You use aeration as one of a sequence of steps to lower alkalinity. Aeration raises PH without changing alkalinity. Acid lowers both PH and alkalinity. The two used together properly will lower alkalinity while leaving PH the same.
Rain is generally a very weak acid, a lot of rain water will lower PH slightly. Meanwhile the aeration from the rain hitting the surface will raise PH. How quickly the PH goes up depends on where you PH and alkalinity start out. The net effect could go either direction.
Rain is generally a very weak acid, a lot of rain water will lower PH slightly. Meanwhile the aeration from the rain hitting the surface will raise PH. How quickly the PH goes up depends on where you PH and alkalinity start out. The net effect could go either direction.