What is the role of greenery in the city? Would Brussels be off worse without the Sonian forest?

Greenery in the city does have a lot of advantages: it has a positive impact on air quality, trees can, through their leaves, filter particulate matter out of the air, and it does decrease the heat island effect. On the other hand, trees in narrow streets with a lot of traffic can, in certain configurations, result in the pollution being spread out worse and thus remaining beneath the foliage. The trees are however NOT the cause of that pollution; that role is reserved for road traffic most certainly. The impact of the Sonian forest on air quality is not so easily calculated. In case there would be no Sonian forest, but the same location would be filled with houses, traffic and/or industry, the resulting air pollution would have a negative effect on air pollution in Brussels in case of southeast winds. In addition, the Sonian forest does filter out part of the regional share of particulate matter entering Brussels in case of southeastern winds. This positive influence, however difficult to estimate, would also disappear together with the Sonian forest.