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 5 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

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... 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 al-10 ways 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 nighttime sinks for NO y . produced from thermal dissociation of N 2 O 5 . Mixing ratios of up to 800 pptv of N 2 O 5 were recently observed in Tokyo, Japan using one such technique (Matsumoto et al., Abstract 25 Fig. 4. LED-BBCEAS measurements of mixing ratios of NO 3 +N 2 O 5 (Panel (a) , red) from the REPARTEE-II campaign. All raw data (15s acquisitions) shown. A quantifiable measurement exists for all times where a LOD (Panel (a) , black) is shown. Data partitioned into N 2 O 5 (Panel (a) , blue) and NO 3 (Panel (c) , green) where required ancillary data (NO 2 and ambient temperature) is available. Panel (b) (grey) shows NO 3 /N 2 O 5 ratio in %. Fig. 8. Scatter plots showing correlation of mixing ratios of NO 3 +N 2 O 5 against (a) O 3 , (b) NO and (c) NO 2 for all night-time data.