In our recent release of AutoCASE for Sites we have included new estimates for the dry deposition removal rates for various vegetation types, including green roofs, maintained and unmaintained grass, plants, and shrubs. Vegetation removes air pollution, also known as criteria air contaminants (CAC’s), from the atmosphere through a process known as dry deposition. Therefore, adding vegetation leads to cleaner urban air and improves the health of a city’s inhabitants. The types of CAC’s we examine in this analysis include sulfur oxides (SO2), particulate matter (PM2.5), ozone (O3), carbon monoxide (CO), and mono-nitrogen oxides (NOX).

With AutoCASE for Sites, an LID’s ability to improve air quality is determined by the combination of vegetation types the LID includes. Below is a comparison of the high, low, and most likely social values for the various vegetation types that a given LID may be composed of.

How do we assign dollar values to each vegetation type? We apply the social cost of each air pollution to the calculated reduction of each pollutant. The social cost used and the full methodology we employ can be found here. This social cost is based on monetizing each pollutant’s effect on human health, agriculture, buildings, and the local ecology. The more CAC’s that can be reduced, the better air quality urban residents enjoy—which leads to positive health, agriculture, and ecological outcomes. Generally, vegetation is able to remove air pollution during non-frost days, so the figures are shown are per non-frost days. This enables us to account for regional variations in the air pollution removal potential of vegetation types, as the number of non-frost days is dependent on a project’s local weather station.

The difference between maintained and unmaintained grass is not large in this analysis, but is much more significant when analyzing carbon emissions, as lawnmowers emit a lot of carbon. The analysis for greenhouse gas emissions is further explored in another blog post and can be found here.

Now let’s consider the impact of different tree sizes and the rate of reduction in air pollution:

*Note: Tree sizes are categorized within a range. Labels show the highest value in the respective categories. DBH is diameter at breast height, a standard measurement for the size of a tree.

After considering the impact that tree size has on improving air quality, it’s made me think twice about the potential that trees actually possess. The graph above demonstrates that bigger trees can reduce quite large amounts of air pollution per year. Trees are pollution-reducing agents with a substantial effect. Consider even the smallest category, a tree that is at most 7cm’s wide—it improves air quality better than the equivalent amount of land covered by any other vegetation type. As the tree grows over time the potential benefit is magnified.

Presently, our research is exploring the inclusion of the air pollution removal rate for different tree species. This will be featured in an upcoming version of AutoCASE for Sites.