Chemical composition and pH of marine aerosols in the summer-time marine boundary layer of the Atlantic sector of the Southern Ocean

Xokashe, Sive

Chemical composition and pH of marine aerosols in the summer-time marine boundary layer of the Atlantic sector of the Southern Ocean

Thesis / Dissertation

2023

Abstract

Aerosol acidity is a fundamental parameter of aqueous chemistry that impacts the lifetime of pollutants, biogeochemical cycles, human health, and climate. However, despite this importance, the effects of aerosol pH on these processes are difficult to constrain, in part because there are no direct methods to measure aerosol pH. The lack of observations in the remote clean atmosphere makes quantification of aerosol pH even more challenging. This dissertation aims to predict the spatial and temporal variation of aerosol pH and its drivers in the marine boundary layer of the summertime Southern Ocean. Results are presented from Ambient Ion Monitor (online method) and high volume sampler (offline method) based chemical analysis of aerosol species sampled during a cruise from Cape Town (34.11°S, 18.03°E) to Antarctica (70°S, 2.11°W). In addition, a thermodynamic model, ISORROPIA-II, is used to predict the pH of aerosol. The thermodynamically predicted aerosol pH from both the online and offline methods are generally acidic (i.e., <7). The modelled pH of particles with a diameter less than 1 µm (submicron) from the offline method ranged from -0.67 to 4.37 with an average value of 0.27, while the pH of particles with a diameter greater than 1 µm (supermicron) ranged from 1.67 to 4.68 with an average value of 2.98. For the online method, which only sampled particulate matter with a diameter less than 2.5 µm in size (PM2.5), the pH ranged from -0.68 to 4.46 with an average value of 2.07. The variation in predicted submicron pH was best explained by the sodium to sulfate (Na+ /SO4 2- ) ratio, while no relationship was observed with temperature or relative humidity. For both supermicron and the PM2.5 aerosols measured online, the pH was best explained by the molar ratio of the inorganic aerosol species. In addition, the supermicron pH varied as a function of relative humidity while for the PM2.5 did not. Using a combination of aerosol concentration, windspeed, and sea ice concentration data, along with air mass back trajectories, it was found that generally, aerosol samples that originated from the open ocean were relatively more acidic compared to those that had air mass back trajectories originating from sea ice covered surface ocean. This was attributed to the partitioning of hydrogen chloride (HCl). One of the major limitations in this study was a lack of aerosol gas concentrations to constrain the model (e.g., ammonia (NH3), nitric acid (HNO3), HCl). Therefore, to improve the predicted aerosol pH, measurements of aerosol gas species is highly recommended. Additionally, it was found that when an offline method is used, an online method proves useful for validation of possible sampling artefacts that may be occurring in the offline method.

Keywords

Ocean and Atmospheric Science

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