Aerosol Particles and their Gas-Phase Precursors in Cold Environments: From Simulated Experiments to Polar Field Observations

Abstract

Our planet is a highly complicated system and the atmosphere – the layer of gases surrounding the globe – enables organisms to breathe and live. Within this layer, aerosol particles can impact human’s health, when inhaled, but they can also interact with the Earth’s climate in many ways and on many different scales.

Aerosols can origin from very different sources, natural or man-made, emitted as is or transformed from gases, through chemical reactions, to particles. These secondary aerosols formed through new particle formation (NPF) have drawn a lot of attention as they can contribute significantly and/or predominantly to the cloud condensation nuclei budget and further impact the climate. For this reason, it is crucial to understand what are the chemicals and physical processes that trigger the formation of new particles. Atmospheric oxidation is an important process that is responsible for a variety of gases and condensable vapors that can initiate atmospheric nucleation and/or contribute to particle growth. Among these vapors, highly oxygenated organic molecules (HOM) are formed by the oxidation of volatile organic compounds via a complex chain reaction yet not fully characterized.

This thesis tackles several aspects linked to aerosol formation and the formation of their gas-phase precursors in cold environment. This work combines experimental work and field observations, with (1) the simulation of an oxidation reaction, alpha-pinene ozonolysis - known to form HOM, at different temperatures, (2) the analysis of the oxidizing agent, ozone, over 20 years at a boreal forest site, and finally (3) the exploration on precursor vapors forming aerosol in the Antarctic peninsula. This work involved the operation of multiple instruments, especially including the state-of-the-art chemical ionization atmospheric pressure interface time of flight mass spectrometer (CI-APi-TOF) to detect HOM and other condensable vapors, or alternatively naturally charged ions (i.e., without chemical ionization).

Using atmospheric chamber simulations, we revealed the temperature dependency of HOM production and hinted the mechanistical steps particularly impacted by cold temperatures. In connection with the aerosol composition, we refuted a direct connection between the formation of HOM and the formation of a class of another oxidation products, namely dimer esters, measured in the particle-phase. From another perspective, we assessed the trends of tropospheric ozone measured from the ground up to 125 m of altitude, in the boreal forest. We also identified and characterized ozone minima and depletion events, typically occurring in the cold season and close to the ground level. Finally, we reported a high frequency of strong NPF events in the Antarctic peninsula, typically on the warmest days of the austral summer. There, we described the aerosol production and discussed the possible NPF mechanisms that did not involve HOM, but sulfuric acid, ammonia, possibly amines, and methane sulfonic acid.

The wide approach of this work has enabled to extend the impact of temperature on multiple components, with, on one side, low temperatures seen with low oxidant (ozone) concentration, and low HOM production in (simulated) vegetated (-like) environment, and, secondly, above-zero temperature occurring simultaneously with NPF in a remote polar site, likely triggered by enhanced emissions with higher temperatures.