night-time chemistry of the urban boundary layer - no 3 and n 2 o 5 measurements from repartee ii...
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Night-time chemistry of the urban boundary layer-NO3 and N2O5 Measurements from REPARTEE II
A.K. Benton, R.L. Jones and
REPARTEE coworkers.
APRIL Meeting, 26th January 2010
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Diurnal physical and chemical profiles
VOC NO
NO2
O3 (ppbv)OH
RO2
HNO3
OH (pptv)
Free troposphere
Nocturnal residual layerDay-time boundary layer
Nocturnal boundary layer ~200m
~1.5km
NO
NO2 (ppt/bv)
O3
NO3 (pptv)
N2O5 (pptv)
O3
NO2
H2O, het
HNO3
CO SO2
•NO3 , N2O5 losses determine how much NOx available the next day•NO3 is a strong oxidant for VOCs
Chemical processes as above.convection ceases stable, stratified
NOx = NO + NO2 , pptv=Parts per trillion by volume, ppbv=parts per billion by volume, VOC=Volatile organic compound
NOx
Oxidising capacity, acidification, tropospheric O3 budgets
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Very sparse measurements:
-of N2O5
-at 50-300m altitude (185m)-in Europe (London)-in Southern Hemisphere-in Cities-on small spatial scale-(cell =1mx3cm, t=15s)
NO3 and N2O5 Measurements to date
NO3N2O5
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transport ‘barrier’
Measurements from Brown et al. 2007
Sources and sinks emitted close to ground Sources and sinks emitted close to ground
peak of NOpeak of NO33 and N and N22OO55 somewhere in NBL but away from surface somewhere in NBL but away from surface
Poorly quantified (one night’s data!)Poorly quantified (one night’s data!)
What are the spatial distributions of NOWhat are the spatial distributions of NO33 and N and N22OO55 in the NBL under different in the NBL under different meteorological, chemical and aerosol conditions?meteorological, chemical and aerosol conditions?
Vertical profiles of NO3 and N2O5
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Questions
• How do profiles of NO3 and N2O5 change as a function of:
-altitude?
-chemistry?
-meteorological conditions?
-aerosols?
• What fraction of NOx is tied up in the NO3/N2O5 equilibrium at night?
• To what extent does this affect NOx/O3 budgets?
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How do we measure these chemicals?
lXIILn t ][/ 0 Beer-Lambert Law:
Cavity throughput light intensities
I
TimeLED on
LED off
km’s of pathlength sensitivity in 1m cell
λ range: ~640-675nm. λcentre~660nm –water vapour and NO3.
detector
Light resonates in high-finesse optical cavity
A LED-BBCEAS absorption spectrum
BBCEAS=Broadband cavity enhanced absorption spectroscopy
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externalinternal
For in-situ calibration
Langridge et al., Rev. Sci. Inst. ’08
LED-BBCEAS
Temporal resolution=15s
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Typical BBCEAS absorption spectrum
NO3 electronic absorbance band
vibrational overtones of water vapour
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•N2O5 does not absorb in visible region
•90°C heated inlet to shift equilibrium (First used by Brown et al., 2001)
•∑[NO3]+[N2O5] measured at ~660nm at height of 185m.
•Data from 19th October – 15th November 2007
•How does chemistry change with altitude in the NBL?
REPARTEE II: Regent’s Park and Tower Experiments
2
1
52
3NO(T)eqKON
NO
NO3 N2O5
185m
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BBCEAS inlet direction 220º
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What does the dataset look like?
Real atmospheric structure, not noise!
[NO3]max ~12ppt
[N2O5]max ~700ppt
[NO3]:[N2O5] ~1-4% due to low ambient T and moderately high NO2
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[NO3]:[N2O5] ~1-4% due to low ambient T and moderately high NO2
Can we summarise the dataset?
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Lifetimes of NO3 and N2O5
(N2O5)~2min-2hours
(NO3 )~1-2min
]][NOk[O
][NO)(NO
ssτ
23
33
]][NOk[O
]O[N)O(N
ssτ
ONO
23
5252
23k
32
ONO
Consistent with strong vertical gradient, particularly for N2O5
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A-Atlantic, P-Polar, NC-Northern Continental, EC-Eastern Continental
REPARTEE-II air trajectories
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No correlation with air mass history
A-Atlantic, P-Polar, NC-Northern Continental,EC-Eastern Continental
Aspects of recent air mass history must important, but no correlation with wind direction
0
100
200
300
400
500
0
30
60
90
120
150
180
210
240
330
pp
t
0
0.13
pro
bab
ility
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A simple function of NO and O3? Not entirely
O3 NO
All night-time data
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An example night: 30-31st Oct
LIDAR*
Physical properties of NBL influencing chemical composition- An indication of NBL top. Are we in/out of NBL?
BT tower height
*J. Barlow, T. Dunbar, University of Reading, U.K.; F. Davies, University of Salford, U.K. LIDAR Data courtesy of the University Facilities for Atmospheric Measurement (UFAM).
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What is the proportion of nitrogen oxide stored in nocturnal reservoir?
Median 0.05, Range 0.01-0.1
c.f. 0.2, McLaren et al. ’09 (Polluted marine environment, Canada)
Lower values of F(NOx) suggests shorter lifetimes
Shorter lifetimes suggest rapid sinks for N2O5
][2][][
][2][)(
5232
523
ONNONO
ONNONOF x
The N2O5 and NO3 partitioning is important for the storage and
removal of nitrogen oxides at night
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• First measurements of NO3 and N2O5 at top of NBL altitude in a urban European site.
• Vertical profile information is important, in addition to ground level.
• Variation in night-time concentrations observed, some correlation with O3/NO but not straightforward. A combination of physical and chemical properties appear to be important in determining [NO3] and [N2O5].
• Fraction of nocturnal nitrogen oxide stored in the NO3↔ N2O5 equilibrium calculated, suggests short lifetimes and high sinks for NOx
• Model fails to reproduce rate of removal and small scale variability.
• N2O5 a source for nitrate in aerosols? aerosols a sink for N2O5
• Vertical profile information is important, in addition to ground level.• Future work:
Extrapolate to global-scale Airborne vertical resolution experiments-RONOCO, (UK based, multi-channel)
Summary
RONOCO=Role Of Night-time chemistry in controlling the Oxidising Capacity of the atmOsphere
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NERC Studentship awarded to A.K. Benton. REPARTEE I and II campaigns funded by the BOC Science Foundation.BT for use of the tower. We gratefully acknowledge the following people for ancillary data used in this work:
R.M. Harrison, W.J. Bloss, University of Birmingham, Birmingham, U.K; M. Dall’Osto, (now at NUI Galway, Eire.) –NO, NO2, O3, met. data.
E. Nemitz, C. di Marco and G. Phillips; Centre for Ecology and Hydrology (CEH), Edinburgh, U.K. –Aerosol data, CO.
J. Barlow, T. Dunbar, University of Reading, U.K.; F. Davies, University of Salford, U.K. LIDAR Data courtesy of the University Facilities for Atmospheric Measurement (UFAM).
J.M. Langridge (now at NOAA, Boulder, CO, U.S.A.) for instrument development
A. Hollingsworth and S. Ball (University of Leicester) for collaborations.
Acknowledgements
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Any questions?Any questions?
8th Nov,
(storm)
Thank-you for your attentionThank-you for your attention