Role of OH-Initiated Oxidation of Isoprene in Aging of Combustion

Article pubs.acs.org/est

Role of OH-Initiated Oxidation of Isoprene in Aging of Combustion Soot Alexei F. Khalizov,† Yun Lin,† Chong Qiu,‡ Song Guo,† Don Collins,† and Renyi Zhang†,‡,* †

Department of Atmospheric Sciences, Texas A&M University, College Station, Texas, 77843, United States Department of Chemistry, Texas A&M University, College Station, Texas, 77843, United States

‡

S Supporting Information *

ABSTRACT: We have investigated the contribution of OHinitiated oxidation of isoprene to the atmospheric aging of combustion soot. The experiments were conducted in a fluoropolymer chamber on size-classified soot aerosols in the presence of isoprene, photolytically generated OH, and nitrogen oxides. The evolution in the mixing state of soot was monitored from simultaneous measurements of the particle size and mass, which were used to calculate the particle effective density, dynamic shape factor, mass fractal dimension, and coating thickness. When soot particles age, the increase in mass is accompanied by a decrease in particle mobility diameter and an increase in effective density. Coating material not only fills in void spaces, but also causes partial restructuring of fractal soot aggregates. For thinly coated aggregates, the single scattering albedo increases weakly because of the decreased light absorption and practically unchanged scattering. Upon humidification, coated particles absorb water, leading to an additional compaction. Aging transforms initially hydrophobic soot particles into efficient cloud condensation nuclei at a rate that increases in the presence of nitrogen oxides. Our results suggest that ubiquitous biogenic isoprene plays an important role in aging of anthropogenic soot, shortening its atmospheric lifetime and considerably altering its impacts on air quality and climate. surfaces,15−18 heterogeneous reactions,19,20 and formation of coatings on soot particles through gas-to-particle conversion of low-volatility vapors from dark and photochemical oxidation of gas-phase pollutants.21−24 Aging may significantly alter both the chemical reactivity16,17 and physical properties25 of soot particles. Coated soot absorbs light more effectively than fresh soot, exerting a higher positive direct radiative forcing.9,26 If the coating material is watersoluble, the particles may grow by absorbing water at high humidity,27 and this results in an additional increase in light absorption and scattering.26 Soot particles coated with sufficient amounts of hygroscopic material may become cloud condensation nuclei (CCN) and form cloud droplets at atmospherically relevant water supersaturations (SS),25 shortening the atmospheric lifetime of soot and contributing to negative climate forcing. Formation of coatings and subsequent absorption of water vapor also influence soot morphology, by transforming fractal, irregular aggregates to more compact, close-to-spherical shapes.21,28 The change in morphology, in addition to the lensing effect by the coating, plays an important role in the evolution of the optical properties of soot with atmospheric aging.26,29,30

1. INTRODUCTION Soot from incomplete combustion of biomass and fossil fuels represents a major constituent of atmospheric aerosols.1,2 As an efficient light absorber, soot contributes significantly to climate change by direct radiative forcing,3 and it has been suggested that soot is the second most important component causing global warming after carbon dioxide.4,5 Light absorption by soot reduces visibility and stabilizes the lower atmosphere, exacerbating accumulation of gaseous and particulate pollutants within the planetary boundary layer.6 UV light absorption by soot may reduce the photochemical activity and tropospheric ozone production.7 Furthermore, the presence of absorbing soot results in a more stable atmosphere due to enhanced surface cooling and atmospheric heating, leading to decreased cloud fraction, optical depth, convective strength, and precipitation.8 The impacts of soot on air quality, cloud processes, and climate depend on its mixing state, being stronger when soot particles are associated with other aerosol constituents.4,9 The mixing state of soot varies greatly among different combustion sources and is further modified during atmospheric aging through interactions with other air pollutants. Field studies show that atmospheric soot particles are often coated by sulfate and organics, with the coating thickness increasing with the age of the air mass.10−13 The aging process is rather complex and may occur via several mechanisms, including coagulation of soot with other aerosol particles,14 dark and photochemical oxidation of soot particle © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2254

