IVb/1 IV. Stratospheric ozone chemistry 1.Basic concepts of atmospheric chemistry 2.Ozone chemistry and ozone distribution 3.Sources and distribution of ozone-related species 4.Ozone trends 5.The ozone hole

IVb/2 Ozone trends Why studying trends?  human hazard by UV radiation  UV-B surface radiation ( nm) is dependent on the ozone column Lauder, Australia

IVb/3 Controlling stratospheric chlorine and excess skin cancer Ozone trends forms the basis for policy making  Montreal protocol and adjustments WMO 2002

IVb/4 Ozone trends Trends in total ozone,  total ozone data from satellites and groundbased data  ‚Scientific assessment of ozone depletion‘, WMO, 2002 global NH midlatitude

IVb/5 Ozone trends trends in vertical distribution of ozone  Largest trends near km and at mid- latitudes  Remember: region where chlorine gas- phase chemistry is important

IVb/6 Ozone trends different time periods = different trends trends in vertical distribution of ozone grey shading: statistically not significantly different from zero trend

IVb/7 Processes affecting stratospheric O3: Quantification How to separate these processes ?  Modelling  Statistical trend analysis 3/1997 3/1999

IVb/8 Statistical analysis of ozone trends Increase in NH total ozone after Mt. Pinatubo  Main contribution from enhanced Brewer- Dobson circulation (HTF) and solar cycle  Moderate contribution from halogens (EESC) Trend since 1995 solar wave driving/BD circulation aerosol EESC (stratospheric chlorine) Dhomse et al. (2006)

IVb/9 Statistical analysis of ozone trends

IVb/10 Statistical analysis of ozone trends Role of major volcanic eruptions  injection of ash and sulfate into the stratosphere  changes in radiation (haze), chemistry, and dynamics Volcanic eruptions detected by aerosol backscatter: El Chichon Pinatubo

IVb/11 Volcanic aerosols and global impact Volcanic eruptions detected by aerosol backscatter

IVb/12 Stratospheric (sulfate) aerosol formation

IVb/13 Global sulfur budget (troposphere) Flux units: Tg/year

IVb/14 Impact of stratospheric aerosol on the chemical composition Pseudo-first order reaction on aerosols: Heterogeneous reaction constant k x :  Unit: 1/sec  A: surface area density [cm 2 /cm 3 ]   : uptake probabibilty  c x : thermal velocity [cm/sec ] Hydrolysis of N2O5:  N2O5 night-time reservoir of NOx  Removal of reactive NOx by transfer into reservoirs HNO3 (NOy) by aerosol reactions Hydrolysis of BrONO2:  Aerosol surface reaction transfers (more stable) BrONO 3 into (reactive) HOBr collision frequency

IVb/15 Increase in aerosol size after Pinatubo Larger surface area means more chemistry on liquid/solid surfaces  more ozone depletion by heterogeneous reactions  more haze

IVb/16 Impact of stratospheric aerosols on the chemical composition Chlorine activation on aerosols: aerosol surface reactions release chlorine from the reservoirs Summary:  Aerosol surface reactions  release chlorine from the reservoirs  release bromine from reservoir  transfer NOx into NOy  all contribute to ozone loss, because at this altitudes, reactions with NOx are the main loss processes for ClOx and BrOx

IVb/17 Impact of stratospheric aerosols on the chemical composition Summary:  Aerosol surface reactions  release chlorine from the reservoirs  release bromine from reservoir  transfer NOx into NOy  all contribute to ozone loss, because at this altitudes, reactions with NOx are the main loss processes for ClOx and BrOx

IVb/18 Temperature dependence of aerosol uptake Aerosol uptake works best at cold temperatures (< 200 K) 

IVb/19 Trends in bromine and total ozone Pinatubo is especially effective for enhanced bromine about 50 % of total ozone trend due to anthropogenic chlorine; 50 % due to anthropogenic bromine! Sinnhuber et al no bromine

IVb/20 Future trends: The Montreal Protocol and its amendements Stratospheric chlorine starts to decline Bromine levels remain high  about 45% more effective in destroying ozone per halogen Effective chlorine includes the effect from bromine

IVb/21 Future trends in ozone O3 increase related to increase in greenhouse gases  Mainly stratospheric cooling  T-dependence of reaction rates recovery of ozone (return to 1980 levels): predictions vary from 2020 to 2100 WMO 2006:  models also predict stronger BD circulation  Resulting in stratospheric warming(!) and more ozone transport (faster recovery) Prediction of future trends using 2D chemical transport models (WMO 2002)

IVb/22 IV. Stratospheric ozone chemistry 1.Basic concepts of atmospheric chemistry 2.Ozone chemistry and ozone distribution 3.Sources and distribution of ozone-related species 4.Ozone trends 5.The ozone hole

IVb/23 Discovery of ozone „hole“ Halley station of the British Antarctic Survey, 75°S, 26°W Measurements adapted from Farman et al, 1985

