Atmospheric molecular hydrogen (H2) at the Shangdianzi regional background station in China

Abstract Atmospheric molecular hydrogen (H 2 ) mole fractions have been continuously measured at the Shangdianzi regional station in China. In this study, we present the atmospheric H 2 time series from January 2015 to April 2016, and investigate the diurnal and seasonal cycles, and the impact of meteorological factors on the observed values. Atmospheric H 2 mole fractions at Shangdianzi vary from a minimum of 381 ppb (parts per billion, 10 −9 dry air mole fraction) to a maximum of 1535 ppb, with a median of 510 ppb and a mean (± standard deviation) of 555 ± 113 ppb during the observation period. The results indicate that H 2 mole fractions at Shangdianzi are frequently influenced by local sources and sinks. Regionally representative conditions account for 44.7% of the total records with a mean mole fraction of 488 ± 20 ppb. The highest regionally representative H 2 mole fraction is observed in July, while the lowest is observed in October. Peak-to-trough amplitude in the seasonal cycle is 63 ± 3 ppb. H 2 mole fractions show nighttime depletion in all seasons, with the lowest values in the morning (7:00–10:00 local time). The H 2 mole fractions are also influenced by local surface wind direction at Shangdianzi. Winds from NW-NNW-N-NNE-NE-ENE-E directions are always associated with negative contribution to atmospheric H 2 loading, whereas winds from SSW-SW-WSW-W directions generally enhance the H 2 values. The results of trajectory clustering analysis demonstrate that air masses from a southerly direction induce high H 2 mole fractions. Conversely, mean H 2 mole fractions are low when air masses are from the north, northwest, and east directions.