Effects of Isoprene on Biogenic New Particle Formation from Pre-Nucleation to CCN Sizes

LEE TISZENKEL, Asadullah Shoaib, Yue Zhao, Yue Zhang, Shanhu Lee, University of Alabama Huntsville

     Abstract Number: 393
     Working Group: Aerosol Chemistry

Abstract
New particle formation (NPF) from biogenic precursors is responsible for a large portion of the sub-micron particle loadings observed in the atmosphere. Oxidation of monoterpenes and their subsequent nucleation and growth are critical first steps to this process. However, the impact on nucleation from the interactions between monoterpenes and isoprene remains unclear. We present results from new particle formation experiments in the Tandem Aerosol Nucleation and Growth Environment Tube (TANGENT) using α-pinene and isoprene oxidized in the presence of ozone and hydroxyl radicals. Particle size distributions ranging from pre-nucleation clusters to CCN-active diameters were measured with a PSM (particle size magnifier) and nano-SMPS. Gas- and aerosol-phase HOMs (highly-oxidized molecules) were measured with CI-APi-TOF (chemical ionization-atmospheric pressure-time of flight mass spectrometer) attached to FIGAERO (a filter inlet for gas and aerosol sampling). Aerosol phase chemical composition measured with CI-APi-TOF was also compared with off-line LC-ESI-Orbitrap mass spectrometer measurements to provide insights into chemical speciation of newly formed particles. Background ammonia and amine concentrations inside the TANGENT were measured with an ethanol-CIMS (chemical ionization mass spectrometer). In this presentation, we will discuss the impact of isoprene on nucleation and growth rates. A direct relationship between HOMs and nucleation and growth rates was observed. Isoprene was found to dramatically increase formation rates of pre-nucleation clusters. Isoprene-only nucleation was observed with size distributions sharply dropping after 3 nm size bins. In the presence of both α-pinene and isoprene, suppression of nucleation rates was observed with the extent of the suppression proportional to the isoprene:α-pinene ratio. Isoprene did not have an effect on observed growth rates. These results serve to explain the presence of sub-2nm clusters combined with the lack of new particle formation events observed in field experiments conducted in isoprene-rich forests of the Southeastern United States and the Amazon.