atmospheric chemistry lecture 5: polar ozone holes & arctic haze dr. david glowacki university...
TRANSCRIPT
Atmospheric chemistry
Lecture 5:
Polar Ozone Holes & Arctic Haze
Dr. David GlowackiUniversity of Bristol,UK
Over the last 4 days…• We’ve discussed:
– Atmospheric structure & transport – Chemical kinetics– Tropospheric oxidation chemistry
– Stratospheric O3 chemistry
Today we’re going to put the pieces together to understand…
• Stratospheric Polar Ozone holes (1995 Chemistry Nobel Prize)
• Arctic Haze (particularly bad in the Northern Hemisphere)
Polar O3 holes
Polar Ozone Holes- Why do we care?
Polar Ozone Holes: Why do we Care?
• The EPA estimates that 60 million Americans born by the year 2075 will get skin cancer because of Ozone depletion
• UV exposure also harms plants & animals
October 2000 “For the Second time in less than a week dangerous levels of UV rays bombard Chile and Argentina, The public should avoid going outside during the peak hours of 11:00 a.m. and 3:00 p.m. to avoid exposure to the UV rays”
Ushaia, Argentina
The most southerly city in the world
Ozone loss does appear in the Arctic, but not as dramatic
Some years see significant
depletion, some years not, and always much less than over
Antarctica
Arctic O3 measurements above Spitzbergen
Catalytic ozone destructionThe loss of odd oxygen can be accelerated through catalytic cycles whose net result is the same as the (slow) 4th step in the Chapman cycle
Uncatalysed: O + O3 O2 + O2 k4
Catalysed: X + O3 XO + O2 k5
XO + O X + O2 k6
Net rxn: O + O3 O2 + O2
X is a catalyst and is reformed
X = OH, Cl, NO, Br (and H at higher altitudes)
• Yesterday, we discussed OH & NO catalyzed loss• What about Br & Cl catalyzed loss?
Catalytic O3 loss via Cl
Catalytic O3 loss at high [ClO]
Br + O3 BrO + O2
Cl + O3 ClO + O2
ALSO BrO + ClO Br + ClOO
ClOO Cl + O2
Net 2O3 3 O2
Br and Cl are regenerated, and cycle does not require O atoms, so can occur at lower altitude
Sources of bromine:
CH3Br (natural emissions from soil and used as a soil fumigant)
Halons (fire retardants)
Catalytic cycles are more efficient as HBr and BrONO2 (reservoirs for active Br) are more easily photolysed than HCl or ClNO3
But, there is less bromine than chlorine
Catalytic O3 loss via Br
Br
BrO
BrO
Br
Cl in the stratosphere: Chloroflorocarbons (CFCs or Freons)
CFC measurements
Niwot Ridge
Pt Barrow
Mauna Loa
Am Samoa
Cape Grim
South Pole
Data from NOAA CMDL
Ozone depleting gases measured using a gas chromatograph with an electron capture detector (invented by Jim Lovelock)
Values in the N hemisphere slightly higher
Global CFC Emissions
These are ground-based measurements. The maximum in the stratosphere is reached about 5 years later
CFC’s are not destroyed in the troposphere. They are only removed by photolysis once they reach the stratosphere.
CFC transport
Simultaneous measurements
of ClO and O3 on the ER-2
Late August 1987 September 16th 1987Still dark over Antarctica Daylight returns
Gas phase chemistry alone cannot explain these observations
Polar Stratospheric Clouds
• PSCs catalyze the conversion of ClNO3 (a Cl reservoir) to Cl2 (and eventually Cl)
• PSC solid phases:
• Formation is more likely in the Antarctic because of Lower T
O3 depletion within the Antarctic vortex
ClO+BrO Cl+Br+O2
In the southern hemisphere, strong westerly winds arise from Coriolis forces because there is little land to induce turbulent mixing
This results in a south polar cell (vortex) which is more isolated from southern mid-latitudes than the northern polar cell, and extreme temperature gradients – especially in winter
Arctic Haze
Arctic Haze
Possible exam trick question: Identify Los Angeles
Observations of Arctic Haze
• First observed in the 1950s during US military weather observation flights from bases in Alaska to the high Arctic
G.E. Shaw, “The Arctic Haze Phenomenon”, Bull. Am. Met. Soc., 1995, 76(12), p 2403-2413
Arctic Haze: Pollutant Transport & buildup
• Pollutants from lower latitudes are transported to the Arctic are oxidized during arctic summer
• During polar winter, OH oxidation ceases • Cold temperatures make the arctic boundary layer very stable
during winter, dramatically slowing mixing
Arctic Haze: Pollutant Transport & buildup
• Peroxyacetylnitrate can transport NO2 long distances from source
• Typical VOC profiles will look something like this
Arctic Haze: In the polar spring the sun returns…
• The boundary layer temperatures increase• OH production begins
• NO2 is released from its reservoirs
• [O3] and [RO2], and [RO] increase rapidly
• The chemical reactor turns ON, making a mixture of reactive chemicals and sticky peroxy radicals that can react with gas phase and aerosol species
OH HO2
RO2 RO
NO NO2
NONO2
oxidation productVOC
VOCOH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O2
OH HO2
RO2 RO
NO NO2
NONO2
oxidation productVOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O2
sunlight
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O2
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O3sunlight
O2
VOC
VOCOH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O2
O3
O3
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O2
sunlight
O3
O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O2
O3
O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3O3
O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O3sunlight
O2
O3
O3
VOC
VOCOH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O2
O3
O3
O3
O3
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O3
O3
O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O2
sunlight
O3 O3
O3O3
O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O2
O3
O3
O3
O3O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O3
O3
O3
O3O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O3sunlight
O2
O3
O3
O3O3
O3
VOC
OH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O3
O3
O3O3
O3
O2
VOC
ARCTIC HAZE!!!!!!!!!
VOCOH HO2
RO2 RO
NO NO2
NONO2
oxidation product
O3
O3
O3
O3O3
O3
Conclusions
• The change in light conditions from polar winter and polar summer makes for some crazy polar pollution
• Atmospheric transport, kinetics, and sunlight influence both stratospheric ozone loss and arctic haze
• The different properties of the arctic and antarctic make for different chemistry