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Name: Rebecca L.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 2001


Question:
Do different weather conditions like snow,frost, dew, humidity,rain affect the amount of acid in air moisture?


Replies:
Rebecca,

The form of precipitation and the amount of water vapor in the air do affect how acid precipitation and water droplets in the air and in clouds are.

Generally, the more water drops or snow flakes, the less acidic the water is, since the acid particles are dissolved and diluted.

The more precipitation and the faster it falls, the less acidic the rainwater or snow is, again, because of the dilution factor. A rapid rain, such as from a thunderstorm, "cleans" the air, washing out much of the pollution and making the air temporarily cleaner.

How acid the precipitation is, is also affected by how much sulfur and nitrogen bearing pollution and soil is in the air. Soil generally helps to neutralize the acidic pollution in rain. Areas of the country with more alkaline soils, such as west of the Rocky Mountains, and where it is windy (so that soil becomes airborne), generally have less acidic precipitation as a result. The western part of the country also has fewer pollution sources and therefore has less acidic precipitation for this reason as well.

Snow is usually less acidic than rain, as it is not very good at "washing" pollution from the air. Snowflakes often form on aerosol particles, which can be made of pollution, but being solid, they cannot capture or absorb other pollution effectively. Rain in Chicago has an average pH of 4.4, whereas the pH for snow is more commonly around 4.8.

A study of the acidity of dew was performed at Argonne Nat. Lab. about ten years ago. It showed, as expected, that the more dew there is, the less acid the dew is. Again, this resulted from the acidic material on the leaves of the grass being diluted by the dew water. Most of the acid material had fallen onto the grass as dry material. Pollution is removed from the air by three natural modes: it is deposited on everything in a dry form of particle, it is absorbed by plants, water bodies, and the soil in a gaseous form, and it is removed from the air by precipitation.

A good web site to look at to see the effects of pollution on precipitation is nadp.sws.uiuc.edu, the site of the National Atmospheric Deposition Program, which I participate in. This Program monitors and tracks acidified precipitation.

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


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