Transcript Two short stories on atmospheric composition

Two short stories on
atmospheric composition
• Re-evaluation on the role of N2O5
• A ‘cheeky’ bottom-up evaluation of global
mean OH
NO + HO2  O3 + OH
Last IPCC report
Sources of oxides of nitrogen
Last IPCC report
Sinks of oxides of nitrogen
• Lots of inter-conversions between
different species
NO, NO2, NO3, N2O5, HONO, HO2NO2, HNO3,
PAN, PPN, MPAN, ‘other PANs’, organic nitrates
…….
• Loss mechanisms dominated by wet
and dry deposition of HNO3
How do we make HNO3 1
1. OH + NO2  HNO3
Looks simple but has caused at least a decade of
controversy
OH+NO2  HOONO  HNO3
OH+NO2  HNO3
Missing chemistry – ‘magic aerosol’ reactions
How do we make HNO3 2
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NO + O3  NO2
NO2 + hn  NO + O3
NO2 + O3  NO3
NO3 + hn  NO2 + O3
NO3 + NO2  N2O5
N2O5 + hn / T  NO3 + NO2
N2O5  2 HNO3 (on aerosols)
How do we parameterize the
uptake?
N2O5
N2O5
How do we parameterize the
uptake?
• How many molecules hit the surface of
the aerosol per second?
– Gas kinetics – mean free path etc etc etc
• What fraction of the molecules that hit
the surface of the aerosol react
g
How do we find the g
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Lab studies
For N2O5 important in the stratosphere
Lots of work done
g = 0.1 ish
What impact does this have on
the tropospheric composition?
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REACTION OF N2O5 ON TROPOSPHERIC AEROSOLS - IMPACT ON THE GLOBAL DISTRIBUTIONS
OF NOX, O3, AND OH, Dentener FJ, CRUTZEN PJ, J.G.R. 1993
Abstract:
Using a three-dimensional global model of the troposphere, we show that the heterogeneous reactions of
NO3 and N2O5 on aerosol particles have a substantial influence on the concentrations of NO(x), O3, and
OH. Due to these reactions, the modeled yearly average global NO(x) burden decreases by 50% (80% in
winter and 20% in summer). The heterogeneous removal of NO(x) in the northern hemisphere (NH) is
dominated by reactions on aerosols; in the tropics and southern hemisphere (SH), with substantial smaller
aerosol concentrations, liquid water clouds can provide an additional sink for N2O5 and NO3. During spring
in the NH subtropics and at mid-latitudes, O3-concentrations are lowered by 25%. In winter and spring in
the subtropics of the NH calculated OH concentrations decreased by up to 30%. Global tropospheric
average O3 and OH burden (the latter weighted with the amount of methane reacting with OH) can drop by
about 9% each. By including reactions on aerosols, we are better able to simulate observed nitrate wet
deposition patterns in North America and Europe. 03 concentrations in springtime smog situations are
shown to be affected by heterogeneous reactions, indicating the great importance of chemical interactions
resulting from NO(x) and SO2 emissions. However, a preliminary analysis shows that under present
conditions a change in aerosol concentrations due to limited SO2 emission control strategies (e.g.,
reductions by a factor of 2 in industrial areas) will have only a relatively minor influence on O3
concentrations. Much larger reductions in SO2 emissions may cause larger increases in surface O3
concentrations, up to a maximum of 15%, if they are not accompanied by a reduction in NO(x) or
hydrocarbon emission.
Cited 234 times
End of story?
• Well g were derived for stratospheric
conditions.
• Cold
• Pure sulfuric acid
• Troposphere though is
– Warm and sulfuric acid aerosol is
neutralized
Rumblings of discontent
• Tie et al., [2003] found gN2O5<0.04 gave
a better simulation of NOx
concentrations during TOPSE
• Photochemical box model analyses of
observed NOx/HNO3 ratios in the upper
troposphere suggested that gN2O5 is
much less than 0.1 [McKeen et al.,
1997; Schultz et al., 2000]
2000
HETEROGENEOUS CHEMISTRY AND TROPOSPHERIC OZONE
Jacob, Atmos. Env., 2000
Ozone is produced in the troposphere by gas-phase oxidation of hydrocarbons and CO catalyzed by hydrogen
oxide radicals (HOx º OH + H + peroxy radicals) and nitrogen oxide radicals (NOx º NO+NO2). Heterogeneous
chemistry involving reactions in aerosol particles and cloud droplets may affect O3 concentrations in a number of
ways including production and loss of HOx and NOx, direct loss of O3, and production of halogen radicals.
