As it turns out, street greenery is even better at reducing pollution than thought. The research that I had found for last week's post showed greenery can filter out some of the worst particles but only up to 8% of the total pollutant plume. New research just released shows that green walls, when employed in an "urban canyon" environment, can filter out up to 60% of the particulate matter in diesel pollution. That, coincidentally, will include the SMART train. The urban canyon environment is when buildings go up on either side of the street and form single walls interrupted only by cross-streets. San Rafael's Fourth Street is a good example of this in Marin, and San Francisco's Market Street is a superb example in any context. These canyons create their own wind environments, circulating air up and then down and then up again in a vertical circulatory pattern.
Green walls are either plants growing out of the wall, like vertical gardens, or plants growing down walls, like ivy from planter boxes or wisteria from the ground. If these walls line a part of the urban canyon, the wind patterns will run polluted air through leaves multiple times, allowing the air to be filtered again and again.
Cities should actively encourage green walls to capture this effect, and SMART should plan for it where trains will run through residential areas. In places where buildings may rise above the freeway, as in the Downtown San Rafael Station Area Plan, green walls could be especially helpful in filtering the worst part of the air. Any investment in greenery for health reasons will be best put here. Similarly, where the freeway runs at ground level, ivy should be encouraged to grow on the sound walls.
As an added bonus, greenery cuts down on urban noise. Given how loud both the freeway is and the SMART train will be, encouraging leafy walls will be able to make our city streets that much more livable.
Investing in greenery is the single most cost-effective way to reduce pollutants and keep our cities healthy. With the new construction and higher-density zoning slated for areas up and down the 101 corridor, city councils and planning agencies need to take it seriously as more than just environmental greenwashing.