Advances in the Chemistry of Atmosphere

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Advances in the Chemistry of Atmosphere
Welcome to

• CHEM-ATOC 419/619

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COURSE OUTLINE

• Introduction Earths atmosphere, chemical
  composition and its vertical structure
• Radiation balance of atmosphere green house
  gases, absorption and photochemistry
• Oxidation potential of the atmosphere
  atmospheric oxidants and homogeneous chemistry
• Aerosols and heterogeneous chemistry
• Selected topics Chemistry of ozone hole and
  air pollution
• Formation process of cloud chemical reactions
  in and on cloud particles
• State-of-the-art field measurement techniques in
  atmospheric chemistry
• Atmospheric modeling 0, 1-D, 2-D and 3-D
  modeling
• Chemistry of the climate change
• Your research topics!

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• Summary of last time

• Introduction to photochemistry and Photochemistry
  of ozone and some other atmospheric oxidants
• Todays Lecture
• More photochemistry
• Photochemical initiated reactions

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Paul Shepson, 95
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Different functional Groups of VOCs

• Alkanes CH4, CH3-CH3, etc.
• Alkenes CH2CH2, CH3CHCH2, etc.
• Alkynes HCCH, etc.
• Aromatics Double-bonded hydrocarbons in a ring
  structure,
• C6H6, CH3-C6H5
• Oxygenated hydrocarbons Aldehydes (R-C(O)-H) and
  ketones
• (R-C(O)-R) eg. HC(O)H and CH3C(O)CH3
• Alcohols R-OH, e.g, ethanol
• Organic acids (RC(O)OH), e.g., HCOOH
• .

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Seinfeld and Pandis, 1998
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Seinfeld and Pandis, 1998
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• Motor vehicles are the dominant contributor to
  anthropogenic VOC emissions in the US.
• It results from the incomplete combustion of
  fuels or
• from its vaporization (exhaust and evaporative
  emissions)
• Exhaust emissions include the unburned and
  partially burned
• fuel and lubricating oil in the exhaust and gases
  that leak
• from engine.
• The evaporative emissions includes vaporization
• of fuel as a result of the heating of the fuel
  tank, vaporization
• of fuel from the fuel system while the vehicle is
  operating (running losses), fuel losses due to
  leaks and diffusion through
• containment materials (resting losses), and fuel
  vapor
• displacement as a result of filling fuel tanks
  (refueling losses).
• Motor vehicles are major sources of alkane and
  aromatic emissions.

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VOCs in downtown Montreal in July 2003
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• Anthropogenic NMHC emission strengths, to a
  reasonable
• approximation, can be assumed to be proportional
  to fossil
• fuel consumption (Muller, 1992).
• The distribution of anthropogenic emissions is
  heavily weighted
• to the northern hemisphere where the worlds
  industrialized
• nations reside.
• Based on the emission estimates of Marland et
  al., (1985),
• eastern and western Europe represent the greatest
  source region
• in the Northern hemisphere (50), followed by the
  eastern
• US and Canada (33), and Japan and China (17).

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The European emissions are concentrated in the
latitude band 35? N to 60? N, while emissions
from eastern US, China and Japan are concentrated
further south (30? to 45? N). Atmospheric
circulation patterns disperse emissions from
these regions through the troposphere. In
winter, northern Canada is heavily impacted by
emissions from Eurasia (Barrie, 1986).
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Biogenic Hydrocarbons

• Vegetation naturally emits organic compounds in
  the
• Atmosphere
• In 1960, Went first proposed that natural foliar
  emissions
• from trees and other vegetation could have a
  significant
• effect on the chemistry of the Earths
  atmosphere.
• Since then there are numerous work on the
  speciation of
• natural VOCs, their emission rate, and the
  distribution of
• these compounds and their oxidation products in
  the
• atmosphere. (Yokouchi, 1994 Fehsenfeld et al.,
  1992,
• Guenther et al., 1995)

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• Many biogenic compounds contain olefinic double
  bond that
• renders the molecule highly reactive in
  atmosphere. With the
• lifetimes of these molecules tend to be quite
  short.
• One compound typically emitted by deciduous trees
  is isoprene
• (C5H8) conifers typically emit terpenes such as
  ?-pinene, and
• ?-pinene.
• Often half or more of the VOC mass emitted by
  vegetation is
• made up of compounds other than ?-pinene, and
• ?-pinene (Singh and Zimmerman, 1990 Placet et
  al., 1990).

