Open vegetation at street level
Loosely planted vegetation creates an optimal contact between air and leaf surface. It is easy to see whether the vegetation is sufficiently sparse, as one will be able to see through it. With dense vegetation the air flows over or alongside the vegetation instead of through it. This means there is less contact and fewer contaminants are captured. This measure will only have a minor effect on local pollution.
Somewhat sparse vegetation does reduce wind speed, through which wind eddies of precipitated particle matter can be reduced.