Airborne particulate matter distribution in urban green space is size-dependent

While the spatial distribution of airborne particulate matter (PM) inside and outside urban green spaces is affected by many environmental factors, the mechanism of the inside-outside difference (IOD) remains unclear. Particle size is an important attribute of PM, but the relationship between IOD and particle size is unknown. By measuring the IOD and external environmental conditions of 188 green spaces in Beijing city (China), we found that the distribution of PM inside and outside green spaces is size-dependent. According to the particle size dependence of PM, they can be divided into three classes (i.e., PM1, PM1_ 5, and PM5_25). The motion characteristics of particles, influence of environmental factors, and their coupling effect might be responsible for this size dependence. This study highlights the importance of size dependence in understanding the mechanism of PM distribution in urban green space through field experiments and puts forward a new concept of particle size classification that might be more suitable for actual environmental conditions and can support the improvement of related models.