Abstract View
Interferences on Aerosol Acidity Quantification due to Gas-phase Ammonia Uptake onto Acidic Sulfate Filter Samples
BENJAMIN A. NAULT, Pedro Campuzano-Jost, Douglas Day, Hongyu Guo, Duseong Jo, Anne Handschy, Demetrios Pagonis, Jason Schroder, Melinda Schueneman, Bruce Anderson, Charles Brock, Michael Cubison, Jack Dibb, Glenn Diskin, Karl D. Froyd, Weiwei Hu, Agnieszka Kupc, J. Andrew Neuman, Brett Palm, Eric Scheuer, Gregory Schill, Christina Williamson, Jose-Luis Jimenez, CIRES, University of Colorado, Boulder
Abstract Number: 124
Working Group: Instrumentation and Methods
Abstract
Measurements of the mass concentration and chemical speciation of aerosol are important to investigate their chemical and physical processing from near emission sources to the most remote regions of the atmosphere. A long-standing, common method to analyze aerosol is to collect it onto filters and to analyze the aerosol off-line; however, this method is prone to biases due to the handling and preparation of the sample. These artifacts affect the chemical composition of aerosol, which in turn affects our understanding of numerous physico-chemical processes and aerosol radiative properties. We show, using filters collected onboard the NASA DC-8 and NSF C-130 during six different aircraft campaigns, a consistent, substantial difference in ammonium mass concentration and ammonium-to-anion ratios, when comparing the aerosols collected on filters versus the Aerodyne Aerosol Mass Spectrometer (AMS). Another on-line measurement is generally in agreement with the AMS, showing the aerosol has lower ammonium-to-anion ratios than obtained by the filters. Using a simple gas uptake model with literature values for accommodation coefficients, we show that for ambient ammonia mixing ratios greater than 10 ppbv, the time scale for ammonia reacting with acidic aerosol on filter substrates is less than 30 s for typical aerosol volume distributions. Measurements of gas-phase ammonia inside the cabin of the DC-8 show ammonia mixing ratios greater than 10 ppbv (45±20 ppbv), consistent with mixing ratios observed in other indoor environments. In addition, using constraints from satellite observations of ammonia indicates that ammonium mass concentrations collected on filters are not thermodynamically possible in remote areas. This analysis provides insight into what the filters’ ammonium limit-of-detection should be when the sample is prone to exposure to ammonia and that this interference is an issue for any filters (ground-based included) when the filter is exposed to ammonia for long periods.