The potential of particulate nitrate photolysis to drive NO_x abundance

William Bloss^a), Leigh Crilley^a), Chris Reed^b), Tomás Sherwen^b), Mathew Evans^b), Katie Read^b), James Lee^b) and Lucy Carpenter^b)

^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 NO_x) to the lower atmosphere are high-temperature combustion, biological soil emissions and lightning.  Other than lightning, these are predominantly terrestrial, and consequently NO_x levels fall in locations remote from land, such as the remote marine boundary layer, as NO_x 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 NO_x 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