[2000], Seinfeld and Pandis [1998], and. Yu et al. [1998]). With the observed anticorrelation of nucleation threshold and dew point, it will be possible to.
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 11, 1585, doi:10.1029/2003GL017000, 2003
Sesquiterpene ozonolysis: Origin of atmospheric new particle formation from biogenic hydrocarbons Boris Bonn and Geert K. Moortgat Atmospheric Chemistry Division, Max-Planck-Institute for Chemistry, Mainz, Germany Received 28 January 2003; revised 22 April 2003; accepted 6 May 2003; published 11 June 2003.
[1] Atmospheric new aerosol particle formation observed in remote areas (e.g., in Finland, Portugal and in the U.S.). is generally attributed to low-volatile oxidation products of monoterpenes (C10H16), which are emitted by the vegetation. In this article we show that this atmospheric new particle formation is not caused by monoterpene products, but most likely initiated by very low-volatile substances produced during sesquiterpene (C15H24)-ozone reactions. For this purpose, the nucleation times of the most abundant monoterpene reactions have been calculated and discussed exemplarily for the Finnish site Hyytia¨la¨, at which nucleation events have been observed. In addition, the important negative influence of water vapor on the nucleation threshold of the b-caryophyllene-ozone reaction has been studied in detail at different dew points in the laboratory. Therein, the saturation vapor pressure of the nucleating compounds was estimated to be less than 1.2 10 10 hPa, which is recommended for atmospheric homogeneous INDEX TERMS: 0305 nucleation of non-volatile organics. Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0317 Atmospheric Composition and Structure: Chemical kinetic and photochemical properties. Citation: Bonn, B., G. K. Moortgat, Sesquiterpene ozonolysis: Origin of atmospheric new particle formation from biogenic hydrocarbons, Geophys. Res. Lett., 30(11), 1585, doi:10.1029/2003GL017000, 2003.
1. Introduction [2] The impact of atmospheric aerosols on the global radiation budget, directly as well as indirectly, is one of the major topics to be solved in order to understand earth’s climate and the anthropogenic contribution to climate change. The organic aerosol fraction including the aerosol formed by nucleation and condensation of gaseous lowvolatile substances (secondary organic aerosol, SOA) is one of the major constituents of atmospheric particulate matter [Andreae and Crutzen, 1997; Andersson-Sko¨ld and Simpson, 2001; Griffin et al., 1999; Kanakidou et al., 2000], especially at remote sites such as boreal or tropical forests. These organic compounds are formed by the atmospheric oxidation reactions of O3, OH and NO3 with different reactive hydrocarbons such as mono- and sesquiterpenes. In this context, the topic highly discussed recently, is the connection of atmospheric new particle formation such as observed at various locations in remote areas (e.g., in Finland, Portugal as well as in the U.S.) [Ma¨kela¨ et al., 1997; Kavouras et al., 1998; Kavouras and Stephanou, Copyright 2003 by the American Geophysical Union. 0094-8276/03/2003GL017000
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2002; Leaitch et al., 1999; Went, 1960] with the oxidation of these biogenic terpenes, i.e., monoterpenes (C10H16) [Hoffmann et al., 1997; Koch et al., 2000]. Unfortunately, it is currently impossible to determine online the aerosol composition of the smallest aerosol particles with a size of 2 nm in diameter. Because of the very small mass of these freshly formed particles, a time consuming accumulation period is needed in order to obtain sufficient material for analysis. During this time unstable products, most likely involved in the initial nucleation process [Bonn et al., 2002], further react with atmospheric oxidants, such as ozone or OH, on the filter surface. This has been found to be a crucial problem especially for organics (e.g., Kirchstetter et al. [2000]). Consequently, the initial products responsible for the nucleation events have not been identified by chemical analysis yet. [3] By contrast, it has most recently been shown by observations of the aerosol particle size distributions, that the homogeneous nucleation of SOA is controlled by the stabilized Criegee intermediate (CI) (see Figure 1), formed in the ozone reaction of biogenic terpenes [Bonn et al., 2002; Bonn and Moortgat, 2002]. This CI further reacts with either carbonyl compounds (which are generated as coproducts in the ozonolysis reaction) forming secondary ozonides (SOZ) or with water vapor [Calvert et al., 2000]. The latter reaction is dominant in the atmosphere due to the high concentration of water vapor. In the atmosphere CI may also react with SO2, which forms organic sulfates. It also has been shown that large SOZ initiate nucleation more effectively in exocyclic reactions (external carbon double bond, e.g., of b-pinene), than in endocyclic ones (internal carbon double bond, e.g., of a-pinene, Figure 1) [Bonn et al., 2002]. Consequently, the intensity of nucleation is found to be anticorrelated with the atmospheric water vapor concentration [Boy and Kulmala, 2002], since water vapor reduces the formation of these SOZ, because of the competitive reactions of CI with carbonyl compounds and water vapor [Bonn et al., 2002; Bonn and Moortgat, 2002]. [4] Consequently, there is a need to study the property of monoterpenes to form new aerosol particles under atmospheric conditions in detail by calculating the nucleation threshold and the time needed to initiate nucleation in the atmosphere.
