Do trees cause air pollution? Not a lot, but high reactivity of compounds is an issue; Joensuu, Finland; 4/27/09

Pining for terpenes separationsNOW.com - Chichester,England,UK
The results illustrate the difficulty in measuring plant emissions in situ but they still provide an indication of the levels of monoterpenes originating from pine forests as a whole and the needles in particular. In a study on the same site in 2003, the net ecosystem production was estimated to be 229 g carbon/m2/year, so, despite the uncertainty in the absolute monoterpene values, it is clear that they constitute a small proportion of the total volatile carbon. Their significance, however, lies in their high atmospheric reactivity and potential for aerosol formation.