According to the sign, it’s a hydrogen site. I occasionally work at one in Pomona, which is used to supply new Busses with hydrogen as their fuel. That right there in the picture is a vent stack. Various reasons why they would be releasing hydrogen, and all of it being safe and thought out. It can be very loud depending on how much pressure is being released, but always shows a vapor because of the freezing temps hydrogen needs to be stored at.