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Understanding the biogenic species responsible for atmospheric new particle growth

 

Authors

Paul Martin Winkler — National Center for Atmospheric Research (NCAR)
John Ortega — National Center for Atmospheric Research
Hans R. Friedli — University Corporation for Atmospheric Research
James Smith — University of California, Irvine

Category

Aerosol Properties

Description

Typical mass spectrum of positive ions obtained from 80-nm particles formed from the oxidation of a-pinene in the NCAR biogenic aerosol chamber. As can be seen a series of peaks separated by m/z, differences of typically 14 amu are observed. While many species as well as a similar pattern were found from the ambient measurements, it is likely that compounds from other precursor gases contribute to the ambient signal as well.

The chemical composition of secondary organic sub-50 nm diameter particles (nanoparticles) is among the key measurements required for understanding the processes responsible for atmospheric nucleation and subsequent growth. While aerosol mass spectrometry has typically been restricted to particle sizes greater than 50 nm due to sampling challenges, few techniques exist for investigating particle composition in a size range close to where nucleation and growth by organic vapours takes place. With the newly developed high-resolution time-of-flight (HTOF) Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS, e.g., Voisin et al. 2003), we now can directly measure chemical compounds of freshly nucleated particles as small as 10 nm.

In this study we investigated biogenic nanoparticles in the atmosphere and laboratory. During summer 2011 we participated in the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen - Rocky Mountain Biogenic Aerosol Study (BEACHON-RoMBAS) field campaign at the Manitou Forest Observatory (MFO) near Woodland Park, Colorado. The site is located in a ponderosa pine-dominated forest at an elevation of roughly 2400 meters above sea level. The remote location relatively far from direct emission sources typically provides clean conditions with substantial monoterpene and sesquiterpene concentrations. New particle formation events starting around noon were observed frequently, and particle diameters around 10–15 nm were found at the low end of the size distribution. Correspondingly, we focused on 20-nm particles as the smallest size for the HTOF-TDCIMS measurements. At this size ambient nanoparticles contained detectable levels of inorganic species such as sulphate, nitrate, and ammonium. In addition, several organic substances were found in significant amounts. The high mass resolution of the HTOF-TDCIMS allows for the identification of the molecular formulae of detected ions.

In order to be able to attribute the organic signals observed at MFO to corresponding precursor gases, we conducted laboratory SOA formation studies using both the NCAR biogenic aerosol chamber as well as a flow tube. Different kinds of monoterpenes and sesquiterpenes representative for MFO conditions were oxidized by ozone under dark conditions. Little difference was found between the particle compositions in the chamber and flow tube experiments, respectively, although reaction rates varied considerably. Current work is focused on identifying these ions and ultimately the precursors and mechanisms responsible for the observed growth.