Abstract
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.
Highlights
The chemical environment in the remote marine boundary layer (MBL) is characterized by very low concentrations of nitrogen oxides (NOx = NO + NO2) i.e. 10 to < 100 pptV (Carsey et al, 1997; Lee et al, 2009; Monks et al, 1998), high concentrations of water vapour, and the presence of inorganic halogen compounds, resulting in net daytime ozone (O3) destruction (Dickerson et al, 1999; Read et al, 2008; Sherwen et al, 2016; Vogt et al, 1999)
In this paper we investigate the budget of NOx in the remote MBL using observations of NOx and HONO collected at the Cape Verde Atmospheric Observatory during 2014 and 2015
The Cape Verde Atmospheric Observatory (CVO), a WMO Global Atmospheric Watch (GAW) station, is located in the tropical North Atlantic (16.864, −24.868) on the island of São Vincente and is exposed to air travelling from the northeast in the trade winds (Carpenter et al, 2010)
Summary
The chemical environment in the remote marine boundary layer (MBL) is characterized by very low concentrations of nitrogen oxides (NOx = NO + NO2) i.e. 10 to < 100 pptV (Carsey et al, 1997; Lee et al, 2009; Monks et al, 1998), high concentrations of water vapour, and the presence of inorganic halogen compounds, resulting in net daytime ozone (O3) destruction (Dickerson et al, 1999; Read et al, 2008; Sherwen et al, 2016; Vogt et al, 1999). Uptake of HNO3 onto aerosol and subsequent rapid (compared to gas phase HNO3) photolysis act to balance this
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