Chemical identification of new particle formation and growth precursors through positive matrix factorization of ambient ion measurements

Abstract

Abstract. In the lower troposphere, rapid collisions between ions and trace gases result in the transfer of positive charge to the highest proton affinity species and negative charge to the lowest proton affinity species. Measurements of the chemical composition of ambient ions thus provide direct insight into the most acidic and basic trace gases and their ion-molecule clusters — compounds thought to be important for new particle formation and growth. We deployed an atmospheric pressure interface time-of-flight mass spectrometer (APi-ToF) to measure ambient ion chemical composition during the 2016 Holistic Interaction of Shallow Clouds, Aerosols, and Land Ecosystems (HI-SCALE) campaign at the United States Department of Energy Atmospheric Radiation Measurement facility in the Southern Great Plains, an agricultural region. Cations and anions were measured for alternating periods of ~24 hours over one month. We use binned positive matrix factorization (binPMF) and scaled Kendrick mass defect (SKMD) analysis to obtain information about the chemical formulas and temporal variation in ionic composition without the need for averaging over a long timescale or a priori high-resolution peak fitting. Negative ions consist of strong acids including sulfuric and nitric acid, organosulfates, and clusters of NO3- with highly oxidized molecules (HOMs) derived from monoterpene and sesquiterpene oxidation. Organic nitrates derived from sesquiterpenes account for most of the HOM signal. Combined with the diel profiles and back trajectory analysis, these results suggest that nitrate radical chemistry is active at this site. Sesquiterpene oxidation products likely contribute to particle growth at the SGP site. The positive ions consist of bases including alkylpyridiniums and amines and a series of high mass species. Nearly all the positive ions contained only one nitrogen atom and in general support ammonia and amines as being the dominant bases that could participate in new particle formation. Overall, this work demonstrates how APi-ToF measurements combined with binPMF analysis can provide insight into the temporal evolution of compounds important for new particle formation and growth.