Nighttime concentrations of NO3 and N2O5 in mixed anthropogenic/biogenic air masses during the ACROSS campaign

The nitrate radical (NO₃) is mainly formed in the atmosphere through the reaction of nitrogen dioxide (NO₂) with ozone (O₃). It accumulates at night because during the day its photolysis in sunlight and its rapid reaction with nitric oxide (NO) can rapidly consume it. NO₃ radical is a major nighttime atmospheric oxidant of biogenic volatile organic compounds (BVOCs), which represent alone 75 to 90% of global non-methane VOC emissions.

Nighttime chemistry between NO₃ and BVOCs has received increased attention in the recent literature due to its potential influences on climate, air quality, health and visibility through the formation of many gaseous and particulate secondary pollutants such as organic nitrates, secondary organic aerosol (SOA), ozone and other functionalized products. In several studies conducted in clean forested environments,  NO₃ concentrations were found to be very low, mostly below the detection limits (2-6pptv), which can be explained by the low concentrations of precursors (NO₂ and O₃) and the high reactivity of NO₃ with unsaturated BVOCs. However, significant contribution of NO₃-BVOC reactions to the secondary pollutants formation in these clean areas have been found.

Since NO₃ radicals are formed more rapidly in polluted air masses, it is expected that reactions with BVOCs are even more important in mixed atmospheres, i.e. influenced by both biogenic and anthropogenic sources and characterized by relatively high concentrations of both VOCs and NO₂. The ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) field campaign was carried out in June and July 2022 in Ile-de-France region and aims, among other objectives, to assess the potential impact of NO₃-BVOC nocturnal processes on SOA formation. For this purpose, a newly developed field instrument, called "NOyBox", was deployed on top of a 40-meter tower, which is above the canopy, located at the Rambouillet Forest supersite southwest of Paris. The instrument allows the measurement of NO₃ radicals, N₂O₅ and HONO (IBB-CEAS, Incoherent Broad Band Cavity Enhanced Absorption Spectroscopy), NO₂ (CAPS, Cavity Attenuated Phase Shift) and NO_y (gold heated converter coupled with a NO chemiluminescence analyzer). Significant concentrations of NO₃ and N₂O₅ were detected on several nights with situations characterized by air masses coming from the direction of Paris, high concentrations of ozone and NO_2, and moderate relative humidity levels. Concentrations of these species as well as concentrations of species of interest for NO₃ chemistry are presented and the resulting implications on nighttime chemistry are discussed.