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Atmospheric aerosols have well documented detrimental effects on human health, ecosystems and air quality and are the key uncertainty in assessing the anthropogenic influence on climate change. Particularly during the cold weather season, urban centers in countries of the Western Balkans region such as Bosnia and Herzegovina (BiH) are experiencing some of the globally poorest air quality due to the extensive use of solid fuels and old vehicle fleet. The city of Sarajevo is the capital of BiH and is situated in Southeastern Europe within a plain surrounded by mountains. In the winter months (domestic heating season), topography and meteorology cause the pollutants to be trapped within the city plain. Recent analysis with the US EPA BenMAP model applied to BiH found that an annual decrease of 50% in fine aerosol would save 4760+ lives and costs of $2.3B annually (Žero et al., 2022). Countries of the Western Balkans lack state‐of‐ the‐ art atmospheric sciences research despite high levels of ambient pollution and position within the EU borders, which makes it imperative to understand the emission sources, processing and the adverse health effects of their atmospheric aerosol pollution. This presentation will highlight the results of the SAFICA 2018 project, the first Sarajevo, BiH project aiming to yield crucial, not previously available information about aerosol emission sources and atmospheric transformations through combination of online field (black carbon and particle number and size distribution) and offline laboratory (physicochemical characterization of daily PM10 filter samples) measurements. Laboratory analyses of PM10 samples determined aerosols’ bulk chemical composition, selected elements (Huremović et al., 2020 ; Žero et al., 2022) and molecular species (Pehnec et al., 2020). Aerosol chemical composition determined by aerosol mass spectrometry was further analyzed by Positive Matrix Factorization to separate organic aerosol (OA) into subtypes characteristic for their sources and atmospheric processes. Aerosol oxidative potential (OP) was also determined to evaluate the ability of SAFICA aerosols to generate reactive oxygen species. Main SAFICA results show that ~3/4 of aerosol mass is carbonaceous (OA + black carbon) and ~2/3 of total carbon (TC) mass is from non‐fossil sources (Figure 1). Aerosol has high loadings of black carbon and toxic species, indicating strong and diverse combustion sources and likely a major public health danger. More work is needed to estimate the contributions of different aerosol sources and species to total aerosol OP. Finally, this presentation will show how SAFICA knowledge gaps will be overcome in a future project, planned to take place during 2022‐2023, entitled Sarajevo Aerosol Experiment: Composition, Sources and Health Effects of Atmospheric Aerosol (SAAERO). We thank Federal Hydrometeorological Institute of BiH, Magee Scientific/Aerosol and TSI for support. We acknowledge the contribution of the COST Action CA16109 COLOSSAL and SEE Change Net. KDž and ASHP acknowledge the grant by the Swiss NSF (Scientific Exchanges IZSEZ0_189495), KDž, GM and ASHP European Commission funding of SAAERO project (EU H2020 MSCA-IF 2020 grant # 101028909), GM the Slovenian Science foundation program P1-0385, and SF by the Croatian Science Foundation (BiREADI IP- 2018-01-3105).

