Biomass burning drives urban PM2.5 across the Western Balkans: evidence from a six-city source apportionment study in North Macedonia.

Biomass burning is widely reported as a driver of wintertime PM2.5 pollution across the Western Balkans, but its residential and open-burning components have rarely been resolved using multi-city source apportionment. We applied EPA Positive Matrix Factorization (PMF) 5.0 to 1195 daily filter samples collected at six urban background sites in North Macedonia during two campaigns (October 2020-October 2021 and March 2023-March 2024). PMF consistently resolved residential wood combustion (RWC) and open/waste biomass burning (OBB) as distinct factors at all sites. RWC profiles were consistent across all 15 city pairs (Pearson distance 0.027-0.236). Campaign-period mean PM2.5 ranged from 29.1 to 62.2 μg/m3, corresponding to 1.2-2.5 times the currently applicable annual limit value of 25 μg/m3. Combined biomass burning contributed 21-35% of campaign-period PM2.5, while heating-season RWC concentrations were up to 33 times higher than non-heating values. An undocumented winter OBB signal in Strumica (12.0 μg/m3) was most consistent with greenhouse boiler combustion and supported by stronger heating-season associations with SO2 (r = 0.587 versus 0.185; p < 0.001). In the three cities with continuous and filter-based monitoring, biomass burning exceeded 50% of PM2.5 on 83% of evaluable episode days in Skopje (Cliff's δ = 0.905) and 71% in Strumica (δ = 0.752). Episode-day wind speeds were up to 52% lower than non-episode means. Bimodal heating-season diurnal patterns, evening peaks reaching 93 μg/m3, and conditional probability function (CPF) analysis provided convergent evidence that RWC and OBB dominate acute pollution episodes. The findings support accelerated EU Ecodesign transposition and stronger regulation of open and waste biomass burning, including greenhouse heating in agricultural areas.