upper troposphere & lower stratosphere
are sulfur dioxide & hydrogen sulfide.
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both
are readily oxidized to sulfur trioxide, by a number of quite
complex mechanisms.
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once
S03 is present, this will quickly hydrolyze to sulfuric acid
in the presence of liquid water.
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if
some ammonia is present in gas phase, this can neutralize part
of the acidity, leading to presence of NH4+,
SO42-, and H+, all of which
are important components of the aerosols in the Junge layer
in the lower stratosphere.
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other
trace chemical constituents of these lower stratosphere
aerosols include Al, K, Ca & Fe.
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these Junge
layer particles are quite hygroscopic and water soluble.
This page last updated:
01/27/04
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