NO3 reactivity in a temperate forest impacted by anthropogenic and biogenic emissions during the ACROSS campaign

Terpenoids are readily released into the environment via biogenic emissions. One of their major nighttime oxidizing agent is the nitrate radical (NO₃), which is formed from the reaction between ozone (O₃) and nitrogen dioxide (NO₂). The NO₃-initiated oxidation of such biogenic volatile organic compounds (BVOCs) leads to the formation of organic nitrates that can deposit on particles to form secondary organic aerosols. This reaction path thus can lead to irreversible removal of NO₂, usually of anthropogenic origin, from the gas phase. Alternatively, NO₃ rapidly reacts with nitric oxide (NO) with reformation of NO₂. Assigning the contribution of BVOCs and NO to the NO₃ reactivity therefore provides insight into the fate of NO_x (= NO + NO₂).

We used a flow-tube coupled to a cavity-ring-down spectrometer (FT-CRDS) to make direct measurements of the NO₃ reactivity in air (at a height of ~ 6 m above ground level) during the ACROSS field campaign in the forest of Rambouillet in the vicinity of Paris, France. Measured reactivities ranged from < 0.006 s^-1 to 2.3 s^-1, with high average daytime and nighttime reactivities of 0.13 s^-1 and 0.25 s^-1, respectively. In addition, vertical profiles of NO₃ reactivity (up to 40 m) were made and are compared to data previously observed in a boreal forest. Reactivities in Rambouillet forest were highest close the ground (0.36 s^-1 at 0 m) and drastically decreased with height (0.08 s^-1 at 24 m).

By comparing direct NO₃ reactivity measurements with those calculated from trace gas mixing ratios and their corresponding rate coefficients, we identify the contributions of BVOCs and NO to NO₃ losses.