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Urban air fine particles are a major health-relating problem. However, it is not well understood how the health-relevant features of fine particles should be monitored. Limitations of PM2.5 (mass concentration of sub 2.5 μm particles), which is commonly used in the health effect estimations, have been recognized and, e.g., World Health Organization (WHO) has released good practice statements for particle number (PN) and black carbon (BC) concentrations (2021). In this study, a characterization of urban wintertime aerosol was done in three environments: a detached housing area with residential wood combustion, traffic-influenced streets in a city centre and near an airport. The particle characteristics varied significantly between the locations, resulting different average particle sizes causing lung deposited surface area (LDSA). Near the airport, departing planes had a major contribution on PN, and most particles were smaller than 10 nm, similarly as in the city centre. The high hourly mean PN (>20 000 1/cm3) stated in the WHO's good practices was clearly exceeded near the airport and in the city centre, even though traffic rates were reduced due to a SARS-CoV-2-related partial lockdown. In the residential area, wood combustion increased both BC and PM2.5, but also PN of sub 10 and 23 nm particles. The high concentrations of sub 10 nm particles in all the locations show the importance of the chosen lower size limit of PN measurement, e.g., WHO states that the lower limit should be 10 nm or smaller. Furthermore, due to ultrafine particle emissions, LDSA per unit PM2.5 was 1.4 and 2.4 times higher near the airport than in the city centre and the residential area, respectively, indicating that health effects of PM2.5 depend on urban environment as well as conditions, and emphasizing the importance of PN monitoring in terms of health effects related to local pollution sources. publishedVersion

Since gas engines are considered a future solution to improve air quality and to mitigate climate impacts, there is an urgent need to understand their emissions. The aim for this study was to understand the phenomena affecting the formation of particulate emissions of a non-road natural gas engine. To achieve this, the engine's exhaust emissions were characterized under different operating conditions. The regulated pollutants (gaseous CO, HC, and NOx; particulate matter (PM) and particle number (PN)) were determined experimentally and a detailed characterization of particulate pollutants over a wide particle size range (particles down to 1.2 nm) was conducted with state-of-the-art instrumentation considering both physical and chemical properties of the exhaust aerosol. The test engine was a prototype non-road spark-ignited natural gas engine, which was studied over the non-road steady test cycle (NRSC). The role of the three-way catalyst (TWC) was studied by sampling and characterizing the exhaust aerosol both with and without the TWC. The TWC was observed to efficiently remove the vast majority of the regulated gaseous (96% CO, 98% HC, 98% NOx) and particulate mass emissions (98%). In general, the measured particle number emission factors were highly dependent on the cut-off sizes of the condensation particle counters. Using CPCs with smaller cut-off sizes resulted in higher particle number emission factors. For black carbon (BC), the intermediate engine speed conditions (modes 5–7) led to lower BC emissions than the high speed conditions (modes 1–3). In contrast, highest BC emissions on a work basis were observed during idling. TWC did not influence BC levels. Without the TWC, PM was comprised mostly of organic compounds (70–100%). Downstream of the TWC, the majority of PM was, depending on the load, composed of organic compounds, sulfate, or black carbon. A statistical source apportionment based on mass spectra revealed that the PM1 was mostly related to unburned and burned lubricating oil, indicating a minor role of fuel in PM formation.

Black carbon (BC) is one of the main components of ambient particulate matter. Previous studies have suggested that BC is more toxic than PM2.5 (mass concentration of all sub-2.5 μm particles). One possible reason for the greater toxicity is that BC is typically in a size range which penetrates easily into lung alveoli and BC particles have a large surface area due to their fractal structure. Due to these properties, toxic gaseous compounds can condensate on the surface of BC particles and then be transported effectively into human lungs, causing a large lung-depositing surface area (LDSA) of particles. In this study, we investigated the relationship between BC and LDSA concentrations in street canyon, highway, and harbour environments in the Helsinki Metropolitan area. In all the studied environments, BC and LDSA concentrations were strongly correlated. In the harbour, cases where marine traffic was considered as the main emission source, the average LDSA per BC mass was 2.4–2.7 times higher than in the road traffic environments. This result was linked to a larger lung depositing size of BC, suggesting that condensation and coagulation of other co-emitted compounds can have a major role in the lung deposition of BC. Thus, BC emissions from marine traffic can cause higher exposure of other co-emitted toxic compounds in the human lungs than the road traffic. The fraction of LDSA linked to BC emissions in the street canyon, the highway, and the harbour were 33%, 30%, and 47%, respectively, whereas the fractions of BC mass in PM1 concentration were 14%, 14%, and 7%, respectively. The results show that BC emissions contribute much more to LDSA than to mass concentration, which indicates that the possible strong negative health effects linked to ambient BC mass concentration could be related to the high LDSA concentration.

The formation of soot, organic, and inorganic aerosols has a profound effect on the environmental and technological feasibility of biomass combustion. In this work, the soot and aerosol processes are examined for a modern pulverized wood-burning 100 MWth district heating plant. Experimental data was collected from two locations inside the furnace (30% and 100% thermal loads), including measurements for fine particle (PM1) number size distribution, number concentration, and chemical composition. The experiments were complemented with Computational Fluid Dynamics (CFD) simulations and Plug-Flow Reactor (PFR) modeling. The measurements and modeling are combined in a comprehensive analysis, providing fundamental understanding on the aerosol processes inside the furnace. The wood-powder combustion is efficient under both thermal loads, indicated by the low unburned carbon content in fly-ash, and the low CO, NO and soot emissions (3). The fine particles consist mainly of K2SO4, and of lesser amounts of alkali salts (NaCl, KCl), and Ca and Mg compounds (oxides or sulfates). A large concentration of KOH/K2CO3 vapor may exist in the flue gas and play a significant role in the heat exchanger fouling. The applied modeling tools are shown to provide accurate estimations for the composition and formation regions of fine particles inside the industrial biomass furnaces.