This study experimentally explores how ammonia (NH3), methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA) affect the chemical formation mechanisms of electrically neutral clusters that contain two sulfuric acid molecules (dimers). Dimers may also contain undetectable compounds, such as water or bases, that evaporate upon ionization and sampling. Measurements were conducted using a glass flow reactor which contained a steady flow of humidified nitrogen with sulfuric acid concentrations of 107 to 109 cm-3. A known molar flow rate of a basic gas was injected into the flow reactor. The University of Minnesota Cluster Chemical Ionization Mass Spectrometer was used to measure the resulting sulfuric acid vapor and cluster concentrations. It was found that, for a given concentration of sulfuric acid vapor, the dimer concentration increases with increasing concentration of the basic gas, eventually reaching a plateau. The base concentrations at which the dimer concentrations saturate suggest NH3 < MA <TMA ≲ DMA in forming stabilized sulfuric acid dimers. Two heuristic models for cluster formation by acid-base reactions are developed to interpret the data. The models provide ranges of evaporation rate constants that are consistent with observations and leads to an analytic expression for nucleation rates that is consistent with atmospheric observations.
Bibliographical noteFunding Information:
The National Science Foundation awards AGS1068201 and AGS0943721 provided funds for measurements and model development. C.N.J. acknowledges support from NSF GRFP award 00006595 and the ARCS Foundation. P.H.M. acknowledges support from the Guggenheim Foundation. The authors would like to thank Parviz Taheripour and Kristine Volz for collecting particle size distribution data with the DEG SMPS. The data from this study are stored on the University of Minnesota sponsored Google Drive and can be found at https://drive.google.com/file/ d/0B5NEDoHzu0KsekFJUlI2VGY1TXc/ edit?usp=sharing.