November 7, 2012 January 30, 2013 February 4, 2013 February 4, 2013 dx.doi.org/10.1021/es3045339 | Environ. Sci. Technol. 2013, 47, 2254−2263

Environmental Science & Technology

Article

experiments in the absence of isoprene and irradiation. Nascent soot particles contained less than 3% organic material that could be volatilized at 300 °C. Water, H2O2, isoprene, and the NO/NO2 mixture were introduced into the chamber in a flow of zero air. The air in the chamber was mixed with a fan for about 30 s and then the black light UV lamps were turned on to initiate OH-induced oxidation of isoprene via H2O2 photolysis. Before and during soot aging, the concentrations of isoprene and its major oxidation products were measured by a compact proton transfer reaction mass spectrometer (PTR-MS, Ionicon Analytik). The concentration of nitrogen oxides (NOx) was measured by a Thermo Scientific model 42i NOx analyzer. Typical concentrations of H2O2, isoprene, and NOx in the chamber were 5 parts per million (ppm), 68−1020 parts per billion (ppb), and 0−317 ppb, respectively; NOx was a mixture of NO and NO2 in a volume ratio of 1:5. Relative humidity (RH) in the chamber was maintained at 20 ± 4%. An integrated instrument including two DMAs (3081, TSI), an aerosol particle mass analyzer (APM, 3600, Kanomax), a condensation particle counter (CPC, 3760A, TSI), a thermal denuder, and a humidifier was used for measurements of the particle size, mass, and hygroscopicity, as described elsewhere.24,27 The size distribution and number concentration of soot particles in the chamber were determined by using the instrument as a scanning mobility particle sizer (SMPS). Particle diameter growth factor

Changes in the properties of soot during aging strongly depend on the composition of the coating material. Coatings composed of sulfuric acid,28,31 glutaric acid,32 the products of αpinene dark ozonolysis,21 and the products of toluene photochemical oxidation24 induce a significant restructuring in fractal soot aggregates and also make coated particles hygroscopic. On the contrary, coatings of oleic acid,33 anthracene,33 succinic acid,32 and dioctyl sebacate (DOS)31 leave the fractal morphology of embedded soot cores and the hygroscopicity of coated particles practically unchanged. Restructuring is generally promoted by liquids with high surface tension, but there are exceptions to this general trend.32 Although some contemporary models simulate the evolution of soot mixing state by explicitly resolving the composition of individual particles,34 the effect of changing morphology is not considered because of the lack of available physical parametrizations. Further experimental research is required to elucidate the sources of atmospheric condensable materials and the factors that control the variation in the properties of aging soot aggregates. Photochemical oxidation of isoprene has been recently recognized as an important source of atmospheric condensable species.35 Isoprene is emitted by biogenic and anthropogenic sources36 and reacts rapidly with atmospheric oxidants to form a broad range of products.37,38 The OH-initiated oxidation represents the dominant pathway of isoprene during daytime,39,40 while isoprene oxidation by ozone and nitrate radicals occurs mainly at nighttime.41,42 Although most of the products are volatile,43 the overall contribution of isoprene to atmospheric aerosol formation may be significant35,44,45 because of its large global emissions, estimated to be around 600 Tg year−1.36 The yield of secondary organic aerosol (SOA) from the OH-induced oxidation of isoprene depends on the relative concentrations of isoprene (i.e., volatile organic compound, VOC) and nitrogen oxides (NOx), and reaches a maximum for VOC/NOx in the range of 1−10 ppbC/ppb.46 Isoprene SOA is moderately water-soluble, with hygroscopicity parameter κ of around 0.1, and may readily serve as cloud condensation nuclei (CCN) at atmospherically relevant water supersaturations.47,48 We have investigated the photochemical aging of sizeclassified soot aerosols in the presence of isoprene, hydroxyl radical (OH), and NOx. The measured evolution in the aging state of soot, expressed through changes in particle size, mass, effective density, dynamic shape factor, and coating thickness, was complemented by measurements of aerosol hygroscopicity, CCN activation, and optical properties.

