Download L30 - UA Atmospheric Sciences

Atmospheric Chemistry
• Texts:
– W+H, Chap 5
– S+P, selected readings from Chaps. 1-7
• Divided into Two Parts
– Gas Chemistry
– Aerosol Chemistry – Next semester
• Relevance
– Pollution/Air Quality/Acid Rain
– Climate
– Ozone layer
Atmospheric Composition Revisited
Atmospheric Composition Revisited
Units
• Normally, units of mixing ratio used, xi
– Mole fraction = Volume Fraction
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ppm = 10-6
ppb = 10-9
ppt = 10-12
“ppx” = “ppxv”
– Mass fraction is different – also called mixing ratio –
beware
– Must multiply volumetric mixing ratio by the ratio of
molecular weights to get mass fraction.
Review of Conversions
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Mixing Ratio (volumetric), xi
Mixing Ratio (by mass), ri
Partial Pressure, pi
Molar concentration (moles/m3), ci
Mass concentration (mg/m3), ri
Molecular concentration (#/m3), ni
xi 
pi ni ci
 
p n c
Mi
ri
ri  x i

M air r
pi  ci R *T
 ni kT

ri
Mi
R *T
ni  N Aci
ri  ci M i
Mixing Ratio vs. Molar
Concentration
• n is a strong function of p (i.e. z)
• Consider vertical profiles in each quantity
Lifetimes and spatial scales of
molecular species
t = Q/R
Q = abundance
R = removal rate
P = production rate
t = mean lifetime
dQ/dt = P – R
Assumes a closed
system
Very often,
R  k rQ
Tropospheric O3
http://aura.gsfc.nasa.gov/science/top10_omi-mls-maps.html
AIRS CO2
Spatial scales in atm. are a function of residence time and spatial scale of source
http://www-airs.jpl.nasa.gov/images/AIRS_CO2_July2003_550x396.jpg
The key elemental players
• Oxygen and Hydrogen
– O 6 valence electrons – highly electronegative
– O2 accumulation the result of photosynthesis
– H2O main reservoir for H
– Key reactive species are O3 O, OH, HO2, all
produced by photochemistry
– Result in an oxidizing atmospheric
environment
The key elemental players
• Nitrogen
– N – 5 valence electrons
– N2 VERY stable in atmosphere
– Rest of species called “fixed nitrogen”, having
oxidation states from +3 to -5
– Key sources of reactive nitrogen are bacteria
lightning, and fertilizer production
– Key atmospheric players are N2O, NO, NO2,
and NH3.
– Also present in many organic species
The key elemental players
• Carbon
– C – 4 valence electrons
– CO2 is the key atmospheric reservoir
– Other players are CO (a pollutant) CH4 (a
greenhouse gas), and the countless organic
species
– Cycles involve the land surface, biosphere,
oceans, and anthropogenic processes
– Much of the atmospheric aerosol is of organic
composition
The key elemental players
• Sulfur
– S – 6 valence electrons; a chalcogen like O
– SO2 and DMS are the key sources
– Strongly influenced by fossil fuel emissions,
esp. coal burning
– Some dry deposition – mostly converted to
sulfate aerosol (SO42-) – source of east-coast
smog
– Much of the atmospheric aerosol is sulfate
Sources
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Biogenic
Terrestrial
Anthropogenic
Oceanic
Sinks
• Deposition
– Dry deposition
– Wet deposition
• In situ reactions