Atmospheric chemistry

Lecture 5:

Polar Ozone Holes & Arctic Haze

Dr. David GlowackiUniversity of Bristol,UK

david.r.glowacki@bristol.ac.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