Heterogeneous reaction and condensation growth observation of NaCl/ dicarboxylic acids nanoparticles by aerosol time-of-flight mass spectrometer with water-based particle size amplifier

摘要

Significant contribution of ultrafine particles in the atmosphere to the cloud condensation nuclei and precipitation has been evidenced. The correlation between the condensation growth and chemical composition of particles is essential in the atmospheric studies. To reduce the lower detection limit of ATOFMS (currently at 200 nm), a water-based particle size amplifier, consisted of a tubing inner-lined with an expanded polytetrafluoroethylene layer and temperature-controlled in three sections, was designed, and its amplification performance was evaluated. With the use of the particle size amplifier, the peaks of Na and Cl in the mass spectra of 50 nm NaCl particles were obviously observed. Further, 50 nm NaCl/Dicarboxylic Acid (DA) particles were generated and characterized by ATOFMS with the amplifier and the sizes of ultrafine particles after the amplifier was measured by an Aerodynamic Particle Spectrometer (APS). It is found that the Cl/Na ratio in 50 nm NaCl + DA particles was negatively correlated with the DA concentration and positively correlated with the acidity coefficient of the DAs, i.e., the high DA mass fraction in particles and low the acidity coefficient of DAs results in strong heterogeneous reaction in the generation of these particles. The amplified sizes of NaCl + DA nanoparticles was also greatly influenced by the degree of the reaction involved. The Cl/Na ratio of NaCl + DA particles was found to have positive correlation with the amplified size of particles.