Environ. Sci. Technol. 2002, 36, 5083-5091
Direct Evidence of Atmospheric Secondary Organic Aerosol Formation in Forest Atmosphere through Heteromolecular Nucleation ILIAS G. KAVOURAS† AND EURIPIDES G. STEPHANOU* Environmental Chemical Processes Laboratory, Department of Chemistry, University of Crete, GR-71409 Heraklion, Greece
reaction products remain in the gas phase, some of them partition between the gas and particulate phase, facilitating thus the formation of ultra-fine particles (aerodynamic diameter < 100 nm). It is presumed that, at the beginning of this process, organic vapors might condense onto “seed” particles if at least one compound attains its saturation concentration (Csat,i) (2-8, 24-28). Once an organic liquid layer is formed, organic compounds can condense even at low gas-phase concentrations. Pankow (26, 27) proposed a mathematical relationship to model the gas/particle partitioning of semi-volatile organic compounds in an amorphous organic particle
log Kp,i ) -logpLo - logγom,i + log
Atmospheric aerosols play a central role in climate and atmospheric chemistry. Organic matter frequently composes aerosol major fraction over continental areas. Reactions of natural volatile organic compounds, with atmospheric oxidants, are a key formation pathway of fine particles. The gas and particle atmospheric concentration of organic compounds directly emitted from conifer leaf epicuticular wax and of those formed through the photooxidation of R- and β-pinene were simultaneously collected and measured in a conifer forest by using elaborated sampling and GC/ MS techniques. The saturation concentrations of acidic and carbonyl photooxidation products were estimated, by taking into consideration primary gas- and particle-phase organic species. Primary organic aerosol components represented an important fraction of the atmospheric gasphase organic content. Consequently, saturation concentrations of photooxidation products have been lowered facilitating new particle formation between molecules of photooxidation products and semi-volatile organic compounds. From the measured concentrations of the abovementioned compounds, saturation concentrations (Csat,i) of R- and β-pinene photooxidation products were calculated for nonideal conditions using a previously developed absorptive model. The results of these calculations indicated that primarily emitted organic species and ambient temperature play a crucial role in secondary organic aerosol formation.
Introduction Atmospheric fine particulate matter plays a key role on climate issues and health related problems. While sulfate dominates the composition of marine aerosols and stratospheric submicrometer aerosols, organic matter frequently makes up the largest aerosol fraction over continental areas (1). Atmospheric reactions of natural volatile organic compounds (VOCs), with OH and NO3 radicals and ozone, are an important formation pathway of organic ultra-fine particles (1). In laboratory and field studies (2-25), it has been shown that monoterpenes rapidly undergo free radical (e.g OH and NO3) and/or ozone (O3) addition to form multifunctional organic compounds such as keto-acids and dicarboxylic acids (2-7, 11-25). Although the majority of * Corresponding author phone: +30 810 393628; fax: +30 810 393678; e-mail:
[email protected]. † Present address: Institute for Environmental Research and Sustainable Development, National Observatory of Athens, GR-15236, P. Penteli, Athens, Greece. 10.1021/es025811c CCC: $22.00 Published on Web 10/23/2002
2002 American Chemical Society
(
)
7.501‚R‚T‚fom 109‚MWom
(1)
where PLo is the vapor pressure (Torr) of the subcooled compound i, γom,i is the dimensionless activity coefficient of the compound i in the aerosol mixture, R is the constant of ideal gases (8.314 J mol-1 K-1), T is the ambient temperature (K), fom is the weight fraction of the total suspended particulate matter that is absorbing organic matter (om) phase (which includes inorganic species and water that may be present), and MWom is the mean molecular weight of the species constituting the liquid organic matter (om) phase (g mol-1). Kp,i is the partitioning coefficient of the i-compound (in m3 µg-1) between the two phases expressed as follows