Gfd = Dp /Do

(1)

characterizes the change in the particle size during aging, where Dp and Do represent the mode of the distributions of soot particles at a given reaction time and of fresh soot particles, respectively. The particle mass was determined from the massmobility measurements (DMA-APM-CPC). The particle mass growth factor, Gfm, and the organic mass fraction, forg, were calculated as,

Gfm = mp /mo

(2)

forg = 1 − 1/Gfm

(3)

where mo and mp are the particle mass before and during aging, respectively. The effective particle density was calculated as

ρeff =

6mp πDp3

(4)

The material particle density ρm, was calculated from the derived forg using material densities of soot (ρsoot, 1.77 g cm−3)50 and isoprene SOA (ρorg, 1.4 g cm−3).51

2. EXPERIMENTAL SECTION Size-resolved soot aging experiments were performed in a 1.2 m3 collapsible environmental chamber, as described previously.24 Briefly, soot aerosol from incomplete combustion of propane was continuously sampled, diluted, dried, and sizeclassified by a differential mobility analyzer (DMA).27,49 The size-classified soot aerosol was charge-neutralized, passed through denuders filled with absorbing media to remove gasphase contaminants, and then introduced into the environmental chamber filled with particle-free zero air. The initial particle number concentration of soot was maintained at 1100 ± 300 particle cm−3 in all chamber experiments. A low concentration and monodisperse size of soot aerosols ensured that particle coagulation was negligible, as confirmed from the observation of a constant particle size and mass in blank

forg 1 − forg 1 = + ρm ρorg ρsoot

(5)

The material density was used to calculate the volume equivalent diameter, Dve =

6mp 3

πρm

(6)

The aging state of soot was expressed as the change in the volume equivalent coating thickness Δrve = (Dve − Dve,o)/2 2255

(7)

dx.doi.org/10.1021/es3045339 | Environ. Sci. Technol. 2013, 47, 2254−2263


Article

where Dve,o and Dve are volume equivalent diameters of fresh and coated soot particles, respectively.33 The irregularity of the particle shape was characterized using the dynamic shape factor, χ, χ=

Dp Cp

×

Cve Dve

(8)

where Dve is the volume equivalent diameter, and Cp and Cve are the Cunningham slip correction factors calculated for Dp and Dve, respectively.52 To characterize the morphology of soot particles, the mass fractal dimension, Dfm, was calculated from the mass-mobility relationship, mp = C × DpDfm

(9)

where C is a coefficient obtained by fitting the data. Tandem DMA (TDMA) measurements were used to quantify the change in mobility diameter of aged soot cores by removing the coating in a thermal denuder at 300 °C or by humidification to 90% RH.27,28 The volatility (vGfd = Dp/Do) and hygroscopicity (hGfd = Dp/Do) diameter growth factors were determined from TDMA distributions, where Do and Dp represent the mobility diameters of coated soot before and after processing, respectively, where processing involves either heating or humidification. The effective density of the soot cores was determined from DMA-APM-CPC measurements of the coated-heated soot according to eq 6. Light scattering, bsca, and extinction, bext, coefficients at 532 nm were measured by an optical system consisting of a threewavelength integrating Nephelometer (3563, TSI) and a Cavity Ring-Down Spectrometer (CRDS), respectively.26 The absorption coefficient babs was calculated from the difference between extinction and scattering coefficients. Aerosol scattering (Csca) and absorption (Cabs) cross sections were calculated by normalizing the corresponding coefficients over the particle number concentration determined by SMPS. Critical supersaturations of size-classified nascent and coated soot aggregates were measured using a CCN counter (Droplet Measurement Technologies, CCN-1) at varied supersaturation. The CCN counter was calibrated with dry ammonium sulfate aerosol, which was produced by a constant output atomizer (TSI 3076), dried to ∼7% RH by passing through a Nafion multitube drier, and then size-classified by the DMA.

