Night-time chemistry above London: measurements of NO[subscript 3] and N[subscript 2]O[subscript 5] from the BT Tower

Broadband cavity enhanced absorption spectroscopy (BBCEAS) has been used to measure the sum of concentrations of NO[subscript 3] and N[subscript 2]O[subscript 5] 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 NO[subscript 3] and N[subscript 2]O[subscript 5] mixing ratios reached 800 pptv, whereas the mean night-time NO[subscript 3] and N[subscript 2]O[subscript 5] was approximately 30 pptv. Additionally, [NO[subscript 3] and N[subscript 2]O[subscript 5]] showed negative correlations with [NO] and [NO[subscript 2]] and a positive correlation with [O[subscript 3]]. Co-measurements of temperature and NO[subscript 2] from the BT Tower were used to calculate the equilibrium partitioning between NO[subscript 3] and N[subscript 2]O[subscript 5] which was always found to strongly favour N[subscript 2]O[subscript 5] (NO[subscript 3]/N[subscript 2]O[subscript 5]=0.01 to 0.04). Two methods are used to calculate the lifetimes for NO[subscript 3] and N[subscript 2]O[subscript 5], 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[subscript y].