Meteorology 465- The Middle Atmosphere

 

The Ozone Hole.

 

Some history of the ozone hole.

 

In 1984, Joe Farman and the British Antarctic Survey noticed yet another year where the springtime ozone was lower than what they had usually measured, starting about 1997.  They submitted a paper to Nature, and it was published in 1985. 

 

Look at the webpages and the chapter on Stratospheric Chemistry to find examples.

 

Ozone is supposed to have a 100-year lifetime in the polar lower stratosphere.  This rapid ozone loss in the lower stratosphere really took everyone by surprise.  The UARS satellite was being built at the time to examine the possibility of ozone loss by chlorine, but at 40 km, when chlorine catalysis is the major catalytic cycle.

 

Many mechanisms were proposed, usually according to the scientists’ specialties:

·       Dynamicists:  upward motion of ozone poor air into the lower stratosphere (N2O was small.)

·       Ionospheric scientists:  ions spiraling into the polar region, causing ozone loss (NOx was very small.)

·       National Inquirer:  Aliens stealing our ozone (No evidence to disprove.)

·       atmospheric chemists:  A new chlorine chemistry (Never seen before.)

 

Chemists immediately zeroed in on chlorine as being the culprit because it is the only chemical that has changed substantially over the last 50 years. 

 

Problems:

 

1.     Normal chlorine chemistry won’t work because at that time of year in that location O atoms are not produced.

2.     Most of the chlorine would be in the forms of HCl and ClONO2 (a partitioning problem).  No mechanism was known to shift HCl and ClONO2 to ClO and then keep it there.

3.     The polar air would need to be mostly isolated from the midlatitude air so that these processes could have the time to work.

 

Both of these problems had to be solved.

 

Let’s start with the second of the three problems.  It was well known that every winter in the polar lower stratosphere, the temperature drops to ~190 K, more in the Antarctic than the Arctic (show figures).  At these temperatures, the ~6 ppmv of water vapor condenses on existing sulfate aerosols, which are composed of sulfuric acid and water.  As temperature decreases, these aerosols take on water. 

 

Once the temperature drops to ~200 K, the particles begin taking on nitric acid as well as water.  At around 195 K, these particles are either super-cooled liquids or frozen, and have a size of about 1 micon.  A large percentage of the nitric acid can be taken up this way. 

 

If the temperature drops to ~190 K, significant water can accumulate on the particles, making them large enough to fall kilometers in days – weeks.

 

Effects of PSCs on chlorine chemistry.

 

Recall the number of molecules striking a surface persecond is given by F =  ¼ <v>A [Molecule], where A is the surface area density (mm2/cm3).  Typically, A is 1 mm/cm3 in the lower stratosphere, but for PSCs, it can be 10 times larger.

 

A reaction does not occur for every collision.  The ratio of reactions (or sticking) to collisions is usually denoted as g.

 

First order rate = ¼ g<v>A.

 

Several important heterogeneous reactions occur:

 

NO2 + O3 ® NO3 + O2

NO3 + NO2 + M ® N2O5 + M

 

This occurs in the gas-phase.  N2O5 is a nighttime reservoir with a lifetime of hours to day in sunlight.

 

However, a heterogeneous reaction can occur:

 

N2O5 (g) + H2O (s) ® 2 HNO3 (s)

 

The reaction coefficient, g, is about 0.06.  The typical aerosols surface area density is 0.6 to 1 mm2 /cm3.  If <v>ClONO2 ~ 200 m s-1, then the first order rate coefficient for this process is about ¼ x 200x102 x 0.06 x .6x10-8 ~ 2x10-6 s-1. 

 

·       HNO3(g) ® HNO3 (s)

 

·       HCl (s)  + ClONO2 (g) ® Cl2 (g) + HNO3 (s)

 

·       ClONO2 (g) + H2O (s) ® HOCl (g) + HNO3 (s)

 

·       HOCl (g) + HCl (s) ® Cl2 + H2O (s)

 

The reaction coefficients are strongly dependent on both temperature and composition of the aerosol.

 

Chlorine species are liberated from their reservoirs and converted into gas-phase Cl2; This can be nearly 100%.  Nitrogen species are converted into HNO3 and sequestered onto the aerosol particles.  If these particles are big, enough, they fall out of the stratosphere, taking nitrogen species with them.