Kp,i )
Caer,i Cgas,i‚TSP
(2)
where Caer,i and Cgas,i are the concentrations (ng m-3) of the i-compound in the aerosol and gas phase and TSP is the concentration (µg m-3) of total suspended particles. On the basis of the above gas/particle partitioning absorption model developed by Pankow (26, 27) Odum et al. (8) expressed the fractional secondary aerosol yield Yi, as a function of formed organic aerosol mass Mo (in µg m-3) as follows
Ri‚Kom,i om.i‚Mo
∑1+K
Yi ) Mo‚
(3)
where Ri is the stoichiometric factor of product i for the reaction scheme and Kom,i is the gas/particle partitioning coefficient in terms of formed organic mass concentration. In laboratory experiments initial monoterpene concentration is very high (up to 700 ppbv) and the air is not circulated or diluted, so saturation concentrations are rapidly surpassed (2-4, 6, 11, 12, 16, 20-25). In addition, according to Kamens and Jang (28) the absence of preexisting aerosol causes that both organic content (fom) and activity coefficient (γi) values to approach unity. Thus, the models derived from experiments under the above-mentioned conditions should be used with caution if an accurate estimate of the aerosol yield from hydrocarbon oxidation is to be made. In addition, the presence of other biogenic semi-volatile organic compounds emitted as primary aerosol cannot be understood and evaluated working under laboratory conditions. Recently, the chemical coupling of newly formed atmospheric particles with specific compounds produced by the photooxidation of monoterpenes (e.g. R- and β-pinene) has been reported for forest areas (14, 19). The monoterpene photooxidation products pinonic and pinic acid, determined in the particulate phase of forest aerosol, were directly linked VOL. 36, NO. 23, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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to the formation of new ultra-fine particles (diameter < 100 nm) and accounted up to 30% of SOA mass (14). In the abovementioned studies the corresponding gas phase of the acidic and polar fraction of forest aerosol was not determined, and thus a mechanism for the organic aerosol formation was not proposed. In the present study, a novel sampling device was used to minimize adsorption, desorption and reaction artifacts (17, 29, 30) occurring during the collection of organic compounds in the gas and particulate phases. Thus, Caer,i, Cgas,i and Yi (See in Experimental Section, eq 5) could be simultaneously and accurately measured in order to examine different nucleation scenarios and propose a mechanism for the formation of SOA based entirely on data collected under real atmospheric conditions. In addition, the measured gas and particle concentrations of terpene photooxidation products and other biogenic semi-volatile polar organic compounds, associated with primary aerosol emission, were utilized to calculate the stoichiometric factor Ri (20) (see eq 3 and in Experimental Section eq 5) and the gas/particle partitioning coefficient Kom,i; (20) (see Experimental Section, eq 6). The saturation concentrations of acidic and carbonyl photooxidation products were estimated to examine five different particle formation scenarios including the following: a) self-nucleation due to the super-saturation of the gas phase (1st scenario); b) nucleation of the photooxidation products by forming a five-component ideal solution (2nd scenario); c) nucleation of photooxidation products, in the presence of preexisting organic aerosol, by forming a multicomponent ideal solution (3rd scenario); d) nucleation of the photooxidation products by forming a five-component nonideal solution (4th scenario); and e) nucleation of photooxidation products in the presence of preexisting organic aerosol by forming a multicomponent nonideal solution (5th scenario). To the best of our knowledge this work represents the first attempt to take into consideration primary organic aerosol in studying secondary organic aerosol formation under real atmospheric conditions.