Current knowledge and hypotheses regarding these processes are reviewed. It is recommended that standard
O3 models include in their chemical mechanisms the following reaction probability parameterizations for reactive
uptake of gases by aqueous aerosols and clouds: gHO2 = 0.2 (range 0.1-1) for HO2 ® 0.5 H2O2, gNO2 = 10-4
(10-6-10-3) for NO2 ® 0.5 HONO + 0.5 HNO3, gNO3 = 10-3 (2x10-4-10-2) for NO3 ® HNO3, and gN2O5 = 0.1
(0.01-1) for N2O5 2 HNO3. Current knowledge does not appear to warrant a more detailed approach.
Hypotheses regarding fast O3 loss on soot or in clouds, fast reduction of HNO3 to NOx in aerosols, or
heterogeneous loss of CH2O are not supported by evidence. Halogen radical chemistry could possibly be
significant in the marine boundary layer but more evidence is needed. Recommendations for future research are
presented. They include among others (1) improved characterization of the phase and composition of
atmospheric aerosols, in particular the organic component; (2) aircraft and ship studies of marine boundary layer
chemistry; (3) measurements of HONO vertical profiles in urban boundary layers, and of the resulting HOx
source at sunrise; (4) laboratory studies of the mechanisms for reactions of peroxy radicals, NO2, and HNO3 on
surfaces representative of atmospheric aerosol; and (4) laboratory studies of O3 reactivity on organic aerosols
and mineral dust.
2003 / 2004
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Working on TRACE-P
NOx too low in the model
What could we do about it?
Normally look at sources
But also look at sinks
New literature
New lab studies
• Warmer temperatures
• More humid conditions
• Tropospherically applicable aerosols
New literature
• Kane et al., 2001 - Sulfate – RH
– JPL
• Hallquist et al., 2003 - Sulfate - temp
– Tony Cox’s group in Cambridge
• Thornton et al., 2003 - Organics - RH
– Jon Abbatt’s group at U Torontio
Parameterization based on best
available literature
Aerosol type
Reaction probabilityb
Reference
Sulfatea
g = a(RH)10b(T)
a = 2.7910-4 +
1.310-4  RH 3.4310-6  RH2 +
7.5210-8  RH3
b = 410-2(T-294) (T ≥ 282K)
b = -0.48
(T < 282K)
[Kane et al., 2001]
[Hallquist et al., 2003]c
Organic Carbon
g = RH  5.210-4 (RH < 57%)
g = 0.03
(RH ≥ 57%)
[Thornton et al., 2003]d
Black Carbon
g = 0.005
[Sander et al., 2003]
Sea-salt
g = 0.005
g = 0.03
Dust
g = 0.01
(RH < 62%)
(RH ≥ 62%)
[Sander et al., 2003]e
[Bauer et al., 2004]f
What gs do we
get?
• Much lower than 0.1
• Dry low values
• Higher at the surface
What is the impact
on composition?
Lower gN2O5 = higher NOx
Higher NOx = Higher O3
Higer NOx = Higher OH
Does this make the model better?
• Complexity in model isn’t automatically
a good thing
• More complex models are not
necessarily better at simulating the
atmosphere
• Complex models take longer to run and
confuse the issue
Compare with observations
Emmons et al. [2000] climatology of NOx
Mass weighted model bias changes from
–14.0 pptv to –7.9 pptv
Mean ratio changes from
0.77 to 0.86
Middle troposphere (3-10km) changes from
0.79 to 0.91
Compare with observations
Logan [1998] Ozonesonde climatology
Mass weighted model bias
-2.9 ppbv to -1.4 ppbv
Mean ratio changes from
0.94 to 0.99.
Ox (odd oxygen) budget
Chemical production increases 7%
3900 Tg O3 yr-1 to 4180 Tg O3 yr-1
Compare with observations
Global annual mean tropospheric OH
0.99106 cm-3 to 1.08106 cm-3
8% increase.
