Quantification of atmospheric organic aerosol sources using on- and off-line aerosol mass spectrometry

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Date

2023-12-19

Authors

Βασιλακοπούλου, Χριστίνα

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Abstract

Quantification of the sources of atmospheric particulate matter and especially its organic fraction has been a major challenge in air quality engineering. In this work new techniques are developed based on the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and they are applied with emphasis on two sources: biomass burning and secondary organic aerosol production from the oxidation of natural emissions of volatile organic compounds. The AMS has been used during the last fifteen years for the source apportionment of organic aerosol (OA) in field studies. The corresponding results have dramatically improved our understanding of particulate matter and especially of organic aerosol. AMS is one of the few instruments which can provide information about the size distribution of aerosols, their concentration, and their chemical composition in high temporal resolution. However, there are circumstances in which its use is impractical. Its weight, size, and power consumption sometimes make its transfer to the field challenging or even impossible for some sites. Also, its high cost makes its use in multiple locations in the same city or region impossible. Off-line AMS measurements can provide valuable information about the ambient organic aerosol in areas and periods in which online AMS measurements are not available. However, these offline measurements have low temporal resolution as they are based on filter samples collected usually over 24 hours. In the first part of this study, we examined whether and how this low time resolution affects source apportionment results to test the feasibility of the off-line approach. We used a five-month period (November 2016-March 2017) of online measurements in Athens and performed positive matrix factorization (PMF) analysis to both the original dataset, which consists of 30 min measurements, and to time averages from 1 up to 24 h. The 30 min results indicated that five factors were able to represent the ambient organic aerosol: a biomass burning organic aerosol factor (BBOA) contributing 16% of the total OA, hydrocarbon-like OA (HOA) (29%), cooking OA (COA) (20%), more oxygenated OA (MO-OOA) (18%), and less oxygenated OA (LO-OOA) (17%). Use of the daily averages resulted in estimated average contributions that were within 8% of the total OA compared with the high-resolution analysis for the five-month period. The error for the low-resolution analysis was much higher for individual days and its results especially for high concentration days are quite uncertain, however for large period the low temporal resolution results were in good agreement with the high-resolution results. These results suggested that the off-line approach could, at least theoretically, work despite its low temporal resolution. Another major limitation of the existing off-line AMS technique is that it is designed to measure the water-soluble organic components, while several atmospheric OA components are partially or completely insoluble in water. In the second part of the study, an improved off-line technique was developed and evaluated in an effort to capture most of the partially soluble and insoluble organic aerosol material, reducing significantly the uncertainty of the corresponding source apportionment. This was achieved by improving the extraction procedure of the insoluble aerosol components from the quartz filters using sonication and making sure that the corresponding suspended particles in the aqueous solution would be transferred with minimal losses to the AMS. The success of the proposed approach was evident by the significant fraction of submicrometer suspended insoluble particles present in the water extract, and by the reduced insoluble material on the filters after the extraction process. Significant elemental carbon concentrations were also measured in the produced aerosol that was used as input to the AMS during the off-line analysis. The improved off-line AMS analysis was tested in three campaigns: two during winter and one during summer. Collocated on-line AMS measurements were performed for the evaluation of the off-line method. The PMF results showed that the fractional contribution of each factor to the total OA differed between the on-line and the off-line PMF results by less than 15%. The differences in the AMS spectra of the factors of the two approaches could be significant suggesting that the use of factor profiles from the literature in the off-line analysis may lead to complications. Aerosol mass spectrometry was used in the next step of the work to study the major sources of OA. Wildfires are a significant air pollution source globally. However, there is a discrepancy between the estimated and the measured biomass burning impacts of wildfires in Europe. Bottom-up estimates of wildfire emissions suggest that they are responsible for more than 85% of the fine particulate matter emissions in Europe during summer. On the other hand, top-down estimates based on measurements of the composition of the aerosol suggest that they are responsible for less than 10% of the OA. This major discrepancy results in significant uncertainty about the current role of wildfires in European air pollution especially far from the corresponding fire areas. Measurements were conducted in a remote continental eastern Mediterranean site (Pertouli, Greece) in the summer of 2022, in order to quantify the impact of wildfires on the air quality in this region (SPRUCE-22 campaign). The results of the field measurements were combined with chemical transport modeling and suggested that the contribution of wildfires to fine particle levels in Europe during summer is underestimated by a factor of 4-7. Wildfires were responsible for approximately half of the total OA in Europe during July 2022. The reason for previous underestimations is that wildfire emissions get rapidly transformed to secondary oxidized organic aerosol with an accompanying loss of its organic chemical fingerprints, resulting in increased difficulty to be identified by even state of the art instruments. These oxidized particles remain in the atmosphere for several days and can travel thousands of kilometers away from the corresponding fire, leading to a regionally distributed background organic aerosol that is responsible for a significant fraction of the health-related impacts caused by fine particles in Europe and probably in other continents. These adverse health effects can occur hundreds or even thousands of kilometers away from the fires. We estimate that wildfire emissions are responsible for 15-22% of the deaths in Europe due to exposure to fine particulate matter during summer. Even if these deaths are currently included in the approximately 300,000 deaths per year in Europe as the result of exposure to ambient fine particulate matter, they have not been associated with summertime biomass burning. Finally, the secondary organic aerosol formed by the oxidation of specific biogenic compounds was studied. Emissions of biogenic volatile organic compounds by trees and plants exceed the anthropogenic ones on a global scale. As a result, oxidation of biogenic organic vapours is an important pathway for the formation of secondary OA both on a global but also on a local scale in forested areas. Atmospheric simulation chamber experiments were conducted in order to study the secondary OA formation during the ozonolysis of four sesquiterpenes (δ-cadinene, β-caryophyllene, α-caryophyllene and α-humulene) and from the reactions of three first generation sesquiterpene ozonolysis products (β-nocaryophyllinic acid, 2-(2-carboxyethyl)-3,3-dimethylcyclobutanecarboxylic acid and 2-(2-methyl-6-oxoheptan-3-yl)-3,6-dioxoheptanαl) with the OH radical. Our results indicate that the produced secondary OA from sesquiterpenes can eventually become quite oxidized having O:C around 0.6. Previous experimental work has suggested that the final O:C of sesquiterpene secondary OA is much lower. Τhe first-generation products continue reacting and the corresponding secondary OA mass spectrum evolves rapidly changing by 20-25 degrees as the fresh secondary OA was aging. This suggests that identification of the corresponding secondary OA under ambient conditions based on the first-generation products may be problematic. A mechanism is proposed and tested for the description of the later generation gas-phase reactions converting the secondary OA to highly oxidized compounds that exist manly in the particulate phase.

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Keywords

Organic aerosol, Sources, Atmosphere

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