Experimental Section Sampling and Analysis of Organic Aerosol. Carbonaceous aerosol samples were collected, using a novel sampling device (17, 29-31), for four 48-h intensive sampling periods from July 27 to August 8, 1998 in the forest of Pertouli in the area of Agrafa Mountains in Greece (17). Details on sampling device and sampling procedures are given in Kavouras et al. (14, 17, 19). Twenty-one (21) samples were collected during three sampling periods on a 24-h basis: in the morning (612 noon; 1st period), in the afternoon (12 noon -18; 2nd period) and in the night (18-6 next morning; 3rd period). In addition, leaves of the forest trees were collected and analyzed in order to determine their epicuticular wax lipid content (32). The polar and acidic fractions of all samples were analyzed according to Kavouras et al. (19) using a GCQ Finnigan ion trap gas chromatograph-mass spectrometer. Vapor pressures (PLo (Torr)) of the subcooled liquid compounds were retrieved from published literature (e.g. pinonic acid (33, 34), pininc acid (34) and pinonaldehyde (12,34)) and/or calculated using the UNIFAC (35) vapor pressure method at temperatures ranging from 277 to 299 K. Calculations. Acidic and carbonyl products typically produced through the photooxidation of R- and β-pinene can be quite precisely described by the following reaction scheme (20):
monoterpenes f R1 pinonic acid + R2 pinic acid + R3 nor-pinonic acid + R4 pinonaldehyde + R5 nopinone + R6 norpinic acid + ......+ Rn productn (4) 5084
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i) The stoichiometric factor or molar yield (36) for each product (R1, R2, R3, R4, R5, R6, ...) was calculated using eq 3. ii) Aerosol yield Yi for the reaction scheme (4), assuming spherical particles and particle density of 1 g cm-3, can be estimated using the following equation
Maer,total ) Ypinonic‚Caer,pinonic + Ypinic‚Caer,pinic + Ynor-pinonic‚Caer,nor-pinonic + Ynor-pinic‚Caer,nor-pinic + Ypinonaldehyde‚Caer,pinonaldehyde + Ynopinone‚Caer,nopinone ) 4 ‚N ‚π‚r3‚d (5) 3 part where Maer,total is the total aerosol mass formed, Npart is the particle number concentration of ultrafine particles having diameter less than 200 nm and r is the radius of ultrafine particles measured with a 3040 Diffusion Battery coupled with a 3022A Condensation Particle Counter (TSI Inc, St. Paul, MN) (17). The analysis of cascade impactor samples, collected during the present study, has shown that secondary organic species are mainly associated with ultrafine particles (37). Yi values were calculated using a multi-linear regression analysis method (SPSS, Version 8.0, Chicago, IL). iii) The gas/particle partitioning coefficient (Kom,i) for each compound was determined according to the following equation:
Kom,i )
Caer,i Cgas,i‚Mo
(6)
Since the partitioning of both primary and secondary organic species takes place in an absorbing organic aerosol phase (8, 21, 26, 27), only the absorbing organic aerosol concentration (Mo) was considered in the present study (reported as organic carbon (µg m-3) from Pio et al. (38)). iv) The basic principle of aerosol formation is that nucleation will be initiated when ambient concentration exceeds the saturation concentration (Csat,i), i.e., having saturation ratios (Cgas,i/Csat,i) higher than one (26, 27). The saturation concentration for a compound (Csat,i) with molar fraction xi, activity coefficient γi, subcooled vapor pressure of PLo (Torr), and molecular weight of MWi was determined as follows (36)
poL‚MWi 9 ‚10 R‚T
Csat,i ) xi‚γi‚C°sat,i ) xi‚γi‚
(7)
where C°sat,i is the saturation concentration of i-compound for ideal conditions. The dimensionless molar fraction xi of the compound in a solution of n-components is given by the following equation (36):
xi )
Caer,i/MWi n
(8)
∑(C
aer,i/MWi)
i)1
For ideal conditions, the activity coefficient (γi) was equal to unity, while for nonideal conditions it was calculated for each photooxidation species reported using the absorptive model developed by Pankow (eq 1) (26, 27).
Results and Discussion Gas/Particle Partitioning. The volatile (compounds with retention time < 20 min) and semi-volatile (compounds with retention time > 20 min) segments of the polar and acidic fractions in both particulate (A,B) and gas (C,D) phase are presented in Figures 1 and 2, respectively. A series of monoterpene-skeleton photooxidation carbonyl and acidic
