PM_2.5 concentrations in three types of urban green spaces (comprehensive park, public utilities green space and road green space) were measured on typical days over the four seasons in Nanjing. In this study, seasonal and diurnal variation in PM_2.5 concentrations in three types of urban green spaces were analyzed and the variation rule for PM_2.5 concentration under different weather conditions was discerned.

The PM_2.5 concentrations in three types of urban green spaces (comprehensive park, public utilities green space and road green space) changed seasonally. The PM_2.5 mean concentration was 96.83 and 104.77 μg/m³ in spring and autumn, whereas it was 175.27 and 146.76 μg/m³ in winter and summer, respectively. At the seasonal scale, there was no significant difference among different plots during the same season, with concentrations mainly in the form of road green space>public utilities green space>comprehensive park. Seasonally, the PM_2.5 mean concentration of road green space was the highest in winter and lowest in autumn, whereas the PM_2.5 mean concentration of the other two types of urban green spaces (comprehensive park and public utilities green space) was the highest in winter and lowest in spring. It was not surprising to see that there were high levels of PM_2.5 caused by ground dust with low soil moisture, while these curves showed “one vale and one peak” in summer, “one peak” or “one vale”in winter. These diurnal variations suggested that anthropogenic activities and weather conditions were major contributors to higher particulate concentration. However, there were no significant differences among the PM_2.5 diurnal variation concentration of the three types of urban green spaces, which may be due to air motion. The mean concentration of PM_2.5 was 44.89% higher on cloudy days than on sunny days, but decreased by 30.94% after rainfall, which might be due to the change in solar radiation and precipitation. The reduction was remarkable after rainfall.

It was showed that the variety characteristic of PM_2.5 concentration was obvious. The PM_2.5 concentrations in comprehensive park was lower than that in other two urban green spaces, which means green spaces with multi-layer structure are very important to improve the quality of air by their PM_2.5 retention capacity.