NOx ground concentration at a station at high altitude in relation to cloud-to-ground lightning flashes

Concentrations of both nitrogen oxides (NOx=NO+NO2) observed at a station at high altitude (Puy de Dome (PDD), 1465 m) have been analyzed in relation with the cloud-to-ground (CG) lightning activity provided by the detection network Météorage. A first analysis of the time series of both types of data issued for 4 months of the 2003 summer shows a good correspondence between high values of NOx and stormy conditions in the area. During this 4-month period, NOx values exceeding 100 μg m−3 have been observed on 7 days and all of these days correspond to lightning activity in a 200×200 km2 area around PDD. Thirty-three days of the period experience lightning activity in this area, and only 14 of these days (42%) correspond to NOx peak values exceeding 50 μg m−3. The presence of lightning activity is a necessary condition to have very large peak values of NOx at PDD, but this condition is not sufficient. Two days showed NOx exceeding 300 μg m−3 which represents a value between 176 and 270 ppbv according to the respective proportion of NO2 and NO. Six days are analyzed in detail, using other information about the convective system and the wind conditions. The data of urban stations close to PDD have also been considered in the analysis for evaluating the possible transport by a plume from a city area. These days include the higher NOx cases, cases with high lightning activity, and cases with remote lightning activity. On each of these days, the location of the CG flashes, the time delays between lightning activity and NOx increase, and the air flow seem consistent with an interpretation of NOx produced by lightning. The higher values of NOx correspond with close and stationary thunderstorms which produced large proportions of positive CG flashes with low wind conditions. However, the characteristic increase seems to depend on several parameters of the CG flashes: their rate, location, and polarity. By comparing with the values given by other authors for aircraft measurements at altitude, our observations indicate that large quantities of NOx could be produced by the CG flash processes.