Particle mass yield in secondary organic aerosol formed by the dark ozonolysis of α-pinene

The yield of particle mass in secondary organic aerosol (SOA) formed by dark ozonolysis was measured for 0.3–22.8 ppbv of reacted α-pinene. Most experiments were conducted using a continuous-flow chamber, allowing nearly constant SOA concentration and chemical composition for several days. For comparison, some experiments were also conducted in batch mode. Reaction conditions were 25°C, 40% RH, dry (NH<sub>4</sub>)SO<sub>4</sub> seed particles, and excess 1-butanol. The organic particle loading was independently measured by an aerosol mass spectrometer and a scanning mobility particle sizer, and the two measurements agreed well. The observations showed that SOA formation occurred for even the lowest reacted α-pinene concentration of 0.3 ppbv. The particle mass yield was 0.09 at 0.15 μg m<sup>−3</sup>, increasing to 0.27 at 40 μg m<sup>−3</sup>. Compared to some results reported in the literature, the yields were 80 to 100% larger for loadings above 2 μg m<sup>−3</sup>. At lower loadings, the yields had an offset of approximately +0.07 from those reported in the literature. To as low as 0.15 μm<sup>−3</sup>, the yield curve had no inflection point toward null yield, implying the formation of one or several products having vapor pressures below this value. These observations of increased yields, especially for low loadings, are potentially important for accurate prediction by chemical transport models of organic particle concentrations in the ambient atmosphere

Cloud droplet activation of mixed organic-sulfate particles produced by the photooxidation of isoprene

The cloud condensation nuclei (CCN) properties of ammonium sulfate particles mixed with organic material condensed during the hydroxyl-radical-initiated photooxidation of isoprene (C<sub>5</sub>H<sub>8</sub>) were investigated in the continuous-flow Harvard Environmental Chamber. CCN activation curves were measured for organic particle mass concentrations of 0.5 to 10.0 μg m<sup>−3</sup>, NO<sub>x</sub> concentrations from under 0.4 ppbv up to 38 ppbv, particle mobility diameters from 70 to 150 nm, and thermodenuder temperatures from 25 to 100 °C. At 25 °C, the observed CCN activation curves were accurately described by a Köhler model having two internally mixed components, namely ammonium sulfate and secondary organic material. The modeled physicochemical parameters of the organic material were equivalent to an effective hygroscopicity parameter κ<sub>ORG</sub> of 0.10±0.03, regardless of the C<sub>5</sub>H<sub>8</sub>:NO<sub>x</sub> concentration ratio for the span of >200:0.4 to 50:38 (ppbv:ppbv). The volatilization curves (i.e., plots of the residual organic volume fraction against temperature) were also similar for the span of investigated C<sub>5</sub>H<sub>8</sub>:NO<sub>x</sub> ratios, suggesting a broad similarity of particle chemical composition. This suggestion was supported by limited variance at 25 °C among the particle mass spectra. For example, the signal intensity at <i>m/z</i> 44 (which can result from the fragmentation of oxidized molecules believed to affect hygroscopicity and CCN properties) varied weakly from 6 to 9% across the range of investigated conditions. In contradistinction to the results for 25 °C, conditioning up to 100 °C in the thermodenuder significantly reduced CCN activity. The altered CCN activity might be explained by chemical reactions (e.g., decomposition or oligomerization) of the secondary organic material at elevated temperatures. The study's results at 25 °C, in conjunction with the results of other chamber and field studies for a diverse range of conditions, suggest that a value of 0.10±0.05 for κ<sub>ORG</sub> is representative of both anthropogenic and biogenic secondary organic material. This finding supports the use of κ<sub>ORG</sub> as a simplified yet accurate general parameter to represent the CCN activation of secondary organic material in large-scale atmospheric and climate models

Formation and occurrence of dimer esters of pinene oxidation products in atmospheric aerosols

The formation of carboxylic acids and dimer esters from α-pinene oxidation was investigated in a smog chamber and in ambient aerosol samples collected during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX). Chamber experiments of α-pinene ozonolysis in dry air and at low NOx concentrations demonstrated formation of two dimer esters, pinyl-diaterpenyl (MW 358) and pinonyl-pinyl dimer ester (MW 368), under both low- and high-temperature conditions. Concentration levels of the pinyl-diaterpenyl dimer ester were lower than the assumed first-generation oxidation products cis-pinic and terpenylic acids, but similar to the second-generation oxidation products 3-methyl-1,2,3-butane tricarboxylic acid (MBTCA) and diaterpenylic acid acetate (DTAA). Dimer esters were observed within the first 30 min, indicating rapid production simultaneous to their structural precursors. However, the sampling time resolution precluded conclusive evidence regarding formation from gas- or particle-phase processes. CCN activities of the particles formed in the smog chamber displayed a modest variation during the course of experiments, with κ values in the range 0.06–0.09 (derived at a supersaturation of 0.19%). The pinyl-diaterpenyl dimer ester was also observed in ambient aerosol samples collected above a ponderosa pine forest in the Sierra Nevada Mountains of California during two seasonally distinct field campaigns in September 2007 and July 2009. The pinonyl-pinyl ester was observed for the first time in ambient air during the 2009 campaign, and although present at much lower concentrations, it was correlated with the abundance of the pinyl-diaterpenyl ester, suggesting similarities in their formation. The maximum concentration of the pinyl-diaterpenyl ester was almost 10 times higher during the warmer 2009 campaign relative to 2007, while the concentration of cis-pinic acid was approximately the same during both periods, and lack of correlation with levels of cis-pinic and terpenylic acids for both campaigns indicate that the formation of the pinyl-diaterpenyl ester was not controlled by their ambient abundance. In 2009 the concentration of the pinyl-diaterpenyl ester was well correlated with the concentration of DTAA, a supposed precursor of diaterpenylic acid, suggesting that the formation of pinyl-diaterpenyl dimer was closely related to DTAA. Generally, the pinyl-diaterpenyl ester was found at higher concentrations under higher temperature conditions, both in the smog-chamber study and in ambient air aerosol samples, and exhibited much higher conc

