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Comprehensive molecular characterization of secondary organic aerosol formation in the atmosphere

Periodic Reporting for period 4 - COALA (Comprehensive molecular characterization of secondary organic aerosol formation in the atmosphere)

Reporting period: 2019-09-01 to 2020-02-29

Particulate matter in Earth's atmosphere impacts society in many ways. It is a pollutant known to be the main cause a variety of detrimental health effects, causing millions of premature deaths each year. In addition, airborne aerosol particles strongly impact the radiation balance of our planet: they can scatter sun light back into space or impact cloud properties and lifetimes. A large source of uncertainty in the anthropogenic influence on climate comes from the lack of understanding of aerosol particle formation, evolution and fate in the atmosphere.

The target of the COALA project was a comprehensive understanding of the role of organic emissions on aerosol formation using novel mass spectrometric techniques. In the atmosphere, volatile emissions are oxidized to produce vapors of various volatilities. The least volatile will condense and form aerosol, but the fraction of products able to do this (for a given molecule) has been hard to determine, in part due to a lack of methods for quantification of these molecules. The main objectives in COALA were to experimentally detect as large fraction as possible of the oxidation products from the most commonly emitted precursors in the atmosphere, and to utilize this data to map out a volatility distribution of the formed vapors. With such information, it is possible to evaluate how much of the organic aerosol is formed directly through condensation and whether additional chemistry occurring on the surface or inside the particles will influence the amount of organics. Constraints on the chemical and physical processes leading to organic aerosol formation will directly translate into better constraints on atmospheric models studying the influence of human activity on climate change.

The primary findings of the COALA project highlight the role of efficient and rapid uptake of the most highly oxygenated molecules (HOM) formed in the oxidation of volatile organic compound (VOC). We found that all major VOC types in the atmosphere, both of biogenic (e.g. monoterpenes) and anthropogenic (e.g. aromatics and alkanes) origin produced HOM at higher yields than had earlier been believed. This has major implications for the dynamics, aging and losses of aerosol particles in the atmosphere.
During the first half of the project, much effort was put into designing appropriate laboratory facilities to study the chemistry of emission oxidation under controlled chamber conditions. In our own chamber, and in collaboration with other groups, we have characterized the complex gas phase chemistry of VOC oxidation in different systems, including chamber oxidation of biogenic and anthropogenic precursors, ambient studies as well as computational work. The general indication is that a large fraction of the atmospheric organic aerosol is formed from highly oxygenated low-volatile vapors formed in the gas phase. This is further in line with our findings on the formation and properties of organic aerosol in both chambers and ambient. We also worked closely with modellers to implement our experimental knowledge on emission oxidation into both atmospheric and chamber models. This work has shown that the low-volatile compounds play a pivotal role in the formation of organic aerosol, but at the same time, the models show that the final impact on climate from these vapors is highly complex, and is sensitive to detailed chemical pathways, many of which remain topics of intensive study. The results from COALA are utilized for modelling efforts both internally and by groups outside of UHEL, based on the published work.
Through
a. the simultaneous deployment of several different types of chemical ionization mass spectrometers, and
b. advanced statistical analysis techniques, together with
c. close collaboration with leading quantum chemists,
we have achieved new understanding on a variety of topics relating to VOC oxidation and SOA formation. These concern
1. the role of temperature on the level of oxygenation achieved during oxidation,
2. direct measurements of the condensability of oxidation products,
3. the role of aerosol particle acidity on reactive uptake of more volatile species,
4. the importance of oxidant concentrations on the potential to form organic aerosol from anthropogenic emissions, and
5. the role of nitrogen oxides on the formation and evolution of organic aerosol.
Overview of the COALA project approach