FIGURE 1. The volatile segments of the gas chromatograms of the carbonyl (A,C) and carboxylic (B,D) fractions in both particulate and gas phases collected over the forest area: (1) nopinone; (2) pinonaldehyde; (3) cis-nor-pinonic acid; (4) trans-nor-pinonic acid; (5) cispinonic acid; (6) trans-pinonic acid; (7) cis-pinic acid.
FIGURE 2. The semi-volatile segments of the gas chromatograms of the alkanol (A,C) and carboxylic (B,D) fractions in both particulate and gas phases collected over the forest area. Compounds are denoted with their carbon atom number. compounds were detected and quantified conjointly with high molecular weight n-alkan-1-ols, n-alkanoic, n-alkenoic and R,ω-dicarboxylic acids in both gas and particles (Table 1). In particular, 6,6-dimethylbicyclo[3.1.1]heptan-2-one (reported as nopinone) (peak No. 1 in Figure 1A,C) and 2,2dimethyl-3-acetyl-cyclobutyl-ethanal (reported as pinonaldehyde) (peak No. 2 in Figure 1A,C), two isomers (cis- and
trans-) of 2,2-dimethyl-3-acetyl-cyclobutyl-formic acid (reported as nor-pinonic acid) (peaks No. 3 and 4 in Figure 1B,D), two isomers (cis- and trans-) of 2,2-dimethyl-3-acetylcyclobutyl-acetic acid (reported as pinonic acid) (peaks No. 5 and 6 in Figure 1B,D) and cis-2,2-dimethyl-3-carboxycyclobutyl-acetic acid (reported as pinic acid) (peak No.7 in Figure 1B,D) were identified on the basis of their mass spectra VOL. 36, NO. 23, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Mean Concentration (ng m-3) with Standard Deviation (σ) of Secondary and Primary Aerosol Components in Gas (Cgas,i) and the Particulate (Caer,i) Phases and Mean Activity Coefficient (γi) with Standard Deviation (σ) of Pinene Photooxidation Products over the Forest Area 6-12
12-18
18-6
294.3 (290.3-295.5)
287.5 (281.0-296.7)
m-3)
mean temp (range)
cis- and trans-pinonic acid cis- and trans-nor-pinonic acid cis-pinic acid pinonaldehyde nopinone
1-eicosanol 1-docosanol azelaic acid (R,ω-C9) lauric acid (C12)
n-tridecanoic acid (C13) myristic acid (C14)
n-pentedecenoic acid (C15:1) n-pentadecanoic acid (C15) palmitoleic acid (C16:1) palmitic acid (C16)
n-heptadecanoic acid (C17) οleic acid (C18:1) stearic acid (C18)
n-nonadecanoic acid (C19) arachidic acid (C20)
n-heneiconaoic acid (C21) behenic acid (C22)
n-tricosanoic acid (C23) n-tetracosanoic acid (C24) n-pentacosanoic acid (C25) n-hexacosanoic acid (C26) n-heptacosanoic acid (C27) n-octacosanoic acid (C28) n-nonacosanoic acid (C29) n-triacontanoic acid (C30)
5086
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K
Ambient Concentrations (ng 287.3 (277.5-295.7)
Photooxidation Organic Compounds Concentrations (ng m-3) Cgas,i 8.52 (10.77) 3.91 (2.00) 9.68 (7.96) 1.16 (0.91) Caer,i γi (10-5) 15.5 (18.4) 10.6 (8.68) Cgas,i 2.65 (1.48) 0.32 (0.13) 1.94 (1.92) 0.14 (0.09) Caer,i γi (10-5) 0.68 (0.67) 0.15 (0.10) Cgas,i 3.77 (3.64) 2.37 (1.38) 2.38 (1.53) 0.49 (0.46) Caer,i γi (10-5) 817 (900) 145 (92.6) Cgas,i 0.73 (1.23) 5.06 (4.17) 0.78 (0.54) 0.27 (0.28) Caer,i γi (10-7) 4.7 (4.9) 9.4 (7.2) Cgas,i 0.56 (1.14) 1.04 0.73) 0.17 (0.22) 0.02 (0.03) Caer,i γi (10-7) 11.0 (16.4) 41.4 (25.6) Directly-Emitted Organic Compounds Concentrations (ng m-3) Cgas,i 0.23 (0.60) 0.39 (0.52) 0.44 (0.24) 0.46 (0.53) Caer,i Cgas,i 0.10 (0.27) 0.65 (1.19) Caer,i 0.74 (0.44) 0.44 (0.42) Cgas,i 2.59 (2.21) 0.69 (0.43) Caer,i 0.47 (0.49) 0.16 (0.11) Cgas,i 0.90 (0.71) 0.71 (0.33) 0.40 (0.42) 0.78 (0.63) Caer,i Cgas,i 1.33 (0.92) 0.45 (0.38) Caer,i 0.81 (0.80) 0.33 (0.19) Cgas,i 54.79 (41.14) 15.59 (12.50) Caer,i 10.52 (11.92) 4.02 (2.49) Cgas,i 13.71 (10.05) 3.23 (3.38) 2.34 (2.72) 0.89 (0.61) Caer,i Cgas,i 38.42 (28.49) 11.11 (8.64) Caer,i 7.04 (7.46) 2.62 (1.53) Cgas,i 16.31 (17.34) 6.01 (6.96) Caer,i 13.06 (14.60) 5.52 (4.15) Cgas,i 170.67 (126.16) 51.47 (38.84) 34.32 (35.62) 12.55 (7.65) Caer,i Cgas,i 8.56 (6.62) 2.05 (2.29) Caer,i 2.52 (2.30) 1.08 (0.67) Cgas,i 24.50 (22.15) 5.76 (8.38) 16.90 (18.56) 7.23 (5.63) Caer,i Cgas,i 29.31 (20.81) 7.82 (8.03) 9.46 (9.13) 3.76 (2.32) Caer,i Cgas,i 3.24 (2.15) 0.82 (0.68) Caer,i 0.60 (0.44) 0.21 (0.14) Cgas,i 5.25 (4.80) 1.32 (1.33) 1.98 (2.00) 0.77 (0.44) Caer,i Cgas,i 1.90 (1.94) 0.44 (0.41) Caer,i 1.28 (1.04) 0.38 (0.38) Cgas,i 6.83 (6.42) 1.46 (1.62) Caer,i 3.92 (3.19) 1.38 (1.11) Cgas,i 2.51 (2.60) 0.71 (0.54) 1.57 (1.14) 0.52 (0.45) Caer,i Cgas,i 10.49 (9.13) 2.57 (2.33) Caer,i 4.24 (3.02) 1.69 (1.07) Cgas,i Caer,i 0.88 (0.99) 0.43 (0.19) Cgas,i Caer,i 1.89 (1.49) 0.81 (0.42) Cgas,i 0.32 (0.14) 0.21 (-) Caer,i 0.25 (0.42) 0.28 (0.18) Cgas,i Caer,i 1.79 (1.22) 0.69 (0.58) Cgas,i Caer,i 0.11 (0.19) 0.03 (0.06) C gas,i Caer,i 1.37 (0.93) 0.45 (0.48)
