Night-time chemistry above London: measurements of NO₃ and N₂O₅ from the BT Tower

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 NO_y.