1. Night-time chemistry of the urban boundary layer - NO 3 and N 2 O 5 Measurements from REPARTEE II A.K. Benton, R.L. Jones and REPARTEE coworkers. APRIL Meeting, 26 th January 2010

2. Diurnal physical and chemical profiles VOCNO NO 2 O 3 (ppbv) OH RO 2 HNO 3 OH (pptv) Free troposphere Nocturnal residual layerDay-time boundary layer Nocturnal boundary layer ~200m ~1.5km NO NO 2 (ppt/bv) O3O3 NO 3 (pptv) N 2 O 5 (pptv) O3O3 NO 2 H 2 O, het HNO 3 COSO 2 NO 3, N 2 O 5 losses determine how much NO x available the next day NO 3 is a strong oxidant for VOCs Chemical processes as above. convection ceases  stable, stratified NO x = NO + NO 2, pptv=Parts per trillion by volume, ppbv=parts per billion by volume, VOC=Volatile organic compound NO x Oxidising capacity, acidification, tropospheric O 3 budgets

3. Very sparse measurements: -of N 2 O 5 -at 50-300m altitude (185m) -in Europe (London) -in Southern Hemisphere -in Cities -on small spatial scale -(cell =1mx3cm, t=15s) NO 3 and N 2 O 5 Measurements to date NO 3 N2O5N2O5

4. transport ‘barrier’ Measurements from Brown et al. 2007 Sources and sinks emitted close to ground  peak of NO 3 and N 2 O 5 somewhere in NBL but away from surface Poorly quantified (one night’s data!) What are the spatial distributions of NO 3 and N 2 O 5 in the NBL under different meteorological, chemical and aerosol conditions? Vertical profiles of NO 3 and N 2 O 5

5. Questions How do profiles of NO 3 and N 2 O 5 change as a function of: -altitude? -chemistry? -meteorological conditions? -aerosols? What fraction of NO x is tied up in the NO 3 /N 2 O 5 equilibrium at night? To what extent does this affect NO x /O 3 budgets?

6. How do we measure these chemicals? Beer-Lambert Law: Cavity throughput light intensities I Time LED on LED off  km’s of pathlength sensitivity in 1m cell λ range: ~640-675nm. λ centre ~660nm –water vapour and NO 3. detector Light resonates in high-finesse optical cavity A LED-BBCEAS absorption spectrum BBCEAS=Broadband cavity enhanced absorption spectroscopy

7. external internal For in-situ calibration T35 Langridge et al., Rev. Sci. Inst. ’08 LED-BBCEAS Temporal resolution=15s

8. Typical BBCEAS absorption spectrum NO 3 electronic absorbance band vibrational overtones of water vapour

9. N 2 O 5 does not absorb in visible region 90°C heated inlet to shift equilibrium (First used by Brown et al., 2001) ∑[NO 3 ]+[N 2 O 5 ] measured at ~660nm at height of 185m. Data from 19 th October – 15 th November 2007 How does chemistry change with altitude in the NBL? REPARTEE II: Regent’s Park and Tower Experiments NO 3 N2O5N2O5 185m

10. BBCEAS inlet direction 220 º

11. What does the dataset look like?  Real atmospheric structure, not noise ! [NO 3 ] max ~12ppt [N 2 O 5 ] max ~700ppt [NO 3 ]:[N 2 O 5 ] ~1-4% due to low ambient T and moderately high NO 2

12. Can we summarise the dataset?

13. Lifetimes of NO 3 and N 2 O 5  (N 2 O 5 )~2min-2hours  (NO 3 )~1-2min  Consistent with strong vertical gradient, particularly for N 2 O 5

14. A-Atlantic, P-Polar, NC-Northern Continental, EC-Eastern Continental REPARTEE-II air trajectories

15. 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

16. A simple function of NO and O 3 ? Not entirely O3O3 NO All night-time data

17. An example night: 30-31 st 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).

18. 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(NO x ) suggests shorter lifetimes Shorter lifetimes suggest rapid sinks for N 2 O 5  The N 2 O 5 and NO 3 partitioning is important for the storage and removal of nitrogen oxides at night

19. First measurements of NO 3 and N 2 O 5 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 O 3 /NO but not straightforward. A combination of physical and chemical properties appear to be important in determining [NO 3 ] and [N 2 O 5 ]. Fraction of nocturnal nitrogen oxide stored in the NO 3 ↔ N 2 O 5 equilibrium calculated, suggests short lifetimes and high sinks for NO x Model fails to reproduce rate of removal and small scale variability. N 2 O 5 a source for nitrate in aerosols?  aerosols a sink for N 2 O 5 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

20. 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, NO 2, O 3, 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

21. Any questions? 8 th Nov, (storm) Thank-you for your attention