A new method to quantify particulate sodium and potassium salts (nitrate, chloride, and sulfate) by thermal desorption aerosol mass spectrometry

The reaction of sea salt (or biomass burning) particles with sulfuric acid and nitric acid leads to the displacement of chloride relative to sodium (or potassium). We have developed a new particle mass spectrometer to quantify non-refractory and refractory sulfate aerosols (referred to as refractory aerosol thermal desorption mass spectrometer: rTDMS). The combination of a graphite particle collector and a carbon dioxide laser enables high desorption temperature (blackbody equivalent radiation temperature of up to 930 ∘C). Ion signals originating from evolved gas molecules are detected by a quadrupole mass spectrometer. Here we propose a new method to quantify the mass concentrations of sodium nitrate (NaNO3: SN), sodium chloride (NaCl: SC), sodium sulfate (Na2SO4: SS), potassium nitrate (KNO3: PN), potassium chloride (KCl: PC), and potassium sulfate (K2SO4: PS) particles by using the rTDMS. Laboratory experiments were performed to test the sensitivities of the rTDMS to various types of particles. We measured ion signals originating from single-component particles for each compound and found a good linearity (r2>0.8) between the major ion signals and mass loadings. We also measured ion signals originating from internally mixed SN + SC + SS (or PN + PC + PS) particles and found that the temporal profiles of ion signals at m/z 23 (Na+) (or 39; K+) were characterized by three sequential peaks associated with the evolution of the desorption temperature. We tested potential interferences in the quantification of sea salt particles under real-world conditions by artificially generating “modified” sea salt particles from a mixture of diluted seawater and SN (or SS) solution. The SS/SC ratios estimated from the ion signals at m/z 23, 36 (H35Cl+), and 48 (SO+) agreed well with those predicted from the solution concentrations to within ∼10 %. The SN/SC ratios estimated from the ion signals at m/z 30 (NO+) and 36 also agreed with those predicted from the solution concentrations to within ∼15 %, whereas the SN/SC ratios estimated from m/z 23 were significantly lower than the predicted values. Based on these experimental results, the applicability of the rTDMS to ambient measurements of sea salt particles is discussed.