Evidence for renoxification in the tropical marine boundary layer

Abstract. We present 2 years of NO_x observations from the Cape Verde Atmospheric Observatory located in the tropical Atlantic boundary layer. We find that NO_x 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_x 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_x 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_x 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_x. This rapid conversion of aerosol phase nitrate to NO_x changes our perspective of the NO_x cycling chemistry in the tropical marine boundary layer, suggesting a more chemically complex environment than previously thought.