Both values are consistent with the current constraints
on global mean OH concentrations based on methylchloroform observations:
1.07 (+0.09 -0.17)  106 cm-3 [Krol et al., 1998]
1.16  0.17
 106 cm-3 [Spivakovsky et al., 2000]
0.94  0.13
 106 cm-3 [Prinn et al., 2001]
Conclusions
• Aerosol reaction of N2O5 is very
important for the atmosphere
• Previous estimates have been too high
• New laboratory data allows a better
constraint
• Sorting out old problems although not
‘sexy’ is important
A ‘cheeky’ bottom-up
evaluation of global mean OH
NO + HO2  O3 + OH
Last IPCC report
Global mean OH
How do they calculate global
mean OH
• Methyl chloroform – solvent in Tippex
• Made by a few large chemical
companies
• Sources are known (nearly)
• Can measure concentrations across the
globe
Measured across the world
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(1) Ireland, Mace Head
(2) USA Cape Meares, California
(3) Barbados
(4) American Samoa
(5) Tasmania
Observations
Inversion
• Have emissions in your Chemistry –
Transport
• Emit the MCF
• Blow it around
• Compare to the observations
• Optimize the OH concentration to get
best possible fit
Top down approach
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Don’t directly observe the OH
Indirect method
Can directly observe OH
But lifetime of OH is ~ 1s
So measurements at one site don’t tell
you much about global concentrations
• Is this true?
NAMBLEX, EASE ’97, SOAPEX
• OH measured by the FAGE group in
chemistry
• Time series of OH
• Can we use this to provide information
about global OH
• ‘Couple’ global atmospheric chemistry
model and the observations
Mace Head - Ireland
Observed vs Modelled OH
Observations dominated by day
What have we learnt?
• In general high OH at solar noon
Model 
• Low OH at night
Model 
• Daily variability
Model ?
More useful comparison
Measured mean is 1.8 × 106 cm-3,
Modelled mean is 2.3 × 106 cm-3
Ratio of 1.56 ± 1.62.
The statistical distribution of the ratio is
not normal and so more appropriate
metrics such as the median (1.13) or the
geometric mean (1.13 +1.44 -0.64 ),
The model simulates 30% of the linear
variability of OH (as defined by the R2).
The uncertainty in the observations (13%)
suggests that the model systematically
overestimates the measured OH
concentrations.
Other HOx components
Sampled for the
EASE ‘97 campaign
Over a year
Smoothed
Sampled
mean
forOH
thefrom
NAMBLEX
model
campaign
Observed Campaign means
Why the annual variability?
All data over a year
R=0.92
All data over a year
Smoothed R=0.98
Is this only at the surface?
Cape Grim - Australia
Other places
So what have we learnt?
• Mace Head we tend to over estimate
• Cape Grim doesn’t seem so bad
• Can we combine this information and
the model to get a global number?
• Very Cheeky!
What do we get?
A Priori
(Model)
Compare
FAGE
A Posteri
Prinn et al.
NH
1.12
-19%
0.91
0.90 ± 0.20
SH
1.02
+1%
1.03
0.99 ± 0.20
Global 1.07
-9 %
0.97
0.95
What does this mean
• Very, very lucky!!!!
• The FAGE OH and the MCF inversions
seem consistent
• Model transfer seems to work
• Uncertainties suggest it could have
gone the other way
Can we do this better?
• Include more data
– Aircraft campaign
– Surface sites
– Ships
• How do we incorporate this information?
One approach
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Have the modelled concentrations
But want to reduce the dimensionality
Principal Components Analysis
Takes concentrations from model
Comes up with different way of thinking
about the model gridboxes
Component 1
Component 2
Component 3
Component 4
How might we use this?
• Compare OH modelled with OH measured
• For each point workout the fraction of that box
represented by each component
• R (Box Model / Measured) = Σ Cstrength Rcomponent
• Find the Rs
• Reapply to the model OH field
• Calculate a global OH
Conclusions
• CTM comparison with OH looks pretty
good
• We can use this information to constrain
the model OH and this gives a
reasonable result
• To take this further requires a bit more
thought