Figure 1. Aging of soot from photochemical oxidation of 340 ppb isoprene for particles with different initial mobility diameter. Changes in the particle (a) size and (b) mass are expressed as diametric (Gfd) and mass (Gfm) growth factors.

into the aging experiment, temporal profiles of Gfd for different particle sizes diverge, showing opposing trends for smaller and larger initial sizes. For 50 nm soot particles, Gfd increases and levels off at about 1.03. For larger particles, Gfd decreases with aging, reaching a value as low as 0.91. This Gfd pattern differs from the one obtained previously in the experiments with toluene, where the growth factor increases continuously with aging for all initial particle sizes.24 Also, soot aging from the condensation of the isoprene oxidation products is significantly slower than the aging from toluene. For instance, for 100 nm soot particles, a consumption of 200 ppb isoprene is required to reach a Gfm of 1.3, whereas a Gfm of 3 is attained previously after oxidation of as little as 12 ppb toluene.24 Soot particles are fractal aggregates composed of smaller primary graphitic spheres. 25 When low-volatility vapor condenses on such aggregates to form a coating, a substantial fraction of the coating material may occupy voids between the primary spheres. Hence, an increase in the particle mass, as reflected by increases in Gfm (Figure 1b) and organic mass fraction forg (Figure 2a), is initially accompanied by little observable change in the mobility diameter (Figure 1a). Because of the voids, the effective density of nascent soot is significantly lower than the material density of the graphitic primary spheres (1.77 g cm−3),50 and is in the range from 0.8 to 0.4 g cm−3 for 50 to 150 nm aggregates, respectively. Figure 2b shows that the effective density increases when the voids fill up with condensing material, reaching values of 1.4 and 0.7 g cm−3 for 50 and 150 nm particles, respectively. Because of the fractal structure of soot aggregates, the mobility diameter is not a good metric of the soot aging state. Instead, we use the volume equivalent coating thickness Δrve calculated using eq 7 to gauge the degree of soot aging because it is independent of the particle morphology.24 As shown in Figure 2c, the change in Δrve during aging is comparable between particles of different initial diameters, when the rate of

3. RESULTS AND DISCUSSION Evolution in the Particle Size, Mass, And Morphology. The aging of size-classified soot in the presence of photochemically oxidized isoprene produces significant changes in particle mobility and mass growth factors, as shown in Figure 1. The increase in Gfm indicates that soot particles gain mass from the condensation of oxidation products of isoprene. When isoprene is nearly consumed, Gfm reaches a maximum and then slowly decreases. Secondary organic material formed from the oxidation of isoprene under our experimental conditions contains high levels of peroxides, which are photolytically unstable.53 Evaporation and photolysis of particle-phase peroxides are the likely cause for the decrease in the particle mass with continued irradiation.45,53 The mass growth factor is the largest for 50 nm particles (Gfm ∼1.9) and it varies between 1.3 and 1.4 for particles of larger initial sizes (Figure 1b). Unlike Gfm, the mobility diameter growth factor Gfd remains practically unchanged during the initial aging stage (Figure 1a). After about 50 min 2256



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Figure 3. Changes in soot particle (a) effective density (ρeff), (b) heated effective density (ρeff(heated)),and (c) dynamic shape factor (χ) as a function of volume equivalent coating thickness (Δrve) upon photochemical aging in the presence of 340 ppb isoprene.

Figure 2. Time-dependent changes in (a) soot particle organic fraction (forg), (b) effective density (ρeff), and (c) volume equivalent coating thickness (Δrve) upon photochemical aging in the presence of 340 ppb isoprene.

aging, the restructuring of soot aggregates also contributes to the decrease in χ. The variation in the particle morphology can be examined using the mass fractal dimension of soot, Dfm, derived from mass-mobility measurements of aging particles with different initial sizes (Supporting Information Figure S1). Figure 4 shows that Dfm initially decreases with coating, because of a joint effect of a larger Gfm for smaller particles and

