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

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

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

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

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?

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

externalinternal

For in-situ calibration

Langridge et al., Rev. Sci. Inst. ’08

LED-BBCEAS

Temporal resolution=15s

Typical BBCEAS absorption spectrum

NO3 electronic absorbance band

vibrational overtones of water vapour

•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

BBCEAS inlet direction 220º

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

[NO3]:[N2O5] ~1-4% due to low ambient T and moderately high NO2

Can we summarise the dataset?

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

A-Atlantic, P-Polar, NC-Northern Continental, EC-Eastern Continental

REPARTEE-II air trajectories

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

A simple function of NO and O3? Not entirely

O3 NO

All night-time data

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).

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

• 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

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

Any questions?Any questions?

8th Nov,

(storm)

Thank-you for your attentionThank-you for your attention