Insights into air pollution sources in urban enviroments during winter 

Fine particulate matter fraction (PM2.5) is a hazardous risk to human health due to its small size, complex chemical composition, and high specific surface area. Different carbonaceous compounds of which some have mutagenic and cancerogenic properties could bind to the PM2.5 surface and by inhalation penetrate the human body which further leads to the development of severe respiratory and cardiovascular illnesses, even premature death. According to the World Health Organization (WHO)  lower air quality due to elevated PM2.5 levels in rural and urban areas worldwide caused around 4.2 million premature deaths in 2019, while the European Environment Agency (EEA) reported around 238 000 of premature deaths in Europe in 2020. The goal of EEA's long-term action plans on zero air pollution considering PM2.5 levels until 2030 is to further lower these numbers by 55%. Due to its high specific surface area, PM2.5 is subjected to aerosol aging processes which can change the PM2.5 properties and further amplify its negative impact on the environment due to eutrophication and acidification.Ambient PM2.5 could be directly emitted from its source as primary or could be produced as secondary from its gaseous pollutants. In an urban environment, anthropogenic sources such as vehicular emissions, industry processes, and fossil fuel combustion are considered predominant to elevate PM2.5 levels while the contribution of natural sources like dust resuspension and lightning as well as the long-range transport should not be neglected. The chemical composition of PM2.5 is mostly related to source characteristics e.g. its type, intensity, temporal, spatial, and/or seasonal distribution, while meteorological parameters such as relative humidity, temperature, wind velocity, and solar radiation index could contribute to PM2.5 gas-phase and aqueous-phase transformation processes.This study aimed to assess the air pollution sources regarding PM2.5 chemical content due to its diverse impact on the environment and human health. Mass concentrations of PM2.5,  as well as, the mass concentrations of water-soluble inorganic and organic ions (Cl-, NO3-, SO42-, Na+, NH4+, K+, Mg2+, Ca2+, acetic (AA), formic (FA), oxalic (OX)) in its content were determined at five measuring sites in different part of Zagreb, capital of Croatia. Daily PM2.5 concentrations were measured by gravimetry and ion chromatography was used to determine water-soluble inorganic and organic ions. Results show that in urban environments mobile and stationary sources as well as primary and secondary sources show the same temporal distribution depending on the day of the week. The mobile and stationary sources both contribute to the overall air pollution in Zagreb at each location, regardless of the station classification. At all measuring sites the higher contribution of primary sources was obtained. Additionally, results indicated  that on certain days, secondary sources were found to be dominant in the northern and western parts of the city. This information highlights the importance of monitoring and regulating both primary and secondary sources of emissions to ensure a healthier environment for all.