AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Rethinking the Concept of pH in Atmospheric Particles
MICHAEL BATTAGLIA JR., Christopher Hennigan, University of Maryland, Baltimore County
Abstract Number: 46 Working Group: Aerosol Chemistry
Abstract The purpose of this study was to compare the acidic conditions inferred from bulk particle distributions with the acidic conditions within individual particles. Specifically, we characterized on an individual particle level the combination of particle size and environmental conditions (T, RH) where a bulk definition of pH may be applied. We find a range of limiting conditions – both particle size and RH – where the scale of individual particles is too small to apply the bulk definition of pH. Such limitations do not exist when aerosol distributions are aggregated, as is typically the approach in using bulk aerosol measurements as inputs into thermodynamic equilibrium models. Using measured values of aerosol water content and thermodynamic equilibrium models, the H$^+ and OH$^- per particle were calculated for monodisperse aerosol size distributions. As a function of particle diameter, for all particles 0-2.5 microns in diameter, pH values from 0-3 were prohibited on the basis of requiring fractional (<1) OH$^- ions per particle. The smallest critical diameter calculated is 0.20 micron, required for pH 7 at 90% RH; no particles smaller than this diameter would meet the bulk definition of pH. As a function of RH, the smallest critical RH value calculated is 62.4%, required for pH 7 and a 250 nm particle diameter. Individual particles at RH levels below this critical value do not contain enough water to apply the bulk definition of pH. These results suggest that the bulk aggregation of particles during sampling leads to a dominating effect of large particles. In the future it may be necessary to develop and utilize a different particle acidity measurement scale, rather than aerosol pH, to characterize the critical factor driving atmospheric processes that are known to be affected by acidity.