A Coupled Condensational-Growth Colorimetric Approach to Direct Measurement of Aerosol Acidity for Field Measurements
EVAN DALTON, Yao Xiao, Cara Waters, Emily Costa, Drew Gentner, Andrew P. Ault, University of Michigan
Abstract Number: 450
Working Group: Urban Aerosols
Abstract
The acidity, or pH, of atmospheric aerosols is a key variable that affects the eventual health and climate impacts of an aerosol. Aerosols are typically highly acidic (pH = 0 – 5), although it is difficult to measure aerosol acidity due to the low water content and high ionic strength of most atmospheric particles, and the non-conservative nature of the H+ ion. As such, aerosol pH is commonly inferred from thermodynamic models, which utilize condensed-phase and gas-phase compositions to derive the aerosol pH. This derivation requires intensive analyses with expensive instrumentation, which has limited the number of reliable aerosol pH determinations during atmospheric field campaigns. Recent advances in direct aerosol pH measurements have the potential to improve our understanding of aerosol acidity but have not yet been broadly applied to the ambient atmosphere. Herein, we present measurements of ambient aerosol pH for particulate matter less than 1 micron in diameter (PM1) in the New York City (NYC) metropolitan area. This method, deployed during a field campaign for the first time, directly measures PM1 pH with colorimetric indicators (thymol blue, pKa = 1.7; methyl orange, pKa = 3.5) following particle growth within a condensational growth tube to ensure sufficient particulate water content to saturate the pH indicator paper. We can derive the pH of the original ambient aerosol by accounting for the well-defined particle volume changes in the growth tube, and knowledge of the original PM1 size distribution and sulfate mass concentration. PM1 was quite acidic during NYC-METS (New York City metropolitan Measurements of Emissions and TransformationS), with pH < 2 in both NYC and at a coastal site near New Haven, CT. We will discuss method optimization, field measurements, and comparison of this pH measurement to thermodynamic models. Measurements of aerosol pH such as these will enable improved model predictions of secondary aerosol formation, a key contributor to atmospheric PM concentrations across urban regions globally.