The potential of particulate nitrate photolysis to drive NOx abundance
William Blossa), Leigh Crilleya), Chris Reedb), Tomás Sherwenb), Mathew Evansb), Katie Readb), James Leeb) and Lucy Carpenterb)
a)School of Geography, Earth & Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK
b)Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
The principal sources of nitrogen oxides (NO + NO2, collectively NOx) to the lower atmosphere are high-temperature combustion, biological soil emissions and lightning. Other than lightning, these are predominantly terrestrial, and consequently NOx levels fall in locations remote from land, such as the remote marine boundary layer, as NOx is converted to organic nitrate reservoir species, and particulate nitrate. Consequently, levels of nitrogen oxides species such as NO and NO2 are expected to be very low in the remote marine atmosphere.
We present observations of NOx and HONO from the Cape Verde Observatory in the tropical Atlantic Ocean which challenge this understanding. Elevated (for the remote MBL) levels of NO and NO2 are observed, with evidence for a photolytic source. Elevated HONO concentrations (for the remote MBL) are also observed, with a diurnal profile which peaks at noon, pointing to a photolytic source (of a magnitude greater than the principal, photolytic, HONO sink). We contrast the observed HONO behavior with that observed for other MBL locations, and show the measurements and associated laboratory measurements are consistent with a photolytic renoxification process releasing reactive NOx species from particulate nitrate