Abstract
Abstract. This study presents measurements of size and time-resolved particle diameter growth rates for freshly nucleated particles down to 1 nm geometric diameter. Novel data analysis methods were developed, de-coupling for the first time the size and time-dependence of particle growth rates by fitting the aerosol general dynamic equation to size distributions obtained at an instant in time. Size distributions of freshly nucleated total aerosol (neutral and charged) were measured during two intensive measurement campaigns in different environments (Atlanta, GA and Boulder, CO) using a recently developed electrical mobility spectrometer with a diethylene glycol-based ultrafine condensation particle counter as the particle detector. One new particle formation (NPF) event from each campaign was analyzed in detail. At a given instant in time during the NPF event, size-resolved growth rates were obtained directly from measured size distributions and were found to increase approximately linearly with particle size from ~1 to 3 nm geometric diameter, increasing from 5.5 ± 0.8 to 7.6 ± 0.6 nm h−1 in Atlanta (13:00) and from 5.6 ± 2 to 27 ± 5 nm h−1 in Boulder (13:00). The resulting growth rate enhancement Γ, defined as the ratio of the observed growth rate to the growth rate due to the condensation of sulfuric acid only, was found to increase approximately linearly with size from ~1 to 3 nm geometric diameter. For the presented NPF events, values for Γ had lower limits that approached ~1 at 1.2 nm geometric diameter in Atlanta and ~3 at 0.8 nm geometric diameter in Boulder, and had upper limits that reached 8.3 at 4.1 nm geometric diameter in Atlanta and 25 at 2.7 nm geometric diameter in Boulder. Nucleated particle survival probability calculations comparing the effects of constant and size-dependent growth indicate that neglecting the strong dependence of growth rate on size from 1 to 3 nm observed in this study could lead to a significant overestimation of CCN survival probability.
Highlights
Atmospheric aerosols influence climate and climate change on local to global scales by affecting the atmospheric radiation balance directly through scattering and absorbing incoming solar radiation and indirectly as cloud condensation nuclei (CCN) (Charlson et al, 1992)
Analyzed nucleation events were acquired during two intensive measurement campaigns: the nucleation and cloud condensation nuclei (NCCN) study that was carried out in Atlanta, Georgia, during July and August 2009 (Jiang et al, 2011b), and a new particle formation and growth study carried out at the Foothill Laboratory of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, during August and September 2010
In the NCAR study, a DEG scanning mobility particle spectrometer (SMPS) was deployed and was operated identically to the system used in the NCCN study, except for the DEG UCPC, which was operated at a higher aerosol flow-rate and saturator temperature for increased particle detection efficiency (Kuang et al, 2012)
Summary
Atmospheric aerosols influence climate and climate change on local to global scales by affecting the atmospheric radiation balance directly through scattering and absorbing incoming solar radiation and indirectly as cloud condensation nuclei (CCN) (Charlson et al, 1992). Atmospheric measurement and modeling studies have shown that new particle formation (NPF), through photochemical reactions of gasphase precursors, greatly increases the number concentration of atmospheric aerosols, and is often followed by rapid growth of the nucleated aerosol to a CCN-active size, significantly increasing the CCN population (Lihavainen et al, 2003; Kerminen et al, 2005; Spracklen et al, 2008; Kuang et al, 2009) This rapid growth, often many times that of the growth assuming the condensation of sulfuric acid alone (Weber et al, 1997), is neither well understood nor represented in regional and chemical transport models (Pierce and Adams, 2007; Wang and Penner, 2009; Spracklen et al, 2010). Methods for obtaining size and time-resolved growth rates are presented, along with insights into the processes of nucleation and growth provided by these measurements
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