Development of an analytical methodology for obtaining quantitative mass concentrations from LAAP-ToF-MS measurements

Laser ablation aerosol particle-time of flight mass spectrometer (LAAP-ToF-MS) measures the size number of particles, and chemical composition of individual particles in real-time. LAAP-ToF-MS measurements of chemical composition are difficult to quantify, mostly because the instrument sensitivities to various chemical species in the multicomponent atmospheric aerosol particles are unknown. In this study, we investigate a field-based approach for quantitative measurements of ammonium, nitrate, sulfate, OC, and EC, in size-segregated atmospheric aerosols, by LAAP-ToF-MS using concurrent measurements from high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), and multi-angle absorption photometer (MAAP). An optical particle counter (OPC) and a high-resolution nanoparticle sizer (scanning mobility particle sizer, or SMPS), were used to measure the particle size distributions of the particles in order to correct the number concentrations. The intercomparison reveals that the degree of agreement of the mass concentrations of each compound measured with LAAP-ToF-MS and HR-ToF-AMS/MAAP increases in the following order NH₄⁺ < SO₄^2- < NO₃^- < EC < OC < Cl^- with r² values in the range of 0.4–0.95 and linear regression slopes ranging between 0.62 and 1.2. The factors that affect the mass concentrations measured by LAAP-ToF-MS are also discussed in details. Yet, the matrix effect remains one of the strongest limiting factor to achieve an absolute quantification of the aerosol chemical composition. In the future we suggest the development of a methodology based on the calculation of the response factors generated by different types of particles, which could possibly resolve certain difficulties associated with the matrix effect.