2.1.3. Daily Variations in Atmospheric Pressure

The daily variations in atmospheric pressure are quite regular at the site and are available from a local meteorological station. Pressure peaks at about 10:00 local solar time, and the steepest part of the subsequent pressure decrease occurs at about 13:00 local solar time. The pressure change over the day is between 4 and 8 mbar. Figure 2A compares the atmospheric pressure variations on 14 August 2018 to the [*H*2] concentrations measured at 1 m depth by Sensor 6 on the same day. Peak *[H2]sensor-6* occurs at ~13:00, coinciding with the maximum in the rate of atmospheric pressure decline.

Figure 2B separates out the diurnal and semidiurnal sinusoidal components of the atmospheric pressure change amplitudes of 1.5 and 1.2 mb and phase lags of 16 and 14 h, respectively. When summed these components replicate the observed pressure variations very closely (compare yellow and blue curves in Figure 2A).

**Figure 2.** (**A**) Observed atmospheric pressure changes at Sao Francisco Basin H2 vent, and the [H2] concentrations measured at Sensor 6. (**B**) The pressure variations can be decomposed into diurnal and s semi-diurnal sinusoidal components which sum to accurately represent the observed variations (yellow curve in A). (**C**) Phase relations between atmospheric pressure and [H2] measured at Sensor 6.

Figure 2C shows the phase relationships between atmospheric pressure and the hydrogen concentration measured at Sensor 6. As in Figure 2A, hydrogen is detected when the atmospheric pressure is decreasing, but the figure shows a clear hysteresis between pressure change and the hydrogen measurements that is diagnostic of pressure wave transmission into the subsurface. By hysteresis we mean that hydrogen concentrations measured in Sensor 6 when the atmospheric pressure is increasing are different from those measured when the atmospheric pressure is decreasing. In this case the contrast is dramatic since the hydrogen concentration is zero when pressure is increasing and positive when it is decreasing. The red circle in Figure 2C emphasizes that the maximum [*H*2] is measured at 13:00 when the surface pressure is decreasing at its maximum rate (Figure 2A).

The regular variations in atmospheric pressure are caused by atmospheric tides. At low latitudes, away from the much larger pressure variations associated with shifts of the polar front jets, the atmospheric tidal variations in pressure are typically mainly semi-diurnal with amplitudes of ~1.6 mb (Le Blancq, 2011 [23]). At our site the tidal atmospheric changes (2 mb amplitude) are similar in magnitude but have a strong diurnal component. We do not know why the daily atmospheric pressure variation at our site are more strongly diurnal than the norm.
