Evidence for renoxification in the tropical marine boundary layer

Abstract

<p><strong>Abstract.</strong> We present two years of NO<i><sub>x</i></sub> observations from the Cape Verde Atmospheric Observatory located in the tropical Atlantic boundary layer. We find NO<i><sub>x</i></sub> 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 NO<sub>2</sub> to HNO<sub>3</sub>. Production of NO<i><sub>x</i></sub> via decomposition of organic nitrogen species and the photolysis of HNO<sub>3</sub> appear insufficient to provide the observed noon-time maximum. A rapid photolysis of nitrate aerosol to produce HONO and NO<sub>2</sub>, however, is able to simulate the observed diurnal cycle. This would make it the dominant source of NO<i><sub>x</i></sub> 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 NO<i><sub>x</i></sub> 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 NO<i><sub>x</i></sub> changes our perspective of the NO<i><sub>x</i></sub> cycling chemistry in the tropical marine boundary layer, suggesting a more chemically complex environment than previously thought.</p>

Constraints on Aerosol Nitrate Photolysis as a Potential Source of HONO and NOx

“…x . An average EF of 10-30 is less than our upper limit of EF and would be within the range found by Reed et al 10 (EF = 10) and Kasibhatla et al 12 (EF = to best explain observations of HONO and NO x at the Cape Verde Observatory.…”

“…[6][7][8] To reconcile models and observations, multiple pathways for the conversion of HNO 3 into NO x or HONO have been proposed, a process termed re-noxification. Various re-noxification pathways have been proposed in areas including the upper troposphere, [7][8][9] the marine boundary layer, 6,[10][11][12] rural forests, 13,14 and areas of continental outflow. 15 Recently, several of these studies have suggested that HNO 3 is rapidly photolyzed in aerosols to form NO 2 or HONO, at a rate between 10 and 300 times faster than the rate of gas-phase HNO 3 photolysis, [10][11][12]15,16 and it is this process that we investigate here.…”

Nitrous acid (HONO) emissions under real-world driving conditions from vehicles in a UK road tunnel

“…Secondary sources were thought to be dominated by homogeneous gas phase reaction of NO and OH during the day (resulting in a null cycle with R1), and heterogeneous production of HONO from NO2 on surfaces at night (Calvert et al, 1994;Finlayson-Pitts et al, 2003;Jenkin et al, 1988;Kleffmann et al, 1998;Stutz et al, 2002). More recently, studies have shown that heterogeneous reactions are also important sources of HONO during the day and include photo-enhanced reduction of NO2 on organic substrates (George et al, 2005;Monge et al, 2010;Stemmler et al, 2006) and nitrate photolysis (Reed et al, 2017;Ye et al, 2017;Zhou et al, 2011). Despite numerous studies over the last few decades, substantial uncertainties remain regarding the relative magnitude of these sources, with models frequently unable to account for total measured HONO concentrations without the inclusion of unknown sources, typically driven by photolysis (e.g.…”