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1.53 (2.40) 3.99 (2.45) 11.5 (15.6) 3.18 (1.63) 1.51 (1.02) 0.58 (0.47) 0.41 (0.78) 3.03 (1.55) 55.0 (18.8) 1.06 (1.01) 0.88 (0.50) 1.9 (1.0) 0.38 (0.31) 0.24 (0.14) 4.5 (6.6) 0.25 (0.23) 0.12 (0.06) 0.25 (0.25) 0.13 (0.03) 0.22 (0.16) 0.31 (0.25) 0.31 (0.34) 1.24 (1.27) 0.20 (0.17) 0.46 (0.43) 11.99 (7.36) 4.86 (3.88) 3.64 (1.76) 1.42 (1.50) 8.33 (5.21) 2.95 (2.20) 7.42 (8.56) 9.93 (11.72) 35.57 (25.36) 13.92 (9.20) 2.10 (1.45) 1.11 (0.82) 7.33 (9.83) 12.37 (13.70) 4.68 (5.07) 4.23 (2.87) 0.95 (0.47) 0.44 (0.48) 1.26 (0.72) 0.97 (0.72) 0.39 (0.26) 0.53 (0.60) 1.21 (0.82) 1.40 (1.02) 0.44 (0.40) 0.71 (0.64) 1.97 (1.32) 1.89 (1.50) 0.82 (0.85) 1.19 (1.17) 0.22 (0.13) 0.86 (1.31) 1.07 (1.11)
0.91 (1.08)
and comparison with authentic standards (19). These compounds have been determined as characteristic products of the photooxidation of R- and β-pinene with ozone, OH and NO3 radicals, in laboratory studies (2-7, 11-23, 25). Furthermore, n-alkan-1-ols (from n-eicosan-1-ol (C20) to n-docosan-1-ol (C22)) (Figure 2A,C) and of n-alkanoic acids (from dodecanoic acid ((lauric; C12) to triacontanoic acid (C30), n-alkenoic acids (e.g. palmitoleic acid (C16:1) and oleic acid (C18:1)), and R,ω-dicarboxylic acids (e.g. azelaic acid (C9)) (Figure 2B,D) were identified and determined in both gas and particles of studied forest atmosphere. The n-alkanols and n-alkanoic and alkenoic acids are known as constituents of epicuticular leaf wax of terrestrial plants (32), and their presence has been confirmed in the leaf extract of the studied forest trees. Pinonic acid, pinic acid, nor-pinonic acid, nopinone and pinonaldehyde mean concentration (and standard deviation, σ) in gas and particulate phase are presented in Table 1. For pinonic and pinic acids, gas mean concentration maxima (pinonic acid 8.52 (10.77) ng m-3; pinic acid 3.77 (3.64) ng m-3; Table 1) were measured in the morning (6-12), when photooxidation reactions of monoterpenes prevail (2-9). Their mean concentration was then reduced (pinonic acid 3.91 (2.00) ng m-3; pinic acid 2.37 (1.38) ng m-3; Table 1) during the afternoon (12-18) most probably due to the decomposition of these products (2-5). The rapid decrease of OH radicals and ozone concentrations in the nighttime (18-6) (35-41) yielded lower gas-phase concentration for both acids (pinonic acid 1.53 (2.40) ng m-3; pinic acid 0.41 (0.78) ng m-3; Table 1). The slower formation pathway of nor-pinonic acid (10) yielded its higher gas-phase mean concentration (3.18 (1.63) ng m-3; Table 1) during the nighttime (18-6) and at the beginning of the next day (612) (2.65 (1.48) ng m-3; Table 1) rather than in the afternoon (12-18) (0.32 (0.13) ng m-3; Table 1). Pinonaldehyde and nopinone reached their maximum gas-phase mean concentration (pinonaldehyde 5.06 (4.17) ng m-3 and nopinone 1.04 (0.73) ng m-3; Table 1) during the afternoon (12-18). Nighttime (18-6) reactions of pinenes with NO3 radicals (1112) eventually contributed to elevated mean concentration of pinonaldehyde (1.06 (1.01) ng m-3; Table 1). Particle-phase mean concentration for all components followed similar diurnal patterns. Two mean concentration maxima were observed: A) The first maximum (pinonaldehyde 0.78 (0.54) ng m-3, nopinone 0.17 (0.22) ng m-3, pinonic acid 9.68 (7.96) ng m-3, nor-pinonic acid 1.94 (1.92) ng m-3 and pinic acid 2.38 (1.53) ng m-3; Table 1) was observed in the morning (6-12). At that time, intense atmospheric reactions of accumulated monoterpenes with OH radicals and low ambient temperature (mean 287 K; range 277-295 K) (39) favored lower saturation concentrations. B) The second maximum (pinonaldehyde 0.88 (0.50) ng m-3, nopinone 0.24 (0.14) ng m-3, pinonic acid 3.99 (2.45) ng m-3, nor-pinonic acid 1.51 (1.02) ng m-3 and pinic acid 3.03 (1.55) ng m-3; Table 1) was observed during the night (18-6) when ambient temperature dropped off (mean 287 K; range 280-296 K) (39). This observable temperature effect might have enhanced the growth of new particles by condensation of organic compounds of high-to-moderate volatility such as pinonaldehyde. The latter does not initiate aerosol formation (1012, 14-19). The elevated gas-phase concentrations of photooxidation products such as pinonaldehyde and nopinone during the afternoon (12-18) may also increase the condensation process and form high concentrations of particles during the night (18-6). A different concentration profile was observed for the semi-volatile polar and acidic organic compounds, associated with primary emissions from plants (17). n-Eicosan-1-ol (C20) (0.23 (0.60) - 0.39 (0.52) ng m-3) and n-docosan-1-ol (C22)
TABLE 2. Mean Stoichiometric Factor (ri) and Gas/Particle Partitioning Coefficient (Kom,i) (m3 µg-1) with Standard Deviation (σ) of Secondary Aerosol Components over the Forest Area stoichiometric partitioning factor (ri; coefficient dimensionless) (Kom; m3 µg-1) compound
mean
σ
mean
σ
Photooxidation Organic Compounds cis- and trans-pinonic acid (C10) 0.078 0.099 cis- and trans-nor-pinonic acid 0.013 0.016 (C9) cis-pinic acid (C10) 0.073 0.060 pinonaldehyde (C9) 0.014 0.009 nopinone (C9) 0.007 0.007
0.253 0.286 0.042 0.039 0.029 0.028
Directly-Emitted Organic Compounds eicosanol docosanol lauric acid (C12) n-tridecanoic acid (C13) myristic acid (C14) n-pentadecenoic acid (C15:1) n-pentadecanoic acid (C15) palmitoleic acid (C16:1) palmitic acid (C16) n-heptadecanoic acid (C17) οleic acid (C18:1) stearic acid (C18) n-nonadecanoic acid (C19) arachidic acid (C20) n-heneiconaoic acid (C21) behenic acid (C22) n-tricosanoic acid (C23) n-tetracosanoic acid (C24) azelaic acid (R,ω-C9)
0.154 0.171 0.049 0.075 0.070 0.086 0.067 0.222 0.081 0.131 0.275 0.127 0.083 0.253 0.179 0.230 0.227 0.256 0.125
0.170 0.242 0.097 0.076
0.275 0.662 0.036 0.088 0.043 0.115 0.044 0.201 0.065 0.116 0.291 0.075 0.076 0.333 0.252 0.196 0.306 0.238 0.184
(0.10 (0.27) - 0.65 (0.1.19) ng m-3) concentrations in the gas phase were noticeably lower than those measured for the fatty acids with the same carbon atom number, such as n-eicosanoic (arachidic) acid (C20) (1.26 (0.72) - 5.25 (4.80) ng m-3; Table 1) and n-docosanoic (behenic) acid (C22) (1.21 (0.82) - 6.83 (6.42) ng m-3; Table 1). This difference in gasphase concentration might reflect the respective proportion of [n-alkan-1-ols/n-alkanoic acids] in leaf epicuticular wax. Indeed, n-alkan-1-ols were determined in the leaf wax of the forest trees in lower amount (10.3 mg per g of leaf organic extract) than n-alkanoic acids (198.2 mg per g of leaf organic extract). n-Alkan-1-ols gas-phase concentration followed a different diurnal profile than the corresponding concentration profile of n-alkanoic acids. This can be due to the different emission mechanisms from either the leaf surface or from the interior of the leaf through the stomata (32). Conversely, particle-phase diurnal concentration profiles for n-alkan1-ols and n-alkanoic acids presented a clear maximum during the first sampling period (n-alkan-1-ols 1.08 (0.44) ng m-3; n-alkanoic acids 324.08 (204.17) ng m-3; Table 1). These observations indicate an important input of directly emitted organic compounds from the epicuticular waxes. The results of the analysis of forest aerosol suggested that carbonyl and acidic products of monoterpenes photooxidation reactions and the primarily emitted semi-volatile organic compounds constitute a considerable fraction of organic aerosol (17, 38). To estimate the relative importance of the identified compounds on the reaction scheme (4) and on the aerosol formation process, stoichiometric factors (Ri) and gas/particle partitioning coefficients (Kom,i) were calculated (Table 2) using the above-obtained gas/particle partitioning results (eqs 5 and 6). Pinonic and pinic acids exhibited similar Ri values (pinonic acid 0.078 (0.099) and pinic acid 0.073 (0.060); Table 2). VOL. 36, NO. 23, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Calculated Saturation Ratios (Cgas,i/Csat,i) of Pinonic Acid, nor-Pinonic Acid, Pinic Acid, Pinonaldehyde and Nopinone Assuming Ideal and Nonideal Conditions (Scenarios 1-5) saturation ratios (Cgas,i/Csat,i) sampling period
