1Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
2Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK
3National Centre for Atmospheric Science, School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, B15 2TT, UK
4Centre for Ecology and Hydrology (Edinburgh), Bush Estate, EH26 0QB, Penicuik, UK
*now at: Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado at Boulder, and NOAA Earth System Research Laboratory (ESRL), Boulder, Colorado, USA
**now at: School of Physics and Centre for Climate and Air Pollution Studies, Environmental Change Institute National University of Ireland, Galway, Ireland
Abstract. Broadband cavity enhanced absorption spectroscopy (BBCEAS) has been used to measure the sum of concentrations of NO3 and N2O5 from the BT (telecommunications) Tower 160 m above street level in central London during the REPARTEE II campaign in October and November 2007. Substantial variability was observed in these night-time nitrogen compounds: peak NO3+N2O5 mixing ratios reached 800 pptv, whereas the mean night-time NO3+N2O5 was approximately 30 pptv. Additionally, [NO3+N2O5] showed negative correlations with [NO] and [NO2] and a positive correlation with [O3]. Co-measurements of temperature and NO2 from the BT Tower were used to calculate the equilibrium partitioning between NO3 and N2O5 which was always found to strongly favour N2O5 (NO3/N2O5=0.01 to 0.04). Two methods are used to calculate the lifetimes for NO3 and N2O5, the results being compared and discussed in terms of the implications for the night-time oxidation of nitrogen oxides and the night-time sinks for NOy.