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Isoprene is unique among the biogenic
hydrocarbons in its relationship to
photosynthetic activity in a plant. It is
emitted from a wide variety of mostly deciduous
vegetation in the presence of photosynthetically
active radiation, exhibiting a strong increase in
emission as temperature increases.
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Jobson et al., 1994
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Not only do the isoprene and terpenoid emissions
vary considerably among plant species, but the
biochemical and biophysical processes that
control the rate of these emissions also appear
to be quite distinct. Isoprene emission appear
to be a species-dependent by-product of
photosynthesis, photorespiration, or both there
is no evidence that isoprene is stored within or
metabolized by plants. As a result, isoprene
emission are temperature and light dependent
essentially no isoprene is emitted without
illumination. However, terpenoid emissions seem
to be triggered by bio- physical processes
associated with amount of terpenoid
material present in the leaf oils and resins and
the vapor pressure of the terpenoid compounds.
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• Terpene emissions do not thus depend strongly on
  light
• their emission continue at night, but they do
  vary with
• ambient temperature.
• Lamb et al., 1987, indicated that an increase in
  ambient
• temperatures from 25 to 35? C can result in a
  factor of 4
• increase in the rate of natural VOC emission from
  
• isoprene-emitting deciduous trees and in a factor
  of 1.5
• increases from terpene-emitting conifers.
• Thus, all other factors being equal, natural VOC
  emissions
• are generally the highest on hot summer days!

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• Owing to the reactivity of isoprene and large
  emission rates,
• isoprene can have a significant impact on
  boundary layer
• photochemistry.
• Reaction with isoprene will be the dominant fate
  of the HO
• radical if isoprene concentrations are greater
  than 0.5 ppbv
• (assuming 1.8 ppmv CH4 and 200 ppbv CO).
• Indeed isoprene is a controlling influence on
  boundary layer
• HO concentrations over tropical forests
  (Rasmussen and
• Khalil 1998 Zimermaann, 1999).
• Formation of CO from isoprene oxidation is
  thought to be a
• significant global source of CO, and therefore,
  the influence
• of isoprene oxidation chemistry can extend beyond
  the
• Boundary layer into the free troposphere.

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Functional VOC Groups undergoing Photolysis
Organic Peroxides The absorption drops off
rapidly at wavelengths above the actinic cutoff
of 290 nm, photolysis is still a significant loss
process for these peroxides in the
troposphere. CH3OOH h?? CH3O HO (Vaghjiani
and Ravishankara, 1990 Thelen et al., 1993)
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• Aldehydes and Ketones
• The photolysis rate for HCHO and the rate of
  production
• of free radicals from photolysis is much higher
  than for
• The larger molecules.
• The absolute values of the absorption cross
  sections of HCHO have been somewhat controversial
• (due to possibly a lack of sufficient resolution
  in some studies
• Since if the spectral resolution is too low
  relative to the
• Bandwidth, non-linear Beer-Lambert plots result).
  

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Finlayson-Pitts and Pitts, 1999
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Halogenated VOCs
Schwedt, 1996
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Table of Rate Laws
A products
A A products
A B products
A A A products
A A B products
A B C products
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Second Order Reactions
(1)
(2)
HO CH3-CH3 ? Products
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Third Order Reactions
e.g., O O2 M ? O3 M
(3)
(4)
(5)
(6)
Low P
High P
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HO-initiated reaction of hydrocarbons
O3 h? ? O2 O(1D)
O(1D) H2O ? 2 HO
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Formation of peroxy radicals
RH HO ? R. H2O
R. O2 M ? RO2. M
R1R2CHO2. R1R2CHO2. ? 2 R1R2CHO. O2 (a)

• 2 R1R2CHOH R1R2CO O2 (b)

? 2 R1R2CHOOCHR1R2 O2 (c)
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Secondary reactions of RO2
RO2 NO ? RO NO2 ( M) ? RONO2
RO2 NO2 M ? ROONO2 M
RO2 HO2 ? ROOH O2
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Schematic of photo-oxidation of organics in
troposphere
Seinfeld and Pandis, 1997
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HO-initiated oxidation of ketones
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Seinfeld and Pandis, 1998
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HO Alkynes
HCCH HO ?HC(OH)C.H (I) ( O2) ?
HC(HO)CHOO. HC(HO)CHOO. (II) ( NO) ?
H(OH)CCHO. ( NO2) ? HC.(OH)-CHO O2 ? (CHO)2
HO2 Hatakeyama et al., 1986 also observed
another product, possibly From rearrangement and
decomposition of intermediate (II)
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HO-initiated reactions of toluene
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Theoretical calculations support that the
preferred attack site is ortho to methyl group
(Andino et al., 1996), but addition to the other
positions also occurs. If the HO-aromatic adduct
which contains about 18 kcal/mol excess energy,
is not stablized, it decomposes back to the
reactants. (e.g., Bjergbakke et al., 1996 for HO
benzene)
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Secondary Reactions
k 3 x 10-11 cm3 molecule-1 s-1
At 300 ppb NO2, this reaction will be
significant
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Secondary reactions of radical formed upon
addition to the aromatic ring
k 5 x 10-16 cm3 molecule-1 s-1
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• Concluding Remarks
• NMHCs have both biogenic and anthropogenic origin
• Biogenic activity can depend on factors such as
  temperature and light.
• They play key roles in production of ozone,
  transport of precursors of ozone
• Some hydrocarbons undergo photolysis though major
  oxidation pathway in troposphere is initiated by
  HO radicals
• NMHCs have local, regions and global impacts
• Transformation of VOCs