Ground-based observation of lightning-induced nitrogen oxides at a mountaintop in free troposphere

Lightning is an important source of nitrogen oxides (LNO_x). The actual global production of LNO_x is still largely uncertain. One of the reasons for this uncertainty is the limited available observation data. We measured the concentrations of total reactive nitrogen (NO_y), nitric oxide (NO) and nitrogen dioxides (NO₂) and then obtained NO_x oxidation products (NO_z: NO_z = NO_y - NO_x) at a station at the top of Mount Fuji (3776 m a.s.l.) during the summer of 2017. Increases in NO_y and NO₂ were observed on 22 August 2017. These peaks were unaccompanied by increases in CO, which suggested that the observed air mass did not contain emissions from combustion. The backward trajectories of the above air mass indicated that it moved across areas where lightning occurred. The NO_y concentration was also calculated by using a chemical transport model, which did not take NO_x produced by lightning into account. Therefore, the NO_y concentration due to lightning can be inferred by subtracting the calculated NO_y from the observed NO_y concentrations. The concentration of NO_y at 13:00 on 22 August 2017 originating from lightning was estimated to be 1.11 ± 0.02 ppbv, which comprised 97 ± 2% of the total NO_y concentration. The fractions of NO₂ and NO_z in the total NO_y were 0.54 ± 0.01 and 0.46 ± 0.03, respectively. The NO concentration was below the detection limit. We firstly observed increase of concentrations of NO_y originating from lightning by ground-based observation and demonstrated the quantitative estimates of LNO_x using model-based calculation.