Quantification of carbonate carbon in aerosol filter samples using a modified thermal/optical carbon analyzer (M-TOCA)

Abstract

Measurement of carbon dioxide (CO₂) gas evolved from acidification is a method to quantify carbonate carbon (CC) in aerosols collected on quartz fiber-filters. This paper describes the installation of an add-on device in a DRI Model 2001 Thermal Optical reflectance (TOR)/Thermal Optical Transmittance (TOT) Carbon Analyzer (M-TOCA) to facilitate a direct CC measurement. In each run, a maximum of 20 filter punches (each of 0.5 cm²) were acidified with 1 mL of 20% v/v phosphoric (V) acid in a vial under a 100% helium gas environment. The CO₂ evolved was reduced to methane (CH₄) and detected by a flame ionization detector (FID). The optimum reaction kinetics were obtained under an operational temperature of 40 °C and ultrasonic agitation. Method precisions were ±3.5% on average for carbonate standards ranging from 3.0 to 60.0 μg and ±3.8% on average for ambient samples in masses ranging from 0.30 to 56.0 μg respectively. Method accuracy was on average 91.9%, ranging from 81.4 to 102.1%. Minimum detection limit (MDL) of the M-TOCA method was 0.048 μg cm⁻², corresponding to an ambient concentration of 0.098 μg m⁻³ for a sampled volume of air of 7.2 m³. The MDL is >22 times lower than the value obtained using the novel method with a regular TOCA. Comparison studies on standards and ambient samples have demonstrated that the two methods do not yield systematic differences in concentrations of the carbonate. The lower MDL value provided by the M-TOCA allows a simple, precise and accurate measurement for ambient samples having a low CC concentration.