Nowadays, urban centres in countries of the Western Balkan (e.g., Bosnia and Herzegovina, B&H) are experiencing some of the poorest air quality worldwide due to the extensive use of solid fuels and an old vehicle fleet. Western Balkan countries lack state-of-the-science atmospheric research despite high levels of ambient pollution, making the efforts to understand the mechanisms of their air pollution imperative. Sarajevo, the capital of B&H, is situated in a basin surrounded by mountains. During the winter months, topography and meteorology cause significant pollution episodes. The Sarajevo Canton Winter Field Campaign 2018 (SAFICA) took place from Dec 04, 2017 to Mar 15, 2018 with online aerosol measurements and collection of daily, continuous filter PM10 samples for offline laboratory analyses. SAFICA aimed to give the first detailed characterization of the Western Balkans aerosol composition including organic aerosol (OA) to elucidate aerosol emission sources and atmospheric processing and to estimate the adverse health effects. PM10 samples (ntotal=180) were collected at four sites in the Sarajevo Canton: a) Bjelave and b) Pofalići (both urban background) ; c) Otoka (urban) ; d) Ivan Sedlo (remote). The urban sites were distributed along the city basin to study the pollutants’ urban evolution and the remote site was chosen to compare urban to background air masses. SAFICA PM10 samples underwent the following offline laboratory chemical analyses: 1) Bulk chemical composition of the total filter-collected water-soluble inorganic and OA by a high- resolution Aerodyne Aerosol Mass Spectrometer (AMS). The measured AMS OA spectra were further analysed by Positive Matrix Factorization (PMF) using the graphical user interface SoFi (Source Finder) to separate OA into subtypes characteristic for OA sources and atmospheric processes. 2) Organic and elemental carbon, water- soluble organic carbon, polycyclic aromatic hydrocarbons (11), levoglucosan, organic acids (16) and 14C total carbon content to evaluate OA chemical composition. 3) Major inorganic anions and cations to evaluate aerosol inorganic species. 4) Aerosol metal content determined by three techniques (AAS, ICP-MS and EESI). 5) Aerosol oxidative potential (OP) by two methods (AA and DTT) to evaluate the ability of particles to generate adverse health effects causing reactive oxygen species. SAFICA online measurements of black carbon (Aethalometer) and the particle number conc. (CPC and OPS) enabled the insights into the daily evolution of primary pollutants and an assessment of aerosol size and number distribution. The combined SAFICA results for field and lab measurements will be presented. Our results show that carbonaceous aerosols make ~2/3 of PM10 mass and the majority are oxygenated, water-soluble OA species with an average OM/OC = 1.9 (Fig.1). Absolute OP levels are very high compared to other sites globally. However, more work is needed to estimate the contributions of different aerosol sources and species to total aerosol OP. Urban air pollution crises in the Western Balkan will be put in the context of local, regional and global air quality. Finally, we will present the scientific questions opened by SAFICA and give suggestions for future studies. We thank Federal Hydrometeorological Institute of B&H, Magee Scientific/Aerosol and TSI for support. We acknowledge the contribution of the COST Action CA16109 COLOSSAL and SEE Change Net. KDz and ASHP acknowledge the grant by the Swiss NSF (Scientific Exchanges IZSEZ0_189495), GM the Slovenian Science foundation program P1-0385, and SF by the Croatian Science Foundation (BiREADI IP-2018-01-3105).

Nowadays, urban centres in countries of the Western Balkan region (including Bosnia and Herzegovina (B&H)) are experiencing some of the poorest European and global air quality due to the extensive use of non-renewable energy sources. Western Balkan countries lack state- of-the-art atmospheric sciences research despite high levels of ambient pollution, which makes the efforts to understand the mechanisms of their air pollution imperative. The city of Sarajevo, the capital of B&H, is situated in a basin surrounded by mountains. Particularly during the winter months, topography and meteorology cause significant pollution episodes. The Sarajevo Canton Winter Field Campaign 2018 (SAFICA) took place from Dec 04, 2017 to Mar 15, 2018 with on-line aerosol measurements and collection of daily, continuous filter PM10 samples for off-line laboratory analyses. SAFICA aimed to give the first detailed characterization of the Western Balkans aerosol composition including organic aerosol (OA) to elucidate aerosol emission sources and atmospheric processing and to estimate the adverse health effects. PM10 samples (ntotal=180) were collected at four sites in the Sarajevo Canton: a) Bjelave and b) Pofalići (urban background) ; c) Otoka (urban) ; d) Ivan Sedlo (remote). The urban sites were distributed along the city basin to study the pollutants’ urban evolution and the remote site was chosen to compare urban to background air masses. SAFICA PM10 samples underwent different off-line laboratory chemical analyses: 1) Bulk chemical composition of the total filter- collected water-soluble inorganic and OA by a high-resolution Aerodyne Aerosol Mass Spectrometer (AMS). The measured AMS OA spectra were further analysed by Positive Matrix Factorization (PMF) using the graphical user interface SoFi (Source Finder) to separate OA into subtypes characteristic for OA sources and atmospheric processes. 