isoprene oxidation is maintained constant. The nearly constant Δrve indicates that soot aggregates in our study have open structures, which allow for comparable mass of material to condense on primary spheres, irrespectively of the initial mobility size of the aggregates. Figure 3 shows the evolution in soot particle properties when referenced to Δrve. The effective density increases almost linearly with Δrve for the 50 nm particles (Figure 3a). For larger particles, ρeff varies weakly when the coating is thin and significant changes in the density occur only for coatings in excess of about 3 nm. Heating the soot particles to remove coatings produced by the aging reveals that the effective density of the soot cores has increased (Figure 3b). The change in the effective density of the soot cores becomes noticeable only for particles coated in excess of Δrve = 3 nm. No change in effective density is observed when nascent soot particles are heated. The increase in the effective density of aged and heated soot is indicative of restructuring that occurs to aggregates after they acquire a sufficient fraction of the coating material.21,24,28,31,32 In the absence of restructuring the effective density returns to a value similar to that for nascent, untreated soot when the coating is removed by heating.31−33 As coatings develop on soot aggregates, the dynamic shape factor is lowered from χ = 1.9−2.4 to 1.2−1.8 (Figure 3c), reflecting the decreased drag on particles. For thicker coatings that form at later stages of

Figure 4. Variation in the mass fractal dimension (Dfm) of soot aerosol with aging. 2257



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independent of the initial isoprene concentration in the range of 68−1020 ppb (SI Figure S3). Previous studies have shown that SOA yield from the OHinduced oxidation of isoprene depends on the VOC/NOx ratio, reaching a maximum at VOC/NOx of 2−10 ppbC/ppb and decreasing at lower and higher values.45,51,64 Figure 5b shows

negligible changes in Gfd for all particle sizes. A similar decrease in Dfm has been observed for soot aggregates that were thinly coated with sulfuric acid.28 However, when the coating thickness approaches 3.5 nm, Dfm reaches its minimum and then increases with increasing Δrve because of restructuring. As follows from Figures 1a, 2c, 3a, 3b, and 4, significant changes in soot properties associated with particle morphology, such as Gfd, ρeff, ρeff(heated), and Dfm, all take place at about the same coating thickness, corresponding to 3−4 nm. The condensation of low-volatility products from isoprene photooxidation occurs preferentially in small angle cavities between primary spheres in aggregates because of the inverse Kelvin effect.54 As the region between adhering spheres corresponds to a coating-vapor interface with negative curvature, the equilibrium vapor pressure over small angle cavities is depressed,55 promoting the condensation. The restructuring of coated aggregates is caused by the surface tension forces that the liquid coating exerts on the aggregate.56 Once a sufficient amount of coating has condensed, the surface tension brings the primary spheres into a closer configuration to minimize the total surface free energy of the coated aggregate. Organic materials of low surface tension, such as oleic acid (32.8 mJ m−2),57 cause little restructuring even when coatings reach 100 nm.33 On the contrary, sulfuric acid and its aqueous solutions, having significantly higher surface tensions (73−75 mJ m−2),58 initiate almost complete restructuring of aggregates already at Δrve ∼ 10 nm.28 The products of isoprene oxidation fall in between the toluene oxidation products24 and sulfuric acid28 in their ability to induce restructuring when present in comparable quantities, as expressed by comparable Δrve. Although isoprene is highly reactive and is readily oxidized by OH, its first generation products, such as formaldehyde, methacrolein, methyl vinyl ketone, glycoaldehyde, hydroxyacetone, methylglyoxal, and hydroxyhydroperoxides are too volatile to condense.59 Second-generation multifunctional compounds, including 2-methyltetrols,44 2-methylglyceric acid, and isoprene epoxydiols60,61 are semivolatile, and can contribute to the coating formation via either partitioning or heterogeneous acid-catalyzed reactions in the particle phase.62 However, since nascent soot particles are not strongly acidic and lack coatings initially to provide a medium for partitioning, it is most probably the third and later generation oxidation products that are responsible for the soot particle coating and growth. To follow the evolution in the chamber gas-phase composition, we routinely used PTR-MS to monitor selected ions corresponding to isoprene (m/z 69) and several of the most abundant/volatile reaction products, such as those with m/z 71 (methyl vinyl ketone and methacrolein) and m/z 61 (acetic acid), as shown in SI Figure S2. Upon UV irradiation, the concentration of isoprene decreases due to oxidation by OH and the volatile products accumulate in the gas phase. In experiments with higher initial concentrations of isoprene, proportionally higher concentrations of first generation products, methyl vinyl ketone and methacrolein, are produced. However, the formation of the later generation products, which were monitored indirectly by following acetic acid formed from isoprene via oxidation of hydroxyacetone,63 varies little with increasing initial isoprene concentrations because a progressively larger fraction of OH is consumed in reactions with isoprene and its first generation products. As a result of this competition for OH, the aging rate of soot is practically

