PhD Thesis Defence Presentations - Ανδρέας Ακτύπης

Atmospheric particulate matter (PM) or aerosol is the air pollutant associated the most with adverse health effects and increased mortality. Yet, in its absence, Earth would be a completely different planet. Aerosols provide the surface upon which ambient water vapor will condense to form cloud droplets. Without this surface, clouds would not form, which would lead to extreme (above 400%) ambient relative humidity. Studying the sources and behavior of atmospheric aerosol is crucial for understanding its role in human health and climate. Sources emitting aerosol directly into the atmosphere (primary particles) are relatively well-documented and targeted for mitigation. However, the production of aerosol from reactions of various gas species (secondary particles) remains poorly understood, despite their significant contribution to particle number and mass concentrations. Secondary sources of particles include new particle formation (NPF) and secondary organic (SOA) and inorganic aerosol formation. Both are largely dependent on the type, the availability and chemical reactivity of various gaseous precursors.

The objective of this thesis is to improve our understanding of the sources of secondary particle number and mass in the atmosphere. The study focuses on new particle formation (NPF), a major source of particle number, and its spatial gradients in Greece, and on the secondary organic aerosol production (SOA), a major source of particle mass. The main instrument used to measure the size distribution and number concentration of particles was the scanning mobility particle sizer (SMPS). The instrument used to measure the particle mass, and its chemical composition was the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). Some of the major scientific questions about NPF are related to identification, occurrence frequency and the factors that affect the formation of new particles. It is clear that sulfuric acid (formed in the atmosphere from the photooxidation of SO₂) is a key component of NPF. The contribution of numerous other pollutants on NPF has been studied in the laboratory, but it is often challenging to observe it in the ambient atmosphere. 

In the first part of this work, the frequency and characteristics of NPF events were investigated for Patras, Greece, during the summers of 2020 and 2021. These periods were characterized by relatively clean air masses, intense solar radiation and adequate sulfuric acid levels, which typically favor the formation of new particles. However, a surprising low NPF frequency was observed in Patras during both summers, with an average of 12% (15 out of 120 days). Although NPF was infrequent in Patras, a significant fraction of days (31%), typically classified as “undefined” in previous studies, was observed. New particles in the range of 20–40 nm appeared during early afternoon and persisted for 5–8 h. Lower PM levels and relative humidity and higher wind speed characterized these days compared to the non-events. The fact that these particles were not associated with simultaneous increases in the concentration of other primary pollutants like black carbon or NO_x suggested that they were not due to nearby combustion sources. The analysis of the air mass origin during the hours these particles appeared, revealed that they had been formed several hours earlier in an area 100–150 km northeast of Patras and had grown significantly by the time they arrived. An improvement of the existing classification method was made, by replacing the term “undefined” with the term “transported events” and studying their characteristics for the first time in Greece. Agricultural areas with elevated ammonia and amine emissions were identified as potential NPF areas, supporting the involvement of these bases in the formation of the fresh nuclei.

            Particle size distribution measurements were conducted during the same periods (summers 2020 and 2021) in 10 more locations in Greece, to explore the spatial variability of the NPF frequency and characteristics. The measurements took place in Athens, Thessaloniki, Patras, Ioannina, Thrace, Lesvos, Sifnos, Finokalia, Chania, Methoni and Mt. Helmos as part of the PANACEA project. Significant spatial gradients in NPF frequency were observed in Greece during the examined period. This frequency varied from close to zero in the southwestern parts of the country to more than 60% in the northern, central, and eastern regions. Not a single simultaneous NPF event was observed over all Greek sites. However, frequent regional events were observed over parts of the Aegean Sea and northern Greece. The sites with the highest NPF frequencies were in close proximity to both coal-fired power plants, which emit SO₂, and agricultural areas with some of the highest ammonia emissions in the country. Sites with infrequent NPF were far from ammonia sources although they had moderate SO₂ concentrations. The few events that occurred in these areas were associated with air masses passing over agricultural areas. These observations support the hypothesis that ammonia and/or amines control new particle formation in the study area.

            Two additional summer campaigns were conducted in 2023 and 2024 in different parts of Greece to study the scale and frequency of NPF events. Measurements during July 2023 took place in two sites in Boeotia (Elateia and Kamena Vourla), Patras and Athens. In September 2024 the sites were Poros, an island in the Saronic Gulf, and Athens. Unexpectedly, NPF in Elateia was infrequent (only one event took place) given that transported events in Patras had been associated with this area. The few transported events that appeared in Patras during this period were still associated with air masses originating from the northeast. These observations suggest that NPF probably took place over the Corinthian Gulf during this period. Transported events frequently occurred in Elateia (30% of all days). Transport of newly formed particles from Elateia to Athens was observed in at least three days. In Poros, transported NPF events were frequent (67% of the days). These events were linked to air masses from the north and northwest, and to slow-moving air over the Saronic Gulf. Many NPF events (6 out of the 14) in Poros did not also occur in Athens, even though the two sites are only 60 km away. When the particles appeared in Athens, they were mostly associated with air masses that had spent some time over the Saronic Gulf. These results suggest that the Saronic Gulf might be a significant hotspot for local (of a few tens of km) NPF. This is supported by the fact that the broader area is a major sea route and has also notable industrial activity, resulting in elevated SO₂ concentrations.

            The more time organic particles spend in the atmosphere, the more they get oxidized. This tends to increase their mass due to SOA formation from the oxidation of various organic vapors. Numerous studies have explored the potential of individual compounds (either anthropogenic or biogenic) to form SOA through lab experiments, but their results often deviate from real ambient observations. Also, a knowledge gap exists regarding the chemical evolution of already aged organic aerosol. A novel dual atmospheric chamber system was deployed in two different environments (one polluted and one in remote forest) to study the potential of ambient air, that was directly injected into the chambers, to form secondary organic and inorganic aerosol. A series of experiments was conducted, using one chamber as the perturbed and the second as a reference. In the polluted environment in northern Italy, highly oxidized (O:C reached 1.05) SOA was rapidly produced (reaching 10 μg m^-3 of SOA formed) in all experiments but one. On the other hand, in all experiments but one, SOA was not formed in the Greek forest. This indicates that SOA formation chemistry had been practically terminated before the beginning of most experiments, so there was little additional SOA formation potential left. The study provides new information about the later stages of secondary aerosol formation in different environments.