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

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