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Name: Jessica I.
Status: student
Age:  16
Location: N/A
Country: N/A
Date: 2000-2001


Question:
I did an experiment on Acid Rain wehre I collected rain water from teh first five minutes of a strom..then from the middle to the end of the storm.

I expected the acid content to be higher in the first 5 minutes, but after 7 tests I have PH readings which, on average go from:

Ph of 6.8/6.9 in the first five minutes

to

PH of 6.4/6.5 in the middle and end of the storm

Why is the acid hihger at the end of the storm? It's contrary to everything I read.


Replies:
Jessica,

The acidity of rain is related to the concentration of air gases (for example: sulfur and nitrogen oxides) that can react with rain (water) to form acids detected by your pH test. As the storm progresses, the rain washes (dissolves) the gases out of the air and thus lowers the concentration of acid in the rain.

As the acid content goes down, the pH of the rain will rise slightly.

Regards,
ProfHoff


Jessica,

I am only making an educated guess, but I assume that the storms that you are talking about are thunderstorms.

Even though where you live is not right on the coast, it is possible that there was a substantial amount of sea salt at the levels where the precipitation was forming, especially as the storm pulled in air from areas outside of where the storm began, as it grew. If so, some of the condensation nuclei for the initial precipitation may have been salt, which would result in higher pH of the precipitation. If there were easterly winds during the beginning of the storm, air with salt in it may have existed below the level of precipitation formation; rain falling though this area would have removed some of the salt, thereby also increasing the pH.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


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