I'm not sure why you might be seeing such a relationship with air quality, but I have heard of such anomalies with energy use.
Specifically, in North America, it is well understood the right tree on the southwest side of your home can help reduce heating and cooling costs. However, if you were to plant an evergreen tree on the side of your house that receives full sun, you might be impeding the natural heating provided by the sun during the winter months. In this case, you might actually use more energy to heat your home in the winter, which could lead to negative values for the tree in this regard.
Whether or not this is common, well understood, or well documented, I'm not sure. Simply, I've seen the issue and this is how it's been explained in the past.