A recent report by the Environmental Protection Agency states that urban areas with extensive green spaces have significantly lower levels of air pollution compared to those with minimal green areas. The report concludes that increasing urban greenery can help improve air quality in cities.
However, some city planners argue that the cost of developing and maintaining green spaces is too high and that resources would be better allocated to other pollution control methods.
Which of the following, if true, would most strengthen the argument that increasing urban greenery is a beneficial method for improving air quality?