Interferences on Aerosol Acidity Quantification due to Gas-phase Ammonia Uptake onto Acidic Sulfate Filter Samples

Abstract

Abstract. Measurements of the mass concentration and chemical speciation of aerosols are important to investigate their chemical and physical processing from near emission sources to the most remote regions of the atmosphere. A common method to analyze aerosols is to collect them onto filters and to analyze filters off-line; however, biases in some chemical components are possible due to changes in the accumulated particles during the handling of the samples. Any biases would impact the measured chemical composition, 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 consistent with the AMS in showing that the aerosol has lower ammonium-to-anion ratios than obtained by the filters. Using a 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 (typical filter handling time in the aircraft) for typical aerosol volume distributions. Measurements of gas-phase ammonia inside the cabin of the DC-8 show ammonia mixing ratios of 45 ± 20 ppbv, consistent with mixing ratios observed in other indoor environments. This analysis enables guidelines for filter handling to reduce ammonia uptake. Finally, a more meaningful limit-of-detection for filters that either do not have an ammonia scrubber and/or are handled in the presence of human emissions is ∼0.2 μg m−3 ammonium, which is substantially higher than the limit-of-detection of the ion chromatography.

Direct measurement of the pH of aerosol particles using carbon quantum dots

“…Properties of the bulk extraction supernatant are then assessed and the pH of the original lter sample is back calculated. 15,[17][18][19] Signicant error can be associated with this back calculation arising from fundamental differences between extract and the particle liquid phase. Proton concentrations do not necessarily scale proportionally with dilution due to gas-phase buffering and the nonconservative extraction of species participating in weak acid equilibria.…”

“…Proton concentrations do not necessarily scale proportionally with dilution due to gas-phase buffering and the nonconservative extraction of species participating in weak acid equilibria. 15,16,[18][19][20][21][22] Further, determining the liquid water content of the original lter sample is not straightforward, so predicting aerosol proton concentration can be a challenge. 1,16 The particle-into-liquid sampler (PILS) is a notable improvement on the lter extraction method where humidied aerosol particles are collected through impaction into a liquid ow for immediate analysis.…”