2. Atmospheric Nucleation Timescales of Monoterpene Oxidation Products 2.1. Calculation of Nucleation Time for Atmospheric Conditions [5] In order to discuss the possibility of atmospheric nucleation events caused by monoterpene ozonolysis products, the nucleation threshold, which is the amount of a - 1
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BONN AND MOORTGAT: ATMOSPHERIC NUCLEATION BY SESQUITERPENES
Figure 1. Exemplary scheme of monoterpene-ozone reactions and the influence of water vapor on homogeneous nucleation using a-pinene. The pathways not assumed to cause homogeneous nucleation are marked with dashed arrows. The star symbolizes the excited and the dagger the stabilized CI. specific terpene to be reacted prior to particle formation, was calculated with a simple FACSIMILE model (kinetic equations solver routine) from earlier published laboratory measurements [Koch et al., 2000]. The amount of monoterpene that was converted at the nucleation time was taken as nucleation threshold, taking into account the OH radicals generated during ozonolysis [Atkinson, 1997; Calvert et al., 2000]. [6] The nucleation thresholds for the atmospheric most relevant monoterpenes, i.e., a-pinene, b-pinene, sabinene, limonene and 3-carene, obtained by Koch et al. [2000] for dry laboratory conditions (dew point of 80C), are displayed in Table 1. From this, it is obvious that all nucleation thresholds of the atmospheric monoterpenes are situated around 2 ppbv. Furthermore, there is a small impact of the monoterpene structure on this value: The nucleation thresholds of ozone reactions with endocyclic compounds (e.g., a-pinene or 3-carene) are slightly higher than those of the reactions with exocyclic compounds (e.g., b-pinene and sabinene). Moreover, it has been shown that nucleation caused by exocyclic reactions (e.g., sabinene-ozone) can totally be suppressed by the presence of atmospheric water vapor concentrations during 50 ppbv conversion of the monoterpene, whereas nucleation originating from endocyclic compounds (e.g., a-pinene and 3-carene) are less affected by water vapor [Bonn et al., 2002; Bonn and Moortgat, 2002]. The nucleation thresholds of endocyclic reactions have been found to be at least 10 ppbv under atmospheric conditions (dew point of 4C and higher) [Bonn et al., 2002]. By contrast, atmospheric mixing ratios of monoterpenes are reported to be in the order of 200 pptv (e.g., for a-pinene) (e.g., Fuentes et al. [2000], Hakola et al. [2000]) or even less at noon and up to 2 ppbv in the night [Harrison et al., 2001]. [7] In order to calculate the time needed for each monoterpene ozonolysis reaction to initiate new particle formation
as observed during the nucleation (‘blue haze’) events under atmospheric conditions (e.g., Ma¨kela¨ et al. [1997]; Kavouras et al. [1998]; Leaitch et al. [1999]), a simple box model has been used, which describes the atmospheric chemistry as well as the meteorology. This box model have been set up earlier by Po¨schl et al. [2000] in order to test the Mainz Isoprene Mechanism (MIM) and was modified for this purpose by incorporation of the primary monoterpene reactions with ozone, OH and NO3. Therein, the meteorological and chemical conditions were chosen as found close to the Finnish site Hyytia¨la¨ [Hakola et al., 2000, 2001], at which these events were observed (e.g., Ma¨kela¨ et al. [1997]; Boy and Kulmala [2002]) except for the relative humidity, which was set to 0% due to the gap of knowledge about the functional dependence of the nucleation threshold on the present vapor. Because of the selected low humidity, the nucleating species will be produced faster and nucleation is setting in earlier than in the situation, in which water vapor is taken into account. The obtained atmospheric nucleation times of the most abundant monoterpenes at this site (e.g., a-pinene) are listed in the right column of Table 1 together with the nucleation thresholds obtained from the laboratory [Koch et al., 2000] and maximum mixing ratios observed at the site [Hakola et al., 2000]. [8] From this table it is obvious that atmospheric monoterpene-ozone reactions have to persist for 6 days (a-pinene) up to 170 days (b-pinene) to form enough nonvolatile material to initiate homogeneous new particle formation even at very dry conditions. During this period the very low-volatile substances produced would be diluted and transported by passing air. Predominantly, they would condense on pre-existing aerosol particles, always present in concentrations around 1000 cm 3 [Ma¨kela¨ et al., 1997], and new particle formation would be suppressed. If the effect of water vapor is considered, then the discrepancy between observed new particle formation and the time needed for nucleation is even much more pronounced. As a result, monoterpene reaction products cannot cause atmospheric nucleation. It is therefore more plausible that nucleation has to be initiated by other reactions of ozone with reactive biogenic hydrocarbons to explain the observed effect of water vapor in the laboratory [Bonn et al., 2002] as well as in the atmosphere [Boy and Kulmala, 2002].
