Does urban vegetation reduce temperature and air pollution concentrations? Findings from an environmental monitoring study of the Central Experimental Farm in Ottawa, Canada

Abstract Urban greenness, or vegetation, plays an important role in reducing ambient air pollution, and urban heat island temperatures. While several studies have developed land use regression surfaces to describe the intra-urban variability of these environmental exposures, it is less well known how green spaces modify these surfaces. In this study, we investigated these environmental benefits of green spaces for the Canadian Central Experimental Farm, a 4 km2 agricultural facility, located adjacent to the urban core of Ottawa, Canada. Fall, winter and summer mobile monitoring campaigns were conducted in 2016/17 within a 40 km2 area centered on the CEF to characterize the spatial variability of: fine particulate matter (PM2.5), ultrafine particles (UFPs), black carbon (BC) and temperature. We concurrently measured two routes each morning (09:30-10:30) and evening (17:00-18:00) for 14 consecutive days in the three seasons. Median exposure levels for each road segment were calculated and used as inputs for season-specific land use regression models. These models were developed for each combination of environmental variable (PM2.5, UFP, BC, and temperature) and season (fall, winter, and summer). Predictors included land and road types as well as season-specific urban greenness represented by the Normalized Difference Vegetation Index (NDVI). Levels and spatial patterns of air pollution and temperature varied by season. We characterized the spatial relationship between the CEF and ambient air pollution and temperature measures by developing linear regression models with the distance to the closest border of the farm as the main predictor. Other land use variable which changed the main effect by ≥ 10% were included in the models. Within the study area, each kilometer of distance from the closest CEF border was associated with increases of PM2.5 in fall (12.1%, 95%CI: 6.1%–18.4%), and winter (6.8%, 95%CI: 2.4%–11.3%). Black carbon increased by 74.1% (95%CI: 13.2%–167.7%) in the summer. After accounting for roadways and other land-use characteristics, proximity to the CEF was also associated with lower temperatures in the fall (2.8%; 95%CI: 1.3–4.2). These results suggest that this unique urban green space has important beneficial impacts on ambient air pollution concentrations and for mitigating extreme heat events for which a large portion of Ottawa's populace is exposed.