# Testing the influence of monthly fluctuations

Let's say we have a dataset that looks like this:

month   year   airquality
jan     2000   36
...     ...    ...
dec     2001   33
jan     2002   44
...     ...    ...
dec     2002   21
jan     2003   44
...     ...    ...
etc.


The airquality in this case is considered to be the number of parts per million of something.

My question is, how would you test if the factor month has an influence on the airquality and how much of an influence? I am having troubles to think of the right method for testing this because besides a monthly difference there may als be an influence of the year factor (ie. airquality may get worse through the years as traffic intensity increases).

Hope you can point me in the right direction (the test/model to use, for example)

M.1=lm(airquality~as.dummy(month)+year)