3. Sesquiterpenes 3.1. Laboratory Measurements and Modeling Results [9] This criterion is fulfilled by the sesquiterpenes, whose reactions with either OH- as well as NO3-radicals are of much less importance in the atmosphere, due to their ca. Table 1. Calculated Nucleation Thresholds and Atmospheric Nucleation Times of Selected Monoterpenes Measured at Hyytia¨la¨ (Finland) monoterpene
max. mixing ratioa [pptv]
nucleation thresholdb [ppbv]
calculated atmospheric nucleation time [days]
a-pinene 3-carene d-limonene b-pinene sabinene
200 35 15 30 35
2.1 2.3 1.9 2.0 1.3
6 66 29 167 17
a
Taken from Hakola et al. [2000]. Calculated using the findings of Koch et al. [2000].
b
BONN AND MOORTGAT: ATMOSPHERIC NUCLEATION BY SESQUITERPENES
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which was obtained in the laboratory, is exceeded at the site of Hyytia¨la¨ within less than 3 min. Therefore the sesquiterpene maximum mixing ratio is deduced to be around 1 pptv at midday. This value has been approximated from recent measurements of emission fluxes of both terpenes, b-caryophyllene and a-pinene [Hakola et al., 2001]. For a better approximation of the sesquiterpene maximum mixing ratio measurements of nucleation events and sesquiterpene fluxes are needed in parallel.
Figure 2. Displayed is the dependence of the nucleation threshold, i. e., the amount of sesquiterpene reacted prior to particle formation, in pptv on the present dew point in C for the gas-phase reaction of b-caryophyllene with ozone: Points symbolize the measured dataset, the dashed line the fitting curve. Nucleation occurs above the dashed line. 100 times higher reactivity with ozone as compared to the monoterpenes [Atkinson, 1997]. Because of their effective reaction with ozone, the influence of water vapor on the process of new particle formation is much less pronounced at mixing ratios similar to the monoterpenes. This effect is attributed to a faster production of competitive reaction partners of the CI. To show the relevance of sesquiterpene (C15H24)-ozonolyses for atmospheric aerosol formation, ozonolysis experiments have been performed with b-caryophyllene, which is the most abundant sesquiterpene at nearly atmospheric conditions. Laboratory experiments with 25 pptv b-caryophyllene and 50 ppbv ozone in a spherical glass vessel of 570 l volume [Bonn et al., 2002; Koch et al., 2000] in the absence of an OH-scavenger due to the low OH yield of 6% [Shu and Atkinson, 1994] showed very intensive nucleation events. These exceeded 160,000 new particles cm 3 at a dew point of 80C in the initial nucleation stage with a particle size distribution centered around 20 nm in diameter around 60 s of reaction time. In order to determine the influence of water vapor on the aerosol formation, the nucleation threshold was observed for various dew points ranging from 80 to + 8C (atmospheric conditions) in a flow reactor, which was already described by Bonn et al. [2002]. Therefore, the ozone concentration was held constant at 50 ppbv and the sesquiterpene concentration was varied by changing the flow rate taken from a pre-diluted mixture of b-caryophyllene and nitrogen. The change in the total flow rate was compensated by reduction or increase of the synthetic air flow. The number concentration of particles larger than 3 nm in radius was measured continuously with a condensation particle counter (CPC, TSI 3025A). The conditions, at which the number concentration exceeded three times the lower instrument concentration limit, were used to calculate the sesquiterpene conversion, the nucleation threshold. By varying the sesquiterpene mixing ratio a different reaction progress was adjusted, while measuring at same reaction time. The results are displayed in Figure 2. Nucleation takes place above the dashed fitted line, not below. This dataset indicates the potential of atmospheric nucleation originating from sesquiterpene ozonolysis products. e.g., a nucleation threshold of 2,5 pptv sesquiterpene conversion at a dew point of 4C,
3.2. Estimation of Saturation Vapor Pressure of Nucleating Species [10] Moreover, Figure 2 was used to estimate the upper limit of the saturation vapor pressure of the nucleating species. Therefore, the nucleation threshold at a relative humidity 0% (dew point = 273C) was obtained from the offset of the fitted line in Figure 2. At this state, no water vapor is present and the stabilized Criegee Intermediate forms almost completely the nucleating species (see Figure 1). For calculation of the saturation vapor pressure of the nucleating species the offset was multiplied by a) the stabilization ratio of the ozone reaction, whose upper limit was estimated to be 85% using the results of an earlier publication of Shu and Atkinson [1994], and b) the