 

Cl2 is not reactive with O3.  However, it is easily photolyzed into two Cl atoms with near visible sunlight (not UV).  As a result, when the airmass with Cl2 is illuminated, we have ppb levels of Cl.

 

What would Cl do?

 

Still we do not have any O to speak of.  We need other reactions to destroy ozone.

 

Molina and Molina proposed the following:

 

ClO + ClO + M ® ClOOCl + M

ClOOCl + hv ® Cl + ClOO

ClOO + M ® Cl + O2 + M

2{Cl + O3 ® ClO + O2}

                    net:       2O3 + hv ® 3 O2

 

It turns out that ClOOCl also thermally decomposes, but only significantly at temperatures above ~ 210 K, well above the temperature of the springtime polar vortices.

 

Can we explain the ozone loss by this mechanism?  Do the calculation.

 

d[O3]/dt = - 2 kClO+ClO{[ClO]}2

 

Go to near the peak in ozone at 50 hPa.  Assume T = 195 K.  [M] = 2x1018 cm-3 .

cClO = 1.3 ppbv; cO3 = 2 ppmv initially

 

kClO+ClO = 1.3 x 10-13 cm3 molecule-1 s-1

 

d[O3]/dt = - 2 1.3x10-13 (2.6x109)2 = - 2x106 moelcules cm-3 s-1

 

If we assume that each day is 40,000 long, then 8x1010 molec cm-3 are lost each day.

 

For [O3] = 4 x 1012  intially, it takes 50 days.  Ozone loss starts in August and continues through September into October.  We have about 50 days.

 

We have a second reaction:

 

ClO + BrO ® BrCl + O2

                 ® Br + ClOO

                 ® Br + Cl + O2

                            Br + O3® BrO + O2

                            Cl + O3 ® ClO + O2

          net:                 2 O3 ® 3 O­2

 

For these conditions, kClO+BrO = 1.3x10-12 cm3 moelcule-1 s-1.  for 7 pptv of BrO, the O3 loss rate is about 1x105 molecules cm-3 s-1, or about 10% of ClO+ClO for this much chlorine.

 

A plot of ClO and ozone shows this anti-correlation that develops over time.

 

We can summarize the reactions with a single plot, or two.

 

We have a number of issues to resolve:

1.  reactive chloine becomes essentially all of the available chlorine, around 1 to 1.5 ppbv

2.  NO and NO2 go into HNO3 and HNO3 goes into PSCs.

3.  If PSCs fall out, < 10% of the NOy can be left.  Water vapor can also be reduced.

4.  If PSCs do not fall out, eventually they will re-evaporate into the air.  This HNO3 can photolyze to produce OH and NO2.  Then almost immediately, ClO + NO2 + M ® ClONO2 + M.  The balance with HCl takes longer to re-establish.

5.  Frequent PSC events over the course of a spring might be necessary for large ozone loss.  So the vortex must remain relatively isolated and cold into well into the spring for ozone loss to be large.

6.  Differences between the Arctic and Antarctic are the temperatures and the length of time in the spring that the vortex is intact.  See figure.

 

Other low temperature regions?

 

The processes that cause the rapid ozone loss made scientists think about what might be happening globally.  Look for regions of low temperature.

 

However, they were aware of another reaction that had implications:

 

NO2 + O3 ® NO3 + O2

NO3 + NO2 + M ® N2O5 + M

 

This occurs in the gas-phase.  N2O5 is a nighttime reservoir with a lifetime of hours to day in sunlight.

 

However, a heterogeneous reaction can occur:

 

N2O5 (g) + H2O (s) ® 2 HNO3 (s)

 

The reaction coefficient, g, is about 0.06.  The typical aerosols surface area density is 0.6 to 1 mm2 /cm3.  If <v>ClONO2 ~ 200 m s-1, then the first order rate coefficient for this process is about ¼ x 200x102 x 0.06 x .6x10-8 ~ 2x10-6 s-1. 

 

This reaction has a significant impact on the nitrogen species in the lower stratosphere.  NOx is reduced by a factor of 4, or more.

 

Two questions:

1.  How does this influence ozone loss?        

2.  What do you think a volcano would do?