High HONO concentrations in aged biomass burning plumes in NOMADSS campaign

More than a dozen of aged biomass burning plumes (BBPs) from various sources in free troposphere were encountered during the aircraft-based NOMADSS (Nitrogen, Oxidants, Mercury and Aerosol Distributions, Sources and Sinks) field campaign from June 1 to July 15, 2013. HCN and CO were used as tracers to identify the BBPs; the ratio of n-butane to propane was used to determine the photochemical age of the BBPs, which was ranging from 5 to 15 days. Significant enhancement in HONO concentration, by 5-56 ppt, was observed with elevated NOx and particulate nitrate concentrations in these BBPs comparing to background air mass. High HONO/NOx ratio up to 0.5 suppresses the importance of NOx as HONO precursors. Further HONO and NOx budget analysis reveals that photo-recycling of particulate nitrate in plumes to NOx was the main chemical source of NOx with HONO as an intermediate. An implication of this recycling NOx source on O3 formation is demonstrated in the correlation of O3 and NOx in aged plumes.