Articles | Volume 17, issue 6
Atmos. Chem. Phys., 17, 4081–4092, 2017
Atmos. Chem. Phys., 17, 4081–4092, 2017

Research article 27 Mar 2017

Research article | 27 Mar 2017

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. Carpenter1 Chris Reed et al.
  • 1Wolfson Atmospheric Chemistry Laboratories (WACL), Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
  • 2National Centre for Atmospheric Science (NCAS), University of York, Heslington, York, YO10 5DD, UK
  • 3School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
  • anow at: Facility for Airborne Atmospheric Measurements (FAAM), Building 146, Cranfield University, Cranfield, MK43 0AL, UK

Abstract. We present 2 years of NOx observations from the Cape Verde Atmospheric Observatory located in the tropical Atlantic boundary layer. We find that 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 noontime 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 according to which the transport of organic nitrogen species, such as PAN, is the dominant source. We show that observed mixing ratios (November–December 2015) of HONO at Cape Verde (∼ 3.5 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 of NOx. 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.

Short summary
The source of ozone-depleting compounds in the remote troposphere has been thought to be long-range transport of secondary pollutants such as organic nitrates. Processing of organic nitrates to nitric acid and subsequent deposition on surfaces in the atmosphere was thought to remove these nitrates from the ozone–NOx–HOx cycle. We found through observation of NOx in the remote tropical troposphere at the Cape Verde Observatory that surface nitrates can be released back into the atmosphere.
Final-revised paper