AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Fine Particle pH and the Partitioning of Nitric Acid during Winter in the Northeastern United States
HONGYU GUO, Amy P. Sullivan, Pedro Campuzano-Jost, Jason Schroder, Felipe Lopez-Hilfiker, Jack Dibb, Jose-Luis Jimenez, Joel A. Thornton, Steven S. Brown, Athanasios Nenes, Rodney J. Weber, Georgia Institute of Technology
Abstract Number: 441 Working Group: Aerosol Chemistry
Abstract Particle pH is a critical but poorly constrained quantity that affects many aerosol processes and properties, including aerosol composition, concentrations, and toxicity. We assess PM$_1 pH as a function of geographical location and altitude, focusing on the northeastern US, based on aircraft measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign (01 Feb to 15 Mar 2015). Particle pH and liquid water were predicted with the ISORROPIA-II thermodynamic model and validated by comparing predicted to observed partitioning of inorganic nitrate between the gas and particle phases. Good agreement was found for relative humidity (RH) above 40%; at lower RH observed particle nitrate was higher than predicted, possibly due to organic-inorganic phase separations or nitrate measurement uncertainties associated with low concentrations (nitrate < 1 micro-gram m-3). Including refractory ions in pH calculations did not improve model predictions suggesting they were externally mixed with PM$_1 sulfate, nitrate, and ammonium. Sample line volatilization artifacts were quantified and found to be minimal. Overall WINTER particle pH for altitudes up to 5000 m ranged between -0.51 and 1.9 (10$^(th) and 90$^(th) percentiles) with a study mean of 0.77 ± 0.96, similar to those reported for the southeastern US ground-based sites and eastern Mediterranean. This expansive aircraft data set is also used to investigate causes in variability in pH and pH-dependent aerosol components, such as PM$_1 nitrate, over a wide range of temperatures (-21 to 19 ºC), RH (20 to 95%) and inorganic gas and particle concentrations. This study provides further evidence that particles with low pH are ubiquitous, which has implications for atmospheric chemistry and environmental and health impacts.