Investigating Aerosol Acidity through Ammonia Measurements Combined with Single Particle-Microspectroscopy
ALI ALOTBI, Yao Xiao, Emily Costa, Kayleigh Reilly, Tiantian Zhu, Mitchell Rogers, Xu He, Cara Waters, Drew Gentner, Rachel O'Brien, Andrew Ault, University of Michigan
Abstract Number: 668
Working Group: Aerosol Chemistry
Abstract
Acidity is an important property of atmospheric aerosols that influences multiphase reactions, secondary organic aerosol (SOA) formation, and particle physicochemical properties. Ammonia-ammonium partitioning plays a crucial role in determining aerosol pH, but combined ambient measurements of this base-conjugate acid pair, which contributes to a range of aerosol types within a diverse particle population, have been limited. Urban ammonia emissions are especially challenging to measure due to its low ambient concentrations, high solubility in water, and interference from other species in the gaseous matrix. Here, we compare high time resolution ammonia measurements from a cavity ring-down spectrometer during the New York City-metropolitan Measurements of Emissions and TransformationS/Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (NYC-METS/AEROMMA) field campaign in July-August 2023 with ambient single-particle composition from a range of microspectroscopy measurements, including simultaneous optical-photothermal infrared (O-PTIR) and Raman microspectroscopy. By examining the ammonia time series in the context of single-particle and bulk ammonium measurements from Aerosol Chemical Speciation Monitoring (ACSM), we evaluate ammonia/ammonium equilibrium and the range of aerosol pH across a heterogeneous particle population. pH measurements are also compared with controlled laboratory measurements of atomized ammonium sulfate at different acidities to gain further insight into the field data. These results provide an improved framework for understanding the distribution of aerosol pH in an urban area and have the potential to help improve atmospheric models by accounting for aerosol acidity impacts.