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Snow, pH and Polution
Name: Reese
Status: Student
Grade: 9-12
Location: CT
Country: United States
Date: Fall 2009
Question:
1. How does the pH level of snow effect people?
2. How does the pollution level of snow effect people?
3. Is there a relationship between pH and pollution?
4. What would cause the snow to have more pollutants?
5. How does snow become more acidic?
Replies:
Reese,
1. How does the pH level of snow effect people?
The pH of snow, as with the pH of rain
effects people indirectly through the acidification
of soil and water (rivers, lakes). I say acidification
because even the most pristine precipitation is acidified,
although not highly in more remote areas. Natural
contaminants such as soil, volcanic eruption products, etc.
are all that is needed to acidify precipitation naturally;
man's contribution is not needed. However, we do add to
the contaminants that get into precipitation, making
it more acidic (by a factor of about 10 for rain; pH 4.4)
in areas like the eastern United States than it is
naturally (pH 5.8 in the Rocky Mountains). Acidification
of soil and water can affect the productivity of plants
and animals. However, some acidification (which occurs
primarily through sulfates and nitrates in precipitation)
can be useful - specifically from nitrates, which can
fertilize the soil slightly.
2. How does the pollution level of snow effect people?
As seen above, in much the same way as rain, but the pH
of snow is usually higher than that of rain, as snow is not
as efficient at scavenging pollutants. For instance, at
our Argonne measurement site, the pH of rain averages around
4.5, whereas the pH of snow averages around 5.0. So, the
acidifying effects of snow on soil and water are less than
the effects from rain.
3. Is there a relationship between pH and pollution?
There is a strong relationship between pH and pollution.
Pollution, depending on the chemistry of it, can lower the
pH from a natural value of 5.8 to a worst case pH of 4.2
in the Unites States (see the 1998 field pH map at
https://nadp.isws.illinois.edu/isopleths/maps1998/phfield.gif).
In the eastern part of the United States, where there is more
pollution, the pH is lower. In the western United States,
particularly in the Rocky Mountains, where there is less pollution,
the pH is higher.
4. What would cause the snow to have more pollutants?
As mentioned in the answer to question 2, snow generally
contains less pollutants than rain. However, if it is "wet"
(partially liquid water) it could contain more pollutants
than if it is "dry" (all ice). See the answer to question 5
below for more of an explanation.
5. How does snow become more acidic?
Snow may be more acidic some times than others for the
same air pollutant levels, by being "wet". Partially
melted snow (for instance falling through a layer of air
near the surface that has a temperature above freezing)
may have an outside layer of water that could make it more
efficient at scavenging pollutants than snow falling through
air that has a temperature well below freezing (keeping it
as all ice). "Wet" snow acts more like rain in it's pollutant
scavenging abilities.
David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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