To investigate the ambient mass concentration, size-distribution and temporal variability of atmospheric particulate matter (PM), a long-term monitoring campaign was undertaken at an urban background site in Como, Northern Italy, from May 2015 to March 2016. A 13-stage Low Pressure Impactor (DLPI) was used for the collection of size-segregated particulates in the 0.028–10 µm size range. The results revealed a good level of agreement between DLPI and a co-located Harvard-type PM2.5 Impactor, allowing them to be classified as comparable and characterized by a reciprocal predictability. The PM concentration levels varied greatly between the different 5-days monitoring sessions, with higher mean mass concentrations during the heating season. Appreciable seasonal differences were found for particles between 0.15 and 1.60 µm that, on average, registered concentration levels 3.5 times higher during the heating season (mean: 28.2 µg m–3; median: 24.4 µg m–3) compared to the non-heating season (mean: 8.3 µg m–3; median: 7.6 µg m–3). No relevant and significant differences were detected for the coarser ranges (> 1.60 µm). Temporal variabilities were influenced by typical PM urban sources (e.g., household heating, traffic), that significantly affected fine and submicrometer particles, and were related to meteorological factors. Ambient air particles exhibited a trimodal distribution: a first and sharp peak more pronounced during the heating season was identified between 0.3 and 0.5 µm and two other slight peaks in the coarse mode were centered on approximately 3 and 8 µm. No relevant differences were found in the shape of the size-distribution between the two investigated seasons. The mean PM2.5 (22.4 µg m–3) and PM10 (27.7 µg m–3) concentrations monitored in the study area exceeded the annual Air Quality Guideline Values (respectively equal to 10 µg m–3 and 20 µg m–3) established by the World Health Organization.