Stephen M. Ball

The first airborne comparison of N2O5 measurements over the UK using a CIMS and BBCEAS during the RONOCO campaign

Dinitrogen pentoxide (N2O5) plays a central role in nighttime tropospheric chemistry as its formation and subsequent loss in sink processes limits the potential for tropospheric photochemistry to generate ozone the next day. Since accurate observational data for N2O5 are critical to examine our understanding of this chemistry, it is vital also to evaluate the capabilities of N2O5 measurement techniques through the co-deployment of the available instrumentation. This work compares measurements of N2O5 from two aircraft instruments on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft during the Role of Nighttime Chemistry in Controlling the Oxidising Capacity of the Atmosphere (RONOCO) measurement campaigns over the United Kingdom in 2010 and 2011. A chemical ionisation mass spectrometer (CIMS), deployed for the first time for ambient N2O5 detection during RONOCO, measured N2O5 directly using I− ionisation chemistry and an aircraft-based broadband cavity enhanced absorption spectrometer (BBCEAS), developed specifically for RONOCO, measured N2O5 by thermally dissociating N2O5 and quantifying the resultant NO3 spectroscopically within a high finesse optical cavity. N2O5 mixing ratios were simultaneously measured at 1 second time resolution (1 Hz) by the two instruments for 8 flights during RONOCO. The sensitivity for the CIMS instrument was 52 ion counts per pptv with a limit of detection of 7.4 pptv for 1 Hz measurements. BBCEAS, a proven technique for N2O5 measurement, had a limit of detection of 2 pptv. Comparison of the observed N2O5 mixing ratios show excellent agreement between the CIMS and BBCEAS methods for the whole dataset, as indicated by the square of the linear correlation coefficient, R2 = 0.89. Even stronger correlations (R2 values up to 0.98) were found for individual flights. Altitudinal profiles of N2O5 obtained by CIMS and BBCEAS also showed close agreement (R2 = 0.93). Similarly, N2O5 mixing ratios from both instruments were greatest within pollution plumes and were strongly positively correlated with the NO2 concentrations. The transition from day to nighttime chemistry was observed during a dusk-to-dawn flight during the summer 2011 RONOCO campaign: the CIMS and BBCEAS instruments simultaneously detected the increasing N2O5 concentrations after sunset. The performance of the CIMS and BBCEAS techniques demonstrated in the RONOCO dataset illustrate the benefits that accurate, high-frequency, aircraft-based measurements have for improving understanding the nighttime chemistry of N2O5.

Product channels in the near-UV photodissociation of ozone

The relative quantum yields for the formation of O2(a 1Δg) from the photolysis of ozone have been measured between 270 and 329 nm at room temperature, and between 300 and 322 nm at 227 K, near the fall-off region for the formation of spin-allowed singlet products O2(a 1Δg) and O(1D2). The molecular fragment was detected by resonance enhanced multiphoton ionisation at 331.5 nm. The measurements were put on an absolute scale by comparison with previous measurements in the short-wavelength region. The results at room temperature are in excellent agreement with the recommended quantum yields for O(1D2) production at wavelengths up to 310 nm, but at longer wavelengths exhibit a pronounced tail of 10–20% out to at least 329 nm. Measurements at 227 K are identical to those at room temperature between 300 and 309 nm, and do not show a shift in the fall-off curve to shorter wavelengths as has been reported in the literature for O(1D2). For wavelengths between 309 and 319 nm the yield of O2(a 1Δg) is smaller than that at room temperature and this, together with the results of measurements at fixed wavelengths as a function of temperature, confirms that the photolysis of internally excited ozone provides a major source of O2(a 1Δg) at wavelengths just beyond 310 nm. For wavelengths 320 nm the quantum yield is found to be approximately constant at the two temperatures, and suggests that spin-forbidden dissociation of ozone is taking place as the dominant process in this long-wavelength region. The results are compared with recent modelling calculations for the formation of the O(1D2) product in the fall-off region which take into account the spinallowed dissociation of internally excited ozone molecules. Good agreement is found at wavelengths up to 320 nm, particularly at room temperature, and suggests that the formation of singlet products extends noticeably beyond the fall-off region. The implications of this for stratospheric and atmospheric modelling are briefly discussed.