cis- and trans-pinonic acid
6-12 12-18 18-6
0.13 × 10-3 0.20 × 10-4 0.11 × 10-4
6-12 12-18 18-6
cis- and trans- nor-pinonic acid
cis-pinic acid
pinonaldehyde
nopinone
0.12 × 10-2 0.23 × 10-3 0.29 × 10-3
0.77 × 10-6 0.73 × 10-8 0.11 × 10-7
1.40 × 10-6 0.71 × 10-6 2.02 × 10-6
Second Scenario xi Calculated using Eq 8 for Five Compounds (Photooxidation Products) and γi)1 0.51 × 10-4 0.79 × 10-5 0.21 × 10-1 0.48 × 10-5 0.10 × 10-4 0.51 × 10-5 0.66 × 10-4 0.12 × 10-6 0.44 × 10-3 0.19 × 10-4 0.98 × 10-2 0.13 × 10-6
0.23 × 10-5 0.19 × 10-5 0.72 × 10-5
First Scenario xi ) 1 and γi ) 1 0.14 × 10-5 0.69 × 10-6 0.15 × 10-5
Third Scenario xi Calculated using Eq 8 for Thirty-Six Compounds (Photooxidation Products and Semivolatile Polar Compounds) and γi)1 6-12 0.11 × 10-2 0.13 × 10-3 0.43 × 10-2 0.80 × 10-4 0.34 × 10-4 12-18 0.56 × 10-4 0.27 × 10-2 0.28 × 10-1 0.92 × 10-6 0.17 × 10-4 18-6 0.10 × 10-3 0.42 × 10-5 2.30 × 10-1 0.32 × 10-5 0.18 × 10-3 6-12 12-18 18-6
Fourth Scenario xi Calculated using Eq 8 for Five Compounds and γi Calculated using Eq 1 for Thirty-Six Compounds 1.25 0.40 3.06 0.44 0.55 0.10 0.01 1.20 0.12 0.29 0.19 1.43 0.11 0.38 0.30
Fifth Scenario xi Calculated using Eq 8 for Thirty-Six Compounds and γi Calculated using Eq 1 for Thirty-Six Compounds 6-12 5.76 0.70 5.71 0.61 0.49 12-18 0.23 0.01 2.02 0.13 0.22 18-6 1.56 9.08 0.70 2.07 1.40
Pinonic acid is produced through the reaction of R-pinene with both OH radicals and ozone, and pinic acid is mainly formed through the reaction of R- and β-pinene with ozone (39). The concentration levels of pinonic acid and pinic acid (Table 1) show that the first is produced faster and in higher amounts than the second under real conditions. Thus, it is clear that the atmospheric concentration of OH radicals and ozone has an effect on the composition of organic aerosol produced through the atmospheric photooxidation reactions (2-8). Conversely, the formation of nor-pinonic acid is apparently less favored as it goes through the decomposition of a substituted high-energy Griegee intermediate, which can also be deactivated leading to the formation of pinonic acid (2). Therefore, nor-pinonic acid Ri values (0.013 (0.016); Table 2) are substantially lower than those estimated for pinonic and pinic acids. In addition, pinonaldehyde and nopinone are formed fairly fast in the first steps of the R-, β-pinenes reactions with radicals and ozone (2, 3, 5). However, their low Ri values (pinonaldehyde (0.014 (0.009) and nopinone (0.007 (0.007); Table 2) are due to the photolysis and/or further photooxidation reactions of pinonaldehyde and nopinone (2, 3, 5, 7, 11, 12). Pinic acid Kom,i mean value was the highest (0.253 (0.286) m3 µg-1; Table 2) in comparison to the corresponding value calculated for the other compounds. A slightly lower value (0.170 (0.292) m3 µg-1; Table 2) was calculated for pinonic acid. These values show the preference of these compounds to the particle phase and eventually their ability to condense under certain atmospheric conditions. A low Kom,i mean value (0.097 (0.076) m3 µg-1; Table 2) was calculated for nor-pinonic acid, denoting a less important contribution to secondary organic aerosol formation. Furthermore, pinonaldehyde and nopinone Kom,i mean values were 0.042 (0.039) m3 µg-1 and 0.029 (0.028) m3 µg-1 (Table 2), respectively, pointing out that these compounds do rather not nucleate to form new particles. Overall, pinonic and pinic acids are the major photooxidation products of R-pinene photooxidation with both ozone and OH radicals and the large Kom,i values for these species suggest that the equilibrium favors the species mass in the particle phase. nor-Pinonic acid, pinonaldehyde and nopinone are also important products of R- and β-pinene reactions. The low Kom,i value for these species suggest that the equilibrium favors the gas-phase for these species and 5088
