The Detection Efficiency of Sulfuric Acid Using a Nitrate-CIMS: Impacts of Base Molecules
JUN ZHENG, Nanjing University of Information Science and Technology
Abstract Number: 136
Working Group: Aerosol Chemistry
Abstract
Nitrate anion (NO3-) based chemical ionization mass spectrometry (i.e., NO3-CIMS) has been extensively employed in atmospheric sulfuric acid (SA, H2SO4) measurements. The ion chemistry is highly sensitive and selective because H2SO4 is one of the select few molecules in the atmosphere that is stronger than nitric acid in gas-phase acidity (GA = 1295 kJ/mol for H2SO4 and 1380 kJ/mol for HNO3). Therefore, the SA - NO3- reaction is considered collision-limited given the relatively significant Gibbs free energy change during the ionization process. However, these estimations were based on the thermodynamic parameters of pure H2SO4 molecules. Under typical ambient conditions, H2SO4 is believed to be present in clusters associated with other ligands, such as water, ammonia, and other basic gases. Thermodynamic calculations indicated that a significant portion of H2SO4 is in the form of SA-amine clusters within amine-rich environments, particularly during nucleation processes. The fact that SA-amine clusters were scarcely detected during nucleation processes by a NO3-CIMS was due to the evaporation of ligands from the SA-amine clusters caused by the large reaction enthalpy released from the nitrate ionization process. Here, using a water cluster-CIMS and proton-transfer-reaction (PTR) based ion chemistry, we demonstrated that SA-amine clusters were indeed present during nucleation processes. The detection efficiency of SA by a NO3-CIMS would increase when trace amounts of amines (such as methylamine, dimethylamine, and trimethylamine) were present due to the formation of SA-amine clusters. These findings indicated that ambient SA concentrations detected by a NO3-CIMS were very likely underestimated because the NO3-CIMS was calibrated under an amine-free condition. The underlying causes and implications of these findings were also further investigated.