Article pubs.acs.org/est
Quantitative Assessment of the Sulfuric Acid Contribution to New Particle Growth Bryan R. Bzdek,† Christopher A. Zordan,† M. Ross Pennington,† George W. Luther, III,‡ and Murray V. Johnston†,* †
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware, United States College of Earth, Ocean, and Environment, University of Delaware, Lewes, Delaware, United States
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S Supporting Information *
ABSTRACT: The Nano Aerosol Mass Spectrometer (NAMS) was deployed to rural/coastal and urban sites to measure the composition of 20−25 nm diameter nanoparticles during new particle formation (NPF). NAMS provides a quantitative measure of the elemental composition of individual, size-selected nanoparticles. In both environments, particles analyzed during NPF were found to be enhanced in elements associated with inorganic species (nitrogen, sulfur) relative to that associated with organic species (carbon). A molecular apportionment algorithm was applied to the elemental data in order to place the elemental composition into a molecular context. These measurements show that sulfate constitutes a substantial fraction of total particle mass in both environments. The contribution of sulfuric acid to new particle growth was quantitatively determined and the gas-phase sulfuric acid concentration required to incorporate the measured sulfate fraction was calculated. The calculated values were compared to those calculated by a sulfuric acid proxy that considers solar radiation and SO2 levels. The two values agree within experimental uncertainty. Sulfate accounts for 29−46% of the total mass growth of particles. Other species contributing to growth include ammonium, nitrate, and organics. For each location, the relative amounts of these species do not change significantly with growth rate. However, for the coastal location, sulfate contribution increases with increasing temperature whereas nitrate contribution decreases with increasing temperature.
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derived component (bisulfate).26−28 Additionally, ambient measurements by the thermal desorption chemical ionization mass spectrometer (TDCIMS), which allows molecular characterization of semivolatile components in collected nanoparticulate samples down to 6 nm in diameter, have indicated the presence of sulfate or bisulfate in newly formed particles.12,13,29 Much effort has focused on quantifying the contribution of sulfuric acid to particle growth.30−35 This work has generally focused on relating gas-phase sulfuric acid concentrations to measured particle growth rates during NPF events. However, to validate such an approach, size-resolved, quantitative nanoparticle chemical composition measurements are required. The Nano Aerosol Mass Spectrometer (NAMS) is capable of single-particle analysis from 7 to 30 nm in diameter and provides a quantitative measure of nanoparticle elemental composition.36,37 NAMS has been deployed to urban,38−40 suburban,41 and rural/coastal42 environments. The present work employs NAMS to quantitatively measure the nanoparticle elemental composition during NPF events. Application
INTRODUCTION Significant uncertainty exists regarding the climate effects of aerosols,1 which influence precipitation patterns and cloud albedo.2,3 New particle formation (NPF) may be an important source of cloud condensation nuclei (CCN).4,5 In order to improve predictions of ambient CCN levels under varying conditions, the contribution of NPF must be determined. NPF is characterized by the nucleation of particles in the low nanometer size range followed by rapid growth to 50−100 nm in diameter.6,7 This growth process requires up to a one million-fold increase in particle mass and the exact mechanisms are unknown, especially during the early steps of particle growth (