Figure 5. Aging of 100 nm soot particles from photochemical oxidation of 340 ppb isoprene in the presence of varying initial levels of NOx. Changes in the particle (a) size and (b) mass are expressed as diametric (Gfd) and mass (Gfm) growth factors.

that additions of NOx increase both the rate and the extent of soot aging, as evidenced by a significantly higher mass growth factor, reaching Gfm = 3.6 in the presence of 317 ppb NOx. The evolution in Gfd is also significantly affected, for example, the addition of 49.7 ppb NOx causes particles to restructure at a faster rate whereas in the presence of 317 ppb NOx, Gfd passes through a minimum and then increases to a value of 1.06, thus resulting in a net positive size change. Such behavior in the high-NOx experiments is mostly caused by fast coating. For instance, the decrease in the dynamic shape factor (SI Figure S4a) and the increase in the effective density (SI Figure S4b) obtained in experiments with different NOx levels overlap well. The deviations from the line may be caused by the differences in the composition of the coating material that is formed in low and high NOx experiments; the latter are expected to produce a larger fraction of organonitrates (e.g., methacryloylperoxynitrate), 2-methylglyceric acid, and its oligoesters.61,65 Optical Properties of Aged Soot. Optical measurements were conducted during the aging of soot particles with initial mobility diameters of 150 and 240 nm. In agreement with previous studies,26,31,66 we find that nascent soot is a strong light absorber and a weak scatterer, showing SSA in the range of 0.19−0.25. During aging, the scattering cross-section remains practically unchanged while the absorption cross-section decreases (SI Figure S5), resulting in a small increase in SSA, reaching values of 0.25−0.33 for about 4−13 nm volume equivalent coating thickness. This variation in the optical properties is consistent with the changes in absorption and 2258



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scattering observed for soot particles thinly coated by oxidation products of toluene.24 Nascent soot aggregates scatter weakly because of the small size (12−20 nm) and open arrangement (Dfm ∼ 2) of primary spheres.24,26 For significant enhancement in scattering to take place, sufficient volume of coating must condense on the aggregates to increase the size of primary spheres and possibly to bring the spheres into a closer arrangement.29 From the knowledge of average diameter of primary spheres (20 nm) and the coating mass fraction, we calculated the actual coating thickness on primary spheres, assuming that the coating material is uniformly distributed on the aggregate surface. For Δrve = 3.5 nm in low-NOx experiments, the actual coating thickness is only 0.7 nm, corresponding to about 2 monolayers. The enhancement in absorption also requires a sufficient coating thickness for lensing and restructuring to occur.29 The decrease in light absorption by thinly coated soot particles in our experiments is related to their aggregated structure. As shown in theoretical calculations by Fuller et al.,67 the interaction between adjacent spheres within the soot aggregate can increase absorption by about 30% relative to absorption by the sum-of-spheres. However, when particles become coated, even thinly, this enhancement factor is lost because typical dielectric coatings reduce the electromagnetic coupling between the primary spheres. Recently, these theoretical calculations have been confirmed by experimental measurements, showing that the removal of thin coatings by thermal denuding of the fresh aggregates leads to a 4−14% increase in absorption.26,29 CCN Activity of Aged Soot. Nascent soot particles produced by our burner are hydrophobic, that is, they exhibit no detectable hygroscopic growth at 90% RH, and less than 0.1% of particles activate to cloud droplets at a 1.6% water supersaturation, in agreement with previous studies.25,68 Condensation of water-soluble products from isoprene oxidation significantly enhances the interaction of soot with water vapor and promotes restructuring of coated aggregates. Hygroscopicity TDMA measurements show hygroscopic growth factors of 1.05, 1.00, 0.96, and 0.95 for aged soot particles with a coating thickness of 5 ± 1 nm and initial mobility diameters of 50, 80, 100, and 150 nm, respectively. The smallest and most compact 50 nm particles grow upon exposure to 90% RH from the absorption of water vapor by the organic coating. Larger particles either remain unchanged or decrease in size because aqueous coatings promote further restructuring of the soot cores. A similar behavior upon humidification has been previously observed for fractal soot aggregates thinly coated by sulfuric acid27 and by oxidation products of toluene.24 Total growth factors, including contributions from coating and humidification are 1.09, 0.99, 0.87, and 0.86 for particles with initial sizes of 50, 80, 100, and 150 nm, respectively. Aging also significantly alters the CCN properties of soot. SI Figure S6a shows temporal profiles of the activated particle number fraction (CCN/CN) at different supersaturations during the aging of soot aerosol with a 100 nm initial mobility diameter. For instance, at SS of 1.0% and 0.6%, half of the particles activate after 35 and 180 min of aging, respectively. We used the data from SI Figure S6a to calculate the CCN/CN ratios as a function of SS for different aging times (SI Figure S6b). Critical supersaturations, Sc, corresponding to a CCN/ CN ratio of 0.5, are presented in Figure 6a as a function of the volume equivalent diameter Dve for soot particles with the

