Abstract
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.
Highlights
The nitrate radical (NO3) is amongst the most important oxidants in the nocturnal boundary layer (NBL)
There are a number of caveats involved comparing BBCEAS-derived water concentrations with relative humidity: evaporation from aerosols can act to increase humidity; pressure needs to be accurately known in order to be able to convert RH to water concentration; and water vapour absorption spectroscopy exhibits non Beer-Lambert behaviour due to the highly structured spectral lines not being fully resolved www.atmos-chem-phys.net/10/9781/2010/
In this paper we have demonstrated the instrumental technique of light emitting diode (LED)-BBCEAS in a sustained deployment in a month-long field campaign
Summary
The nitrate radical (NO3) is amongst the most important oxidants in the nocturnal boundary layer (NBL). It is responsible for initiating the processing of a wide range of anthropogenic and biogenic emissions and in its reactivity to some VOCs can be considered the night-time analogue of the hydroxyl radical. Understanding the atmospheric cycle of NO3 -its formation and loss pathways, spatial variability, and role in the regulation of NOx (=NO+NO2) and budgets of volatile organic compounds (VOCs) (Atkinson, 2000), is of key importance to understanding processes impacting surface ozone formation and air quality
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