Night-time vertical profiles of nitrate radical concentrations in urban environment (Paris, France)

The NO₃ radical is the main atmospheric oxidant at night. The night period is favorable to the formation and accumulation of NO₃ radicals in the atmosphere. On the one hand, it is formed by the reaction of nitrogen dioxide with ozone while, on the other hand, NO₃ being highly photosensitive, it cannot accumulate significantly during the day (S. S. Brown and J. Stutz, Chem. Soc. Rev. 2012). In addition, the reaction between NO and NO₃ is very fast and so, urban environment is considered so far, being not favorable to the occurrence of NO₃ radicals. However, atmospheric nitrogen chemistry near the earth surface is strongly linked to the dynamics of the boundary layer and in summer NO is rapidly depleted by ozone. A large variability of the mixing ratios for NO₃ as a function of height above the ground is thus expected with non-negligible concentrations in altitude (Brown et al., Atmos. Chem. Phys., 2007). The contribution of NO₃ radical to the atmospheric evolution of VOCs in urban and sub-urban areas may therefore also be influenced by this vertical distribution.

To demonstrate the potential importance of NO₃ radical even in urban environment, a field campaign was carried out at night during July 2018 inside Paris. A newly developed field instrument dedicated to the measurement of NO₃ radical was deployed on a high payload touristic tethered balloon located in Paris 15^th district that was used as vertical vector. The NO₃ instrument is a compact, robust and easily deployable on field instrument based on the IBB-CEAS (Incoherent Broad band Cavity Enhanced Absorption Spectroscopy) technique. NO₃ measurements were completed by ground and airborne measurements of NO (chemiluminescence analyzer), NO₂ (CAPS cavity) and O₃ (absorption analyzer) concentrations as well as particle number concentrations (OPC GrimmTM) and 355 nm lidar (Leosphere ALS300) measurement for mixing layer probing.

Vertical profiles from 0 to up to 300 m were obtained at night characterized by high concentrations of ozone and moderate humidity. In this presentation, vertical profiles of the species measured and implications for VOC oxidation in urban environment will be discussed.