Figure 6. Increase in the cloud forming ability of soot particles with photochemical aging in the presence of 340 ppb isoprene and different NOx levels. Solid lines are for zero NOx and dashed lines are for 50 ppb NOx experiments. (a) Critical supersaturations as a function of volume equivalent diameter (Dve) for particles of 100 and 150 nm initial mobility diameters (Do). Initial (nascent) soot Dve is given above each curve. (b) Dependence of the hygroscopicity parameter (κ) on the coating volume fraction (ε).

initial mobility diameter of 100 and 150 nm in experiments with and without the addition of NOx. Table 1 shows the Table 1. Relationship between Initial Mobility and Volume Equivalent Diameters in CCN Experiments Do, nm

Dve, nma

NOx, ppb

100 100 150 150

67 62 88 95

0 50 0 50

a

The variation in the particle Dve for the same Do is caused by some variability in the effective density of soot between different experiments.

relationship between the initial mobility and volume equivalent diameters in the experiments. The critical supersaturation decreases with increasing Dve, as particles are coated. In zero NOx experiments, Sc becomes as low as 0.60% and 0.50% for particles with Do of 100 and 150 nm, respectively. In the presence of 50 ppb NOx, aging occurs at a faster rate, producing thicker coatings, and Sc values decrease to 0.46% and 0.24% for 100 and 150 nm particles, respectively. The decrease in the critical supersaturation is caused jointly by the increases in the particle size and fraction of water-soluble material, with the latter factor being more prevalent. As shown in Figure 6a, coated soot particles of the same Dve of 100 nm activate at very different critical supersaturations, when they have different coating thicknesses, reflecting a different water-soluble content. Specifically, particles of the initial Dve = 88 nm acquire a 6 nm coating and activate at a 0.47% Sc, whereas the coating formed on initially 95 nm particles is significantly thinner (2.5 nm) and 2259



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they activate at a much higher Sc (0.82%). Thus, our results suggest that the cloud forming properties of soot are mainly controlled by the composition and thickness of coatings formed during aging, rather than by the size of the original particles. For a submicrometer CCN with a substantial soluble fraction, the hygroscopic growth parameter, κ, can be derived from CCN measurements following the hygroscopicity parametrization introduced by Petters and Kreidenweis,69 κ=