present reactor pressure. With the corresponding nucleation threshold of 0.15 pptv (4 106 molecule cm 3 at atmospheric pressure and room temperature) at the offset and atmospheric pressure an upper limit of the saturation vapor pressure was obtained to 1.2 10 10 hPa (9.0 10 11 Torr). This value needs to be corrected for molecular properties of the nucleating species such as surface tension, its density and its bondings, which increase the needed supersaturation for homogeneous nucleation (Kelvin effect) and therefore lowers the obtained saturation vapor pressure [Pruppacher and Klett, 1997]. Due to the incomplete knowledge of the chemical properties of the nucleating species, which might be oligomeres etc. of the stabilized CI, the Kelvin effect was not corrected. This result fulfills the recommendations of Kerminen et al. [2000] for atmospheric homogeneous nucleation events of non-volatile organics. By contrast, this is not reached by very low-volatile compounds of monoterpenes such as pinic acid, a dicarboxylic acid, whose vapor pressure is reported to be between 2.4 10 6 hPa (1.8 10 6 Torr) (calculated from Asher et al. [2002]) and 7.5 10 8 hPa (5.6 10 8 Torr) [Koch et al., 2000]. Nor is the vapor pressure, obtained in this study, reached by dicarboxylic acids of sesquiterpene reactions, such as 7-keto-b-caryophyllic acid with an estimated saturation vapor pressure of 1. 110 9 hPa (8. 110 10 Torr) at room temperature (see Asher et al. [2002] for details). [11] Consequently, the reaction of the sesquiterpenes with ozone will most likely be the origin of the observed atmospheric biogenic secondary organic aerosol (SOA) formation. In a later stage, more volatile reaction products such as those arising from the monoterpene reactions (e.g., dicarboxylic acids) will contribute to a particle growth, as observed by numerous studies (e.g., by Hoffmann et al. [1997], Koch et al. [2000], Seinfeld and Pandis [1998], and Yu et al. [1998]). With the observed anticorrelation of nucleation threshold and dew point, it will be possible to predict atmospheric nucleation events such as found in Finland [Ma¨kela¨ et al., 1997] in combination with terpene
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BONN AND MOORTGAT: ATMOSPHERIC NUCLEATION BY SESQUITERPENES
emission rates, SO2 concentrations, meteorological conditions in the boundary layer (e.g., mixing state, origin of air masses), background aerosol concentrations and solar radiation intensity. Consequently, clean air masses containing low SO2- and water vapor as well as aerosol number concentrations would favor new particle formation.
4. Future Needs and Outlook [12] Unfortunately, it is currently not possible to directly measure sesquiterpene mixing ratios because of their high reactivity with ozone during the collection period, since the terpene has already completely reacted prior to their analysis. However, the anticorrelation of the nucleation threshold and water vapor mixing ratio, displayed as dashed line in Figure 2, can serve as an approximation of the atmospheric mixing ratio of sesquiterpenes, if the nucleation is assumed to occur only by the sesquiterpene ozonolysis products and not by e.g., sulphuric acid [Kulmala et al., 2000]. Furthermore, Figure 2 allows to compare sesquiterpene mixing ratios obtained from the analysis of nucleation events and future sesquiterpene mixing ratio measurements, so to bring final evidence for the presented hypothesis. [13] Recent observations of sesquiterpene emissions fluxes from the biosphere [Ciccioli et al., 1999; Hakola et al., 2000] indicate that the role of sesquiterpenes is by far underestimated. With respect to these observations, secondary organic aerosol formation by sesquiterpene ozonolyses will be a significant contribution to the organic aerosol mass as indicated by earlier laboratory measurements regarding the aerosol volume yield. The latter was found to be close to 100%, obtained under unrealistic high initial sesquiterpene concentrations of 100 ppbv [Hoffmann et al., 1997]. Future investigations of the emission behavior of the vegetation under different meteorological conditions and detailed studies of sesquiterpene oxidation mechanisms will lead to a significantly better understanding of the impact of atmospheric aerosols on the earths climate and will hopefully reveal the important role of sesquiterpenes in this context. [14] Acknowledgments. B. B. would like to thank T. Laurila for supporting material. This study was funded by the European Commission within the project OSOA (EVK2-1999-00016).
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B. Bonn and G. K. Moortgat, Air Chemistry Division, Max-PlanckInstitute for Chemistry, P. O. Box 3060, 55020 Mainz, Germany. (bonn@ mpch-mainz.mpg.de)
