Day and night-time formation of organic nitrates at a forested mountain site in south-west Germany
Abstract. We report in situ measurements of total peroxy nitrates (ΣPNs) and total alkyl nitrates (ΣANs) in a forested–urban location at the top of the Kleiner Feldberg mountain in south-west Germany. The data, obtained using thermal dissociation cavity ring-down spectroscopy (TD-CRDS) in August–September 2011 (PARADE campaign) and July 2015 (NOTOMO campaign), represent the first detailed study of ΣPNs and ΣANs over continental Europe. We find that a significant fraction of NOx (up to 75 %) is sequestered as organics nitrates at this site. Furthermore, we also show that the night-time production of alkyl nitrates by reaction of NO3 with biogenic hydrocarbons is comparable to that from daytime OH-initiated oxidation pathways. The ΣANs ∕ ozone ratio obtained during PARADE was used to derive an approximate average yield of organic nitrates at noon from the OH initiated oxidation of volatile organic compounds (VOCs) of ∼ 7 % at this site in 2011, which is comparable with that obtained from an analysis of VOCs measured during the campaign. A much lower AN yield, < 2 %, was observed in 2015, which may result from sampling air with different average air mass ages and thus different degrees of breakdown of assumptions used to derive the branching ratio, but it may also reflect a seasonal change in the VOC mixture at the site.