A=

4A3 27Dd3 ln 2 Sc

particles deprived of coatings would be more absorptive than the freshly emitted soot, which is always coated, at least thinly. Thus, the surprisingly low absorption enhancement for aged soot recently observed by Cappa et al.13 during the 2010 CalNex and the Carbonaceous Aerosols and Radiative Effects (CARES) campaigns may be biased by the use of thermal denuding to obtain the reference absorption for nascent soot. The actual enhancement may have been underestimated by a factor of 1.15−1.30. Another important implication of our study is that aging of soot from photooxidation of atmospherically abundant isoprene transforms initially hydrophobic particles into efficient CCNs that activate at atmospherically relevant water vapor supersaturations. Furthermore, since a mixture of the isoprene photo-oxidation products and ammonium sulfate forms a miscible liquid phase even at low RH,70 it is reasonable to expect that coatings developed on soot particles from co-condensation of the isoprene oxidation products and sulfuric acid25 remain liquid even after their neutralization by ammonia. The persistence of soot particles with an aqueous-phase coating over a broad range of atmospheric RH has a significant impact on their radiative properties and on the subsequent aqueous-phase processing of coatings.71 We show that the rate of soot aging increases and the changes in soot properties become more pronounced in the presence of NOx, a common combustion copollutant of soot. A similar enhanced SOA formation from mixed anthropogenic and biogenic emissions has been recently observed during CARES on the days when anthropogenic emissions from Sacramento mix with isoprene-rich air from the foothills.72 Emissions of biogenic hydrocarbons have been shown to play an important role in the nucleation and growth of atmospheric aerosols.73−75 Our findings from the present work indicate that ubiquitous biogenic isoprene also plays a significant role in atmospheric aging of anthropogenic combustion soot, shortening its atmospheric lifetime and considerably altering its impacts on air quality and climate. To accurately model the atmospheric impacts of biogenic emissions, an explicit treatment of the variation in morphology, hygroscopicity, and optical properties of soot may be required.

(10)

4M w σs/a RTρw

(11)

where Dd is the volume equivalent diameter of the dry coated soot particle, Mw = 0.018 kg mol−1 is the molar mass of water, σs/a = 0.072 J m−2 is the surface tension of the water/air interface, R = 8.31 J mol−1 K−1 is the ideal gas constant, T = 298 K is the temperature, and ρw = 1000 kg m−3 is the density of water. Figure 6b shows the evolution of κ with the increasing coating volume fraction of soot particles, ε, for aging experiments presented in Figure 6a. For a multicomponent system, such as the coated soot, the overall κ is, κ=

∑ εiκi i

(12)

where εi is the volume fraction and κi is the hygroscopicity parameter of the individual component, i. Since for hydrophobic soot κsoot = 0, eq 12 can be simplified as κ = εcoatingκcoating (13) Figure 6b shows that in experiments in the absence of NOx the dependence of κ versus ε is linear (r2 = 0.997−0.998), and the hygroscopic parameter of the coating material can be derived from the slope, κcoating = (0.15−0.19) ± 0.03. These values are comparable with an average κSOA = 0.14 ± 0.02 obtained for the isoprene SOA in low-NOx oxidation experiments by Engelhart et al.48 and are somewhat larger than the value of 0.10 ± 0.02 derived by King et al.47 The nonlinear increase in κ in the experiments with addition of NOx may be caused by oxidative processing of the coating constituents in the gas and particle phases by the higher concentration of OH, and also by ozone, which is formed in significant amounts in the presence of NOx. As shown by Engelhart et al.,48 the hygroscopicity parameter for the watersoluble fraction of isoprene SOA can be as high as 0.31 ± 0.03. Atmospheric Implications. The initial aging of soot from OH-initiated oxidation of isoprene forms relatively thin coatings on particles because of the high volatility of the isoprene SOA. However, the impacts of these coatings on the particle morphology and light absorption are profound and comparable to those from condensation of sulfuric acid. For instance, coated particles take up water vapor at subsaturated relative humidity that increases the coating volume and promotes an additional compaction of soot. Our experimental data confirm the results of previous theoretical calculations that the interaction between adjacent spheres within the soot aggregate increases light absorption relative to that by the sumof-spheres. An important implication of this observation is that aged atmospheric soot particles cannot be restored to their “nascent” state by thermal denuding in order to quantify the enhancement in light absorption caused by aging. The heated

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ASSOCIATED CONTENT

S Supporting Information *

Six additional figures show the change in the particle morphology with aging, the evolution in concentrations of isoprene and several gas-phase oxidation products in the chamber, the aging of soot in the presence of different initial concentrations of isoprene, the dynamic shape factor and effective density of soot in the presence of different initial concentrations of NOx, the light absorption and scattering cross sections of soot, and the activation of soot to cloud droplets with aging. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2260



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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CBET-0932705 and AGS-0938352) and the Robert A. Welch Foundation (A-1417).

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