Green spaces, particularly around schools, improve cognitive development in children 7-10 years old. In this Barcelona study, reduced exposure to elemental carbon (a traffic-related air pollutant) associated with green spaces accounted for some, but not all of this improvement.
Dadvand, Payam, Mark J. Nieuwenhuijsen, Mikel Esnaola, Joan Forns, Xavier Basagaña, Mar Alvarez-Pedrerol, Ioar Rivas, Mónica López-Vicente, Montserrat De Castro Pascual, Jason Su, Michael Jerrett, Xavier Querol, and Jordi Sunyer. (2015). Green spaces and cognitive development in primary schoolchildren. PNAS, published ahead of print June 15, 2015, doi:10.1073/pnas.1503402112