Quantifying spatiotemporal variability of fine particles in an urban environment using combined fixed and mobile measurements

Spatiotemporal variability of fine particle concentrations in Indianapolis, Indiana is quantified using a combination of high temporal resolution measurements at four fixed sites and mobile measurements with instruments attached to bicycles during transects of the city. Average urban PM2.5 concentrations are an average of ∼3.9–5.1 μg m−3 above the regional background. The influence of atmospheric conditions on ambient PM2.5 concentrations is evident with the greatest temporal variability occurring at periods of one day and 5–10 days corresponding to diurnal and synoptic meteorological processes, and lower mean wind speeds are associated with episodes of high PM2.5 concentrations. An anthropogenic signal is also evident. Higher PM2.5 concentrations coincide with morning rush hour, the frequencies of PM2.5 variability co-occur with those for carbon monoxide, and higher extreme concentrations were observed mid-week compared to weekends. On shorter time scales (<one day), spatial variability is 2–3 times greater than temporal variability. Spatial variability along the transect (within 10 km of the fixed measurement sites) was within ±3.6 μg m−3 of the stationary measurements (once the temporal variability is removed). Localized extreme values of PM2.5 concentrations ranged in the spatial dimension from a few hundred meters up to 2 km, and contributed an average of 4.4 μg m−3 to ambient concentrations.