2) Organic and elemental carbon (OC/EC), water-soluble organic carbon, polycyclic aromatic hydrocarbons, levoglucosan, and 14C content of total carbon to evaluate OA chemical composition. 3) Major inorganic anions and cations to evaluate aerosol inorganic species. 4) Metal content in aerosol determined by two analytical techniques (AAS and ICP-MS). SAFICA on-line measurements of black carbon (Aethalometer) and the particle number concentration (Condensation Particle Counter and Optical Particle Sizer) enabled the insights into the daily evolution of primary pollutants and an assessment of aerosol size and number distribution. The combined SAFICA results for on- and off-line measurements will be presented. Our results show that the carbon- containing species make ~2/3 of PM10 mass and the majority are oxygenated, water-soluble OA species with an average OM/OC = 1.9 (Fig.1). Urban air pollution crises in the Western Balkan will be put in the context of local, regional and global air quality. Finally, we will present the scientific questions opened by SAFICA, including the advantages and limitations of SAFICA data set, and give the recommendations for future studies. Figure 1. Selected results of the SAFICA field measurements (a) and laboratory analyses (b). We thank the Federal Hydrometeorological Institute of B&H, Magee Scientific/Aerosol d.o.o. and TSI for SAFICA support. We acknowledge the contribution of the COST Action CA16109 COLOSSAL and SEE Change Net Foundation. KDz and ASHP acknowledge the grant awarded by the Swiss National Science Foundation (Scientific Exchanges IZSEZ0_189495) and SF by the Croatian Science Foundation (BiREADI IP- 2018-01-3105).

Atmospheric fine aerosol (PM2.5) is important to Earth’s biogeochemical cycles and climate, and adversely impacts air quality and human health. Specifically, organic aerosol (OA) substantially contributes to PM2.5 mass but remains poorly characterized due to the diversity and complexity of its sources, formation mechanisms, and chemical composition. A suite of state-of-the-art mass spectrometry (MS) methods were deployed and optimized to analyze distinct sets of source-specific OA samples collected from field studies and laboratory experiments in order to chemically characterize OA constituents at the molecular level. First, over 200 archived marine aerosol samples collected from Cape Grim, Tasmania, Australia under remote background conditions from 1991-2015, were analyzed using ultra-performance liquid chromatography interfaced to a high-resolution quadrupole time-of-flight mass spectrometer equipped with an electrospray ionization source (UPLC/ESI-HR-QTOFMS) and gas chromatography interfaced to a quadrupole mass spectrometer equipped with electron ionization source (GC/EI-MS). Several ng m-3 of biogenic (e.g., isoprene- and monoterpene-derived) secondary organic aerosol (SOA) tracers were quantified from 29 summer and winter seasons. Biogenic SOA tracers were enhanced during summer seasons and had moderate-to-strong correlations with marine bioactivity indicators such as methanesulfonic acid and chlorophyll-a. In addition, UPLC/ESI-HR-QTOFMS coupled with inline diode array detection (DAD) were utilized to assess light-absorbing brown carbon (BrC) OA from over 100 systematically-performed laboratory-simulated primary and aged wildfire emissions during the 2016 Fire Influence on Regional and Global Environments Experiment (FIREX) at the US Forest Service Fire Science Lab in Missoula, Montana. 37 solvent-extractable BrC constituents were characterized in terms of their composition, contributions to PM2.5 mass, emission factors, light absorbance, and evolution due to photochemical aging. Furthermore, we developed and optimized a versatile hydrophilic interaction liquid chromatography (HILIC)/ESI-HR-QTOFMS method that can efficiently resolve and measure the major isoprene epoxydiols (IEPOX)-derived and several other water-soluble SOA constituents with enhanced resolution of separation, sensitivity of detection, and accuracy of measurements. These findings provide detailed chemical composition of the atmospheric OA constituents that originate from varying sources, and thus, can serve as inputs for future studies on atmospheric analytical chemistry, earth science modeling, environmental management, and health effects of PM2.5.