solution to homework problem set 1 - eth z · stratospheric chemistry hs 2017 solution to homework...

Stratospheric Chemistry HS 2017

Solution to Homework Problem Set 1

For questions:[email protected] (CHN O15.2)

Problem 1: Explosive ozone

Ozone is explosive because the reaction:

O3 + O3 → 3 O2

is exothermic (i.e. releases heat).

Given a volume with sufficiently high concentration of ozone, the heat released by each reaction increases the rates of the subsequent reactions (T increases → P increases → chain reaction).

The energy released is the enthalpy of reaction:

ΔHReaction = CProducts ∙ ΔHF(Products) − CReactants ∙ ΔHF(Reactants)

(ΔHF = Enthalpy of formation, C = Stechiometric coefficients)

Problem 1: Explosive ozone

From Brasseur and Solomon [1986]:

ΔHF(O3) = 34.1 kcal / molΔHF(O2) = 0 kcal / mol

→ ΔHReaction = −68.2 kcal / mol = −2.98 kJ / g

Problem 2: The “good” tropospheric ozone

Ozone in the troposphere acts as a «cleaning agent» because it leads to theformation of OH radicals, which oxidate the pollutants (e.g. methane: CH4 + OH → CH3 + H2O) and increase their solubility, facilitating their removal by wet deposition in rain droplets.

Important reactions are ozone photolysis (requires UV radiation)

O3 + hv → O2 + O1D (λ < 310 nm)

Followed by OH formation

O1D + H2O → 2 OH

O1D is the excited state of the oxygen atom that has sufficient energy tobreak a water molecule: the ground state (O3P) cannot react with H2O.

Problem 3: Surface UV radiation

Surface UV reduction potential = Fraction of UV radiation which is removed (i.e. absorbed and/or scattered) from the incoming sunlight in the atmosphere before it reaches the surface.

a) Clouds: Generally between 15-30%, with strong variations depending on cloud type, depth and distribution across the sky. Up to 90% in case ofstrongly overcast conditions.

Aerosols: Generally around 15% in urban areas, up to 25% in strongly pollutedareas. Note that these values are larger than the expected UV response tochanges in stratospheric ozone.

[Ozone Scientific Assessment, chapter 2]


Surface albedo = Fraction of the incoming sunlight (in this case UV) which is reflected from the Earth’s surface without being absorbed.

b) Snow-free surfaces: Less than 0.1


Snow-covered surfaces: Between 0.6-1 depending on the type and age ofsnow. It can reach value of 1 (i.e. all the UV radiation is reflected) in case offresh, clean snow.

[Blumthaler and Ambach, 1988]


c)Globally ozone recovery is predicted for the next century, so more stratospheric ozone→ less UV radiation reaching the surface

At mid and high latitudes even a “super-recovery” is predicted, which implies less UV radiation reaching the surface compared to 1960.

At low latitudes recoveryis not fully accomplished, which implies sligthlymore UV reaching thesurface than in 1960.


c) Other effects:

Surface albedo (global warming): reduction of sea ice and snow-coveredareas at the the poles, reduction of mountain glaciers → Decresed surfacealbedo → More UV radiation absorbed at the surface

Clouds: global cloud coverage is predicted to increase over high latitudesand decrease over low latitudes → More UV radiation (10-15%) reaching thesurface in tropical regions, less (10-15%) in high latitudes

Impact on human health: less UV radiation reaching the surface impliesdecreasing impact of skin cancer, but also less in vitamin D production. Impact on terrestrial and marine ecosystems are largely unknown.


Problem 4: The world avoided of Montreal Protocol

Problem 4: The world avoided of Montreal ProtocolCarbon dioxide (CO2) Methane (CH4 )Nitrous oxide (N2O)

Greenhouse gases but not ozone depleting substances (ODS) therefore not affected by Montreal Protocol

→ Increased concentration due to 6 more years of emissions (2011-2017)

→ Increased warming

Halocarbons

Greenhouse gases and strong ODS: without Montreal Protocol, concentration would have increased by more than a factor of 2 (see Q15)

→ Strongly increased concentration

→ Strongly increased warming



Ozone

Besides UV ozone absorbs IR (longwave radiation from the surface) so it is also a greenhouse gas

For 2011 compared to 1750:

Less O3 in the stratosphere due to ODS emissions → Cooling effect

More O3 in the troposphere due to precursor gases (NOx, VOCs) emissions → Warming effect Strat. O3 Trop. O3

Stratospheric Ozone

For 2017 compared to 2011 without Montreal Protocol:

→ Decreased concentration due to 6 more years of ODS emissions

→ Less IR absorption in the stratosphere

→ More cooling


Strat. O3 Trop. O3


Tropospheric Ozone

Mainly formed by reaction of precursor gases (NOx, VOCs) which are not affected by Montreal Protocol

→ Increased concentration due to 6 more years of precursor gases emissions

→ More IR absorption in the troposphere

→ More warmingStrat. O3 Trop. O3

solution to homework problem set 1 - eth z · stratospheric chemistry hs 2017 solution to homework...

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