Seasonal changes in urban PM2.5 hotspots and sources from low-cost sensors
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Abstract
PM2.5 concentrations in urban areas are highly variable, both spatially and seasonally. To
assess these patterns and the underlying sources, we conducted PM2.5 exposure measurements at
the adult breath level (1.6 m) along three ~5 km routes in urban districts of Mainz (Germany) using
portable low-cost Alphasense OPC-N3 sensors. The survey took place on five consecutive days
including four runs each day (38 in total) in September 2020 and March 2021. While the between sensor accuracy was tested to be good (R2 = 0.98), the recorded PM2.5 values underestimated the
official measurement station data by up to 25 µg/m3. The collected data showed no consistent PM2.5
hotspots between September and March. Whereas during the fall, the pedestrian and park areas
appeared as hotspots in >60% of the runs, construction sites and a bridge with high traffic intensity
stuck out in spring. We considered PM2.5/PM10 ratios to assign anthropogenic emission sources
with high apportionment of PM2.5 in PM10 (>0.6), except for the parks (0.24) where fine particles
likely originated from unpaved surfaces. The spatial PM2.5 apportionment in PM10 increased from
September (0.56) to March (0.76) because of a pronounced cooler thermal inversion accumulating fine
particles near ground. Our results showed that highly resolved low-cost measurements can help to
identify PM2.5 hotspots and be used to differentiate types of particle sources via PM2.5/PM10 ratios.