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that their presence in particles should be rather due to the absorption in formed organic particle surface. Nucleation Scenarios. The mean saturation concentration of each species calculated (eq 7) for the five different nucleation scenarios, described above, and the mean gas and particle concentrations corresponding to each sampling period were used to calculate saturation ratios (Cgas,i/Csat,i). If a species is present in both particle and gas phase the saturation ratio is equal to unity. The saturation ratios are presented in Table 3. The hypothesis for the 1st scenario is that new particles are entirely formed through the gas-phase nucleation of each single compound under ideal conditions (xi)1 and γi)1). Pinonic acid saturation ratios ranged from 0.11 × 10-4 to 0.13 × 10-3 (Table 3) being up to hundred times lower than the saturation ratios of nor-pinonic acid (0.69 × 10-6 - 0.15 × 10-5; Table 3). The saturation ratios of the monocarboxylic acids, pinonic and nor-pinonic acids, were lower than the saturation ratios of pinic acid (0.23 × 10-3 - 0.12 × 10-2; Table 3). These differences show that pinic acid could reach its saturation concentration faster than pinonic and norpinonic acids. The saturation ratios of carbonyl products were the lowest (pinonaldehyde 0.73 × 10-8 - 0.77 × 10-6 and nopinone 0.71 × 10-6 - 2.02 × 10-6; Table 3). Therefore, the role of these two products might be insignificant for this nucleation scenario, unless extremely high concentrations of monoterpenes react with atmospheric oxidants. The calculated saturation ratios, for both acidic and carbonyl photooxidation products, are considerably lower than unity (Table 3). Thus, nucleation between the molecules of a single compound, to form secondary organic aerosol formation is not achievable under ambient conditions. In the 2nd scenario, the hypothesis was that photooxidation products could form an ideal solution between each other, in the absence of preexisting organic aerosol. In this case, the saturation concentration might be significantly reduced (xi was calculated according to eq 8 and γi)1). Thus, the calculated saturation ratios for pinonic acid (0.10 × 10-4 - 0.44 × 10-3; Table 3) and pinic acid (0.66 × 10-4 0.21 × 10-1; Table 3) were higher (i.e. at least 1 order of magnitude for pinic acid) than their corresponding 1st scenario saturation ratios (Table 3). A similar trend was also observed for nor-pinonic acid (0.51 × 10-5 - 0.19 × 10-4;
Table 3), pinonaldehyde (0.12 × 10-6 - 0.48 × 10-5; Table 3) and nopinone (0.19 × 10-5 - 0.72 × 10-5; Table 3). Saturation ratios show that saturation concentrations of all identified compounds, in the absence of preexisting aerosol, were still substantially higher than the corresponding gas phase concentration measured under field conditions. These results suggest that taking into consideration only the combined presence of monoterpene photooxidation compounds would result in no nucleation of secondary organic aerosol under real atmospheric conditions. For the 3rd scenario, the measured concentrations of monoterpene photooxidation products and semi-volatile organic compounds (associated with primary organic aerosol emission) in the gas and particles were taken into account for the calculation of their respective saturation concentrations and consequently saturation ratios. The abovementioned organic compounds in aerosol phase were considered to compose an ideal solution with molar fraction xi (calculated using eq 8) and activity coefficient γI equal to unity (Table 3). Saturation concentrations of monoterpene photooxidation products further decreased and thus saturation ratios increased for all considered species (pinonic acid 0.56 × 10-4 - 0.11 × 10-2, nor-pinonic acid 0.42 × 10-5 0.27 × 10-2, pinic acid 0.43 × 10-2 - 2.30 × 10-1, pinonaldehyde 0.92 × 10-6 - 0.80 × 10-4, and nopinone 0.17 × 10-4 0.18 × 10-3; Table 3). This saturation ratio increase (or saturation concentration decrease) is due to the low contribution of photooxidation products to the total aerosol molar fraction of carbonaceous particles indicating that preexisting organic aerosol (e.g. from primary emissions) plays a significant role on SOA formation even assuming ideal conditions. The measured gas phase concentration was still significantly lower than the calculated saturation concentration (exhibiting saturation ratios
