Investigating Possible Biases of Filter Measurements of Submicron Inorganic Salts onboard the NASA DC-8

The mass concentration and speciation of accumulation and coarse mode aerosol is important to investigate the chemical and physical processes controlling aerosol and gas species from emission sources to the most remote regions of the atmosphere. Collection of aerosol onto filters to be analyzed off-line by chromatography is one widely used technique, especially for analysis of refractory salts (e.g., NaCl, NaNO₃, MgSO₄, etc.). These filters are typically also used for the analysis of the inorganic species that are generally dominant in the submicron mode (NH₄NO₃, (NH₄)₂SO₄, HNH₄SO₄, and H₂SO₄). These filters, during the recent remote NASA ATom studies, consistently show ammonium-to-sulfate molar ratios of 0.5 (equivalent to NH₄HSO₄) or higher. However, the AMS and a single particle mass spectrometer (PALMS) indicate often more acidic aerosols. The implied differences in pH are large, which would result in radically different rates for chemical uptake and reactions.

Here, we use observations collected onboard the NASA DC-8 during recent NASA campaigns along with laboratory results, theoretical calculations, and measurements of NH₃ in the cabin and in the area of filter preparation to investigate potential bias from the uptake of NH₃ to acidic aerosol on filters. Using literature uptake coefficients for NH₃(g) onto sulfuric acid particles with a measured range of aircraft cabin NH₃ concentrations and typical measured sulfate concentrations and mode diameters, we show that the substantial uptake of NH₃ onto H₂SO₄ particles on the filters to form NH₄HSO₄ can occur in 20 s or less. Further, we are validating these calculations through additional experiments. We plan to collect filters of sulfuric acid, NH₄HSO₄, and (NH₄)₂SO₄ particles in a controlled environment with comparable levels of NH₃ and other parameters as observed on the DC-8 to investigate the chemical change of these sulfate particles, both during the initial collection to the filter and the processing of the filter. These results will provide further insights on the extent to which particles on the filters may be partially taking up cabin NH₃. These results will allow improved interpretation of ammonium concentrations, acidity, and pH from filter and real-time measurements, especially over remote regions.