A review of the anthropogenic influence on biogenic secondary organic aerosol

Because of the climate and air quality effects of organic aerosol, it is important to quantify the influence of anthropogenic emissions on the aerosol burden, both globally and regionally, and both in terms of mass and number. Methods exist with which the fractions of organic aerosol resulting directly from anthropogenic and biogenic processes can be estimated. However, anthropogenic emissions can also lead to an enhancement in secondary organic aerosol formation from naturally emitted precursors. We term this enhanced biogenic secondary organic aerosol (eBSOA). Here, we review the mechanisms through which such an effect may occur in the atmosphere and describe a work flow via which it may be quantified, using existing measurement techniques. An examination of published data reveals support for the existence of the enhancement effect

The 1-by-3 Tandem Differential Mobility Analyzer for Measurement of the Irreversibility of the Hygroscopic Growth Factor

A new instrument, namely the 1 × 3 tandem differential mobility analyzer (1 × 3-TDMA), has been developed. Its primary measurement is the irreversibility of the hygroscopic growth factor of aerosol particles. The instrument uses the hysteresis of phase transitions to infer the solid or aqueous state of the particles. A first DMA passes particles of a selected electric mobility at relative humidity RH0. Exiting this DMA, the particles are split into three separate flows. The first flow is exposed to RH0 → (RH0+ δ) → RH0 in a deliquescence test before passing through a second DMA that is set to the same electric mobility as the first DMA. The second flow passes directly to a third DMA without change in RH, thereby serving as a reference arm. This DMA is also set to the same electric mobility as the first DMA. The transmission ratio of the 1 × 3-TDMA is defined as the particle concentration passing the deliquescence test divided by that passing through the reference arm. The transmission ratio is unity in the absence of deliquescence and zero when a phase transition occurs, at least for ideal instrument performance in application to a test aerosol of fully deliquesceable particles. For the third flow passing out of the first DMA, an efflorescence test is run by using the RH profile of RH0 → (RH0-δ) → RH0 before passing through a fourth DMA. A full data set for the 1 × 3-TDMA is obtained by scanning RH0, typically from 20 to 85%. In the present paper, the 1 × 3-TDMA instrument is described, and laboratory data are presented for the phase transitions of externally mixed aerosols of aqueous and solid sodium chloride particles, aqueous and solid ammonium sulfate particles, and their mixtures, as well as a mixture of aqueous and solid sea salt particles. The observed transmission ratio is compared to a model analysis. The intent behind the development of this instrument is to deploy it for field measurements and use observations of the irreversibility of the growth factors of atmospheric particles as markers of their physical state.Earth and Planetary SciencesEngineering and Applied Science

Increased cloud activation potential of secondary organic aerosol for atmospheric mass loadings

The effect of organic particle mass loading from 1 to ≥100 μg m<sup>−3</sup> on the cloud condensation nuclei (CCN) properties of mixed organic-sulfate particles was investigated in the Harvard Environmental Chamber. Mixed particles were produced by the condensation of organic molecules onto ammonium sulfate particles during the dark ozonolysis of α-pinene. A continuous-flow mode of the chamber provided stable conditions over long time periods, allowing for signal integration and hence increased measurement precision at low organic mass loadings representative of atmospheric conditions. CCN activity was measured at eight mass loadings for 80- and 100-nm particles grown on 50-nm sulfate seeds. A two-component (organic/sulfate) Köhler model, which included the particle heterogeneity arising from DMA size selection and from organic volume fraction for the selected 80- and 100-nm particles, was used to predict CCN activity. For organic mass loadings of 2.9 μg m<sup>−3</sup> and greater, the observed activation curves were well predicted using a single set of physicochemical parameters for the organic component. For mass loadings of 1.74 μg m<sup>−3</sup> and less, the observed CCN activity increased beyond predicted values using the same parameters, implying changed physicochemical properties of the organic component. A sensitivity analysis suggests that a drop in surface tension must be invoked to explain quantitatively the CCN observations at low SOA particle mass loadings. Other factors, such as decreased molecular weight, increased density, or increased van't Hoff factor, can contribute to the explanation but are quantitatively insufficient as the full explanation

A review of the anthropogenic influence on biogenic secondary organic aerosol

Because of the climate and air quality effects of organic aerosol, it is important to quantify the influence of anthropogenic emissions on the aerosol burden, both globally and regionally, and both in terms of mass and number. Methods exist with which the fractions of organic aerosol resulting directly from anthropogenic and biogenic processes can be estimated. However, anthropogenic emissions can also lead to an enhancement in secondary organic aerosol formation from naturally emitted precursors. We term this enhanced biogenic secondary organic aerosol (eBSOA). Here, we review the mechanisms through which such an effect may occur in the atmosphere and describe a work flow via which it may be quantified, using existing measurement techniques. An examination of published data reveals support for the existence of the enhancement effect. © 2011 Author(s)