A.K. Benton, R.L. Jones and REPARTEE coworkers.

1. 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

2. Diurnal physical and chemical profiles Free troposphere ~1.5km Day-time boundary layer Nocturnal residual layer H2O, het Oxidising capacity, acidification,tropospheric O3 budgets N2O5(pptv) HNO3 NO2 NO3(pptv) •NO3 , N2O5 losses determine how much NOx available the next day •NO3 is a strong oxidant for VOCs HNO3 O3 NO2(ppt/bv) OH (pptv) O3 NO2 NO RO2 O3(ppbv) OH Nocturnal boundary layer ~200m VOC NO CO NOx SO2 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

3. NO3 and N2O5 Measurements to date 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) N2O5 NO3

4. Vertical profiles of NO3 and N2O5 transport ‘barrier’ Sources and sinks emitted close to ground  peak of NO3 and N2O5 somewhere in NBL but away from surface Poorly quantified (one night’s data!) What are the spatial distributions of NO3 and N2O5 in the NBL under different meteorological, chemical and aerosol conditions? Measurements from Brown et al. 2007

5. Questions • How do profiles of NO3 and N2O5 change as a function of: -altitude? -chemistry? -meteorological conditions? -aerosols? • What fraction of NOxis tied up in the NO3/N2O5 equilibrium at night? • To what extent does this affect NOx/O3 budgets?

6. How do we measure these chemicals? Beer-Lambert Law: Light resonates in high-finesse optical cavity detector km’s of pathlength sensitivity in 1m cell Cavity throughput light intensities A LED-BBCEAS absorption spectrum LED off I Time LED on λ range: ~640-675nm. λcentre~660nm –water vapour and NO3. BBCEAS=Broadband cavity enhanced absorption spectroscopy

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

8. Typical BBCEAS absorption spectrum NO3 electronic absorbance band vibrational overtones of water vapour

9. REPARTEE II: Regent’s Park and Tower Experiments • Data from 19th October – 15th November 2007 • How does chemistry change with altitude in the NBL? 185m • 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. NO3 N2O5

10. BBCEAS inlet direction 220º

11.  Real atmospheric structure, not noise! [NO3]max ~12ppt [N2O5]max ~700ppt What does the dataset look like? [NO3]:[N2O5] ~1-4% due to low ambient T and moderately high NO2

12. Can we summarise the dataset? [NO3]:[N2O5] ~1-4% due to low ambient T and moderately high NO2

13. Lifetimes of NO3 and N2O5 (N2O5)~2min-2hours (NO3 )~1-2min Consistent with strong vertical gradient, particularly for N2O5

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

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 O3? Not entirely O3 NO All night-time data

17. LIDAR* BT tower height Physical properties of NBL influencing chemical composition - An indication of NBL top. Are we in/out of NBL? An example night: 30-31st Oct *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?  The N2O5 and NO3 partitioning is important for the storage and removal of nitrogen oxides at night 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

19. Summary • 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) RONOCO=Role Of Night-time chemistry in controlling the Oxidising Capacity of the atmOsphere

20. Acknowledgements 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.

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