Evidence for renoxification in the tropical marine boundary layer

• Main Authors: C. Reed, M. J. Evans, L. R. Crilley, W. J. Bloss, T. Sherwen, K. A. Read, J. D. Lee, L. J. Carpenter
• Format: Article
• Language: English
• Published: Copernicus Publications 2017-03-01
• Series: Atmospheric Chemistry and Physics
• Online Access: http://www.atmos-chem-phys.net/17/4081/2017/acp-17-4081-2017.pdf

We present 2 years of NO_<i>x</i> observations from the Cape Verde Atmospheric Observatory located in the tropical Atlantic boundary layer. We find that NO_<i>x</i> 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 NO_<i>x</i> 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 NO_<i>x</i> 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 NO_<i>x</i> production. Reactions between the nitrate radical and halogen hydroxides which have been postulated in the literature appear to improve the box model simulation of NO_<i>x</i>. This rapid conversion of aerosol phase nitrate to NO_<i>x</i> changes our perspective of the NO_<i>x</i> cycling chemistry in the tropical marine boundary layer, suggesting a more chemically complex environment than previously thought.