Atmospheric aerosol particles are a complex combination of primary emitted sources (biogenic and anthropogenic) and secondary aerosol resulting from aging processes such as condensation, coagulation, and cloud processing. To better understand their sources, investigations have been focused on urban areas in the past, whereas rural-background stations are normally less impacted by surrounding anthropogenic sources. Therefore, they are predisposed for studying the impact of long-range transport of anthropogenic aerosols. Here, the chemical composition and organic aerosol (OA) sources of submicron aerosol particles measured by an aerosol chemical speciation monitor (ACSM) and a multi-angle absorption photometer (MAAP) were investigated at Melpitz from September 2016 to August 2017. The location of the station at the frontier between western and eastern Europe makes it the ideal place to investigate the impact of long-range transport over Europe. Indeed, the station is under the influence of less polluted air masses from westerly directions and more polluted continental air masses from eastern Europe. The OA dominated the submicron particle mass concentration and showed strong seasonal variability ranging from 39 % (in winter) to 58 % (in summer). It was followed by sulfate (15 % and 20 %) and nitrate (24 % and 11 %). The OA source identification was performed using the rolling positive matrix factorization (PMF) approach to account for the potential temporal changes in the source profile. It was possible to split OA into five factors with a distinct temporal variability and mass spectral signature. Three were associated with anthropogenic primary OA (POA) sources: hydrocarbon-like OA (HOA; 5.2 % of OA mass in winter and 6.8 % in summer), biomass burning OA (BBOA; 10.6 % and 6.1 %) and coal combustion OA (CCOA; 23 % and 8.7 %). Another two are secondary and processed oxygenated OA (OOA) sources: less oxidized OOA (LO-OOA; 28.4 % and 36.7 %) and more oxidized OOA (MO-OOA; 32.8 % and 41.8 %). Since equivalent black carbon (eBC) was clearly associated with the identified POA factors (sum of HOA, BBOA, and CCOA; R²= 0. 87), eBC's contribution to each of the POA factors was achieved using a multilinear regression model. Consequently, CCOA represented the main anthropogenic sources of carbonaceous aerosol (sum of OA and eBC) not only during winter (56 % of POA in winter) but also in summer (13 % of POA in summer), followed by BBOA (29 % and 69 % of POA in winter and summer, respectively) and HOA (15 % and 18 % of POA in winter and summer, respectively). A seasonal air mass cluster analysis was used to understand the geographical origins of the different aerosol types and showed that during both winter and summer time, PM₁ (PM with an aerodynamic diameter smaller than 1 µm) air masses with eastern influence were always associated with the highest mass concentration and the highest coal combustion fraction. Since during wintertime CCOA is a combination of domestic heating and power plant emissions, the summer contribution of CCOA emphasizes the critical importance of coal power plant emissions to rural-background aerosols and its impact on air quality, through long-range transportation.

Uncertainties persist in estimating the radiative forcing of black carbon (BC) due to an incomplete understanding of its microphysical properties. This study investigated the physical properties of refractory black carbon (rBC) at the central European background site Melpitz during summer and winter, using a single-particle soot photometer coupled with a thermodenuder. Different air masses associated with distinct rBC properties were identified in both seasons. In summer, rBC exhibited a similar mass concentration (∼0.16 µg m⁻³) among different air masses, with the smallest mass median diameter (MMD) of rBC observed in the long transportation from the northwest (140 nm), while in winter, the highest concentration (1.23 µg m⁻³) and largest MMD (216 nm) were both observed in the air mass influenced by the easterly winds. Thickly coated rBC fractions increased during the daytime in summer, indicating that the photochemical processes significantly influence the rBC mixing state. In winter, a higher fraction (27 %) of rBC, with thick coatings in the cold air mass compared to the warm air mass (14 %), suggests the contribution of residential heating emissions to the mixing state. Most rBC retained a low-volatility coating in the thermodenuder samples (63 % mass fraction). In summer, photochemical processes also contribute to coating volatility, showing a higher fraction of rBC particles containing low-volatility coatings during the daytime. In winter, low-volatility coatings showed no significant diurnal variation and were more dependent on ambient temperature. Therefore, rBC coating volatility in winter is more influenced by emission sources, particularly residential heating, rather than atmospheric processes.

Fine particulate matter (PM) poses a major threat to public health, with organic aerosol (OA) being a key component. Major OA sources, hydrocarbon-like OA (HOA), biomass burning OA (BBOA), and oxygenated OA (OOA), have distinct health and environmental impacts. However, OA source apportionment via positive matrix factorization (PMF) applied to aerosol mass spectrometry (AMS) or aerosol chemical speciation monitoring (ACSM) data is costly and limited to a few supersites, leaving over 80% of OA data uncategorized in global monitoring networks. To address this gap, we trained machine learning models to predict HOA, BBOA, and OOA using limited OA source apportionment data and widely available organic carbon (OC) measurements across Europe (2010–2019). Our best performing model expanded the OA source data set 4-fold, yielding 85 000 daily apportionment values across 180 sites. Results show that HOA and BBOA peak in winter, particularly in urban areas, while OOA, consistently the dominant fraction, is more regionally distributed with less seasonal variability. This study provides a significantly expanded OA source data set, enabling better identification of pollution hotspots and supporting high-resolution exposure assessments.

Submicron particles (PM₁) play a crucial role in air quality and human health. This study investigates the influence of long-range transport (LRT) on urban aerosol levels in Leipzig, Germany, using high-resolution aerosol mass spectrometry measurements at two sites: an urban traffic site (Eisenbahnstrasse, Eiba) and a rural background site (Melpitz), located ∼50 km apart. The sites were analyzed during winter 2017 under two dominant wind regimes: East and West. These sites were directly linked to each other, which was supported by cross-correlation analysis, with a typical time lag of −2 h in East and +4 h in West. Eastern winds brought higher concentrations (Melpitz: 35.50 μg m^–3, Eiba: 37.47 μg m^–3), while Western winds led to cleaner conditions. After being corrected for time lag, the Urban Increment (UI) was estimated, showing that during Eastern wind, only ∼9% of PM₁ mass measured at Eiba was attributed to urban sources, highlighting the dominant contribution of regionally transported aerosol. Furthermore, source apportionment of organic aerosol (OA) identified five major factorsthree primary OA and two oxygenated OAat both sites. The findings underscore the significant role of regional pollution in shaping urban air quality and the need for cross-border emission reduction strategies.