Evidence for renoxification in the tropical marine boundary layer
Chris Reed1,a, Mathew J. Evans1,2, Leigh R. Crilley3, William J. Bloss3, Tomás Sherwen1, Katie A. Read1,2, James D. Lee1,2, and Lucy J. Carpenter11Wolfson Atmospheric Chemistry Laboratories (WACL), Department of Chemistry, University of York, Heslington, York, YO10 5DD, United Kingdom 2National Centre for Atmospheric Science (NCAS), University of York, Heslington, York, YO10 5DD, United Kingdom 3School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom anow at: Facility for Airborne Atmospheric Measurements (FAAM), Building 146, Cranfield University, Cranfield, MK43 0AL, United Kingdom
Received: 09 Dec 2016 – Accepted for review: 08 Jan 2017 – Discussion started: 09 Jan 2017
Abstract. We present two years of NOx observations from the Cape Verde Atmospheric Observatory located in the tropical Atlantic boundary layer. We find NOx mixing ratios peak around solar noon (at 20–30 pptV depending on season), which is counter to box model simulations that show a midday minimum due to OH conversion of NO2 to HNO3. Production of NOx via decomposition of organic nitrogen species and the photolysis of HNO3 appear insufficient to provide the observed noon-time maximum. A rapid photolysis of nitrate aerosol to produce HONO and NO2, however, is able to simulate the observed diurnal cycle. This would make it the dominant source of NOx at this remote marine boundary layer site overturning the previous paradigm of transport of organic nitrogen species such as PAN being the dominant source. We show that observed mixing ratios (Nov–Dec 2015) of HONO at Cape Verde (~ 2.5 pptV peak at solar noon) are consistent with this route for NOx production. Reactions between the nitrate radical and halogen hydroxides which have been postulated in the literature appear to improve the box model simulation. This rapid conversion of aerosol phase nitrate to NOx changes our perspective of the NOx cycling chemistry in the tropical marine boundary layer, suggesting a more chemically complex environment than previously thought.
Reed, C., Evans, M. J., Crilley, L. R., Bloss, W. J., Sherwen, T., Read, K. A., Lee, J. D., and Carpenter, L. J.: Evidence for renoxification in the tropical marine boundary layer, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1111, in review, 2017.