IVb/24 Antarctic ozone depletion Ozone depletion mainly confined to altitudes between 12 and 20 km (T<-195 K ~ -78° C)

IVb/25 Definition of an ozone hole Ozone hole: area with ozone column below 220 DU

IVb/26 Where is the ozone hole located? Ozone hole mainly confined to the polar vortex  winter polar vortex is characterized by high potential vorticity values (PV)  stratospheric low pressure system (cyclone)

IVb/27 Potential vorticity Potential vorticity:   potential temperature  p pressure,  f Coriolis parameter,   latitude, longitude,  g Earth acceleration,  R Earth radius,  u zonal wind speed,  v meridional wind speed region of high absolute PV delineates polar vortex rotation of air mass

IVb/28 Potential vorticity ca. 20 km PV is conserved for adiabatic processes Remember: adiabatic parcel motion conserves potential temperature  and potential vorticity P vortex edge (green color):  region with largest gradient in PV (coincides with region of highest wind speeds=polar night jet)  transport barrier: isolates the polar vortex from middle latitudes (no mixing across ientropes)

IVb/29 Polar vortex and stratospheric temperature Region with T<195 K may contain polar stratospheric clouds (PSCs) polar night PSC region T<195K~-78°C

IVb/30 Polar stratospheric clouds Type I PSCs  Nitric acid trihydrate (NAT):  HNO33H2O  Ternary solution:  (H2O, H2SO4, HNO3)  Formation temperature: 195 K  Particle diameter: 1  m  Altitudes: km  Settling rates: 1 km/month Type II PSCs  water ice  Formation temperature: 188 K  Particle diameter: >10  m  Altitudes: km  Settling rates: >1.5 km/day

IVb/31 PSC types PSC type 1a and 1b have different polarisation signal in lidar backscatter PSC type Ia PSC type Ib PSC type II volcanic aerosol

IVb/32 Composition of liquid aerosol PSC type II PSC type I liquid aerosol Carslaw et al. 1994

IVb/33 PSC area Large PSC areas (T<195 K) above Antarctica  up to 30 Mio km 3 (~area of Russia or N-America!)

IVb/34 Dehydration after conversion into PSC type II POAM III water vapor observations during Antarctic winter  Removal of H2O water vapor  Sedimentation of ice particles

IVb/35 chlorine & bromine activation on PSC surface Chlorine activation: Denoxification: Bromine activation Reactions on PSC surfaces are much faster than on background aerosol (T-dependence): all chlorine can be activated! reservoir gas solid/liquid on PSCs active halogens

IVb/36 Gaseous ClOx and HCl T nat depends on the mixture of HNO3 and H2O (T~195 K) Below T nat, ClOx increases and gaseous HCl decreases

IVb/37 satellite observations of ClOx Reservoir removal (ClONO2) and chlorine activation (ClO) inside polar vortex denitrification: subsidence of HNO3 containing particles  prevents NO2 from deactivating ClO back to ClONO2

IVb/38 Polar ozone loss Chlorine catalytic cycles initiated when sun returns during spring: Ozone loss: Note this catalytic cycle does not require atomic oxygen (reduced during polar night!) ClO-BrO cycle: Only half of the inorganic bromine is bound in the reservoirs BrONO2 and HBr ClO dimer

IVb/39 Time evolution of ozone hole above Antarctica

IVb/40 Time evolution of ozone hole above Antarctica

IVb/41 Ozone hole area above Antarctica

IVb/42 Arctic ozone hole?

IVb/43 Inter-annual ozone variability 63°N-90°N 63°S-90°S Northern polar latitudes spring Southern polar latitudes spring ‚ozone hole‘: TOZ < 220 DU

IVb/44 Inter-annual ozone variability 63°N-90°N 63°S-90°S chemical ozone loss inter-hemispheric differences in transport inter-annual variability in ozone chemistry & transport in each hemisphere

IVb/45 Ozone variability in northern hemisphere 63°N-90°N

IVb/46 Ozone variability High inter-annual ozone variability in winter/spring NH  Cold (stratospheric) Arctic winters with low ozone:  1996, 1997, 2000, (2003), 2005  Warm Arctic winters with high ozone  1998, 1999, 2001, 2002, 2004

IVb/47 Arctic ozone hole? PSC occurences are sporadic in NH  higher planetary wave activity  larger inter-annual variability Almost no PSC type II (ice) observed in NH

IVb/48 PSC Area (T<195, Type I) 475 K~19km 70 hPa~17 km ice PSC (type II): dotted NAT PSC (type I): solid) PSC occurences are sporadic in NH  higher planetary wave activity  Larger inter-annual variability Almost no PSC type II (ice) observed in NH

IVb/49 Hemispheric differences in chlorine activation OClO observations from GOME

IVb/50 Inter-annual variability in Arctic ozone loss

IVb/51 Polar ozone loss in the Arctic Arctic winter/spring 1999/2000 is considered a winter with high ozone loss compared to other Arctic winters