GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L10808, doi:10.1029/2006GL025672, 2006
Evaluating aerosol nucleation parameterizations in a global atmospheric model Donald D. Lucas1 and Hajime Akimoto1 Received 6 January 2006; revised 1 April 2006; accepted 14 April 2006; published 23 May 2006.
[1] Numerically efficient parameterizations based on theories of binary, ternary and ion-induced aerosol nucleation (BN, TN and IN) enable online calculations of new particle formation in high resolution atmospheric models. These parameterizations are evaluated interactively in a 3D global chemical transport model that simulates gaseous sulfuric acid, ammonia and the ion-pair production rate. The BN and IN parameterizations produce new particles only in the cold upper troposphere, with IN giving relatively lower nucleation rates due to parameterization errors. In contrast, the TN parameterization predicts new particle formation throughout the troposphere, but gives unrealistically high nucleation rates. New and revised TN theories from recent studies instead give much lower rates and indicate that TN is not likely in the lower troposphere. Nucleation parameterizations can be useful for global modeling applications, but the current schemes should be used cautiously given their large uncertainties. Citation: Lucas, D. D., and H. Akimoto (2006), Evaluating aerosol nucleation parameterizations in a global atmospheric model, Geophys. Res. Lett., 33, L10808, doi:10.1029/ 2006GL025672.
1. Introduction [2] Knowledge of the rate of forming new aerosols in the atmosphere is vital for assessing highly-uncertain climate issues related to clouds and cloud formation. Two such issues are, for example, the postulated links between clouds and dimethyl sulfide (DMS) oxidation [Charlson et al., 1987] and galactic cosmic rays (GCRs) [Carslaw et al., 2002]. New atmospheric aerosols are believed to form by nucleation of water vapor, gaseous sulfuric acid H2SO4(g), and possibly other species (e.g., ammonia, ions, halogens, or organic vapors). Although aerosol nucleation is recommended for atmospheric models [Pirjola et al., 2004], the calculations can be computationally expensive. [3] Parameterizations have therefore been developed that enable nucleation computations in high resolution atmospheric models. Convenient parameterizations are currently available for binary (BN, H2SO4-H2O) [Vehkama¨ki et al., 2002], ternary (TN, H2SO4-H2O-NH3) [Napari et al., 2002], and ion-induced (IN, H2SO4-H2O-ions) nucleation [Modgil et al., 2005]. These schemes give the formation rates of nanosized clusters as functions of temperature (T), relative humidity (RH), H2SO4(g), NH3(g), and the ion-pair production rate (Q). Nucleation also depends indirectly on 1
Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan. Copyright 2006 by the American Geophysical Union. 0094-8276/06/2006GL025672
the existing aerosol surface area (ASA) upon which nucleation precursor gases can condense rather than nucleate. As a function of these variables, nucleation is generally most rapid at low values of T and ASA, and high values of RH, H2SO4(g), NH3(g) and Q. [4] Aerosol nucleation occurs for atmospheric conditions [Berndt et al., 2005], but the exact process remains elusive. BN is mainly ineffective in the lower troposphere because T is too high and H2SO4(g) too low [Lucas and Prinn, 2003]. Measurements have also found negligible [Eisele et al., 2006] and minor [Laakso et al., 2004] contributions of IN to ground-level nucleation, but more studies are needed before ruling out IN in the lower troposphere. Other mechanisms (e.g., TN in Jung et al. [2006]) or combinations of mechanisms (e.g., H2SO4-NH3-organics-ions in Laakso et al. [2004]) have therefore been suggested to explain nucleation in lower tropospheric regions. In the upper troposphere and lower stratosphere, in contrast, IN is believed to be the dominant mechanism [Lee et al., 2003] because Q reaches a maximum at high altitudes [Heaps, 1978], and because the high-altitude levels of other nucleating species (e.g., NH3) are thought to be too low. [5] We use a 3D global chemical transport model with detailed emission inventories of nucleation precursor gases and assimilated meteorology to simulate interactively the nucleation variables and nucleation rates. This is the first study, to our knowledge, that evaluates these BN, TN and IN parameterizations throughout the lower atmosphere for realistic conditions. Our results depend strongly on these specific nucleation schemes and will likely differ using other schemes.
2. Method [6] The 3D global MATCH model is used to simulate H2SO4(g), NH3(g), Q and the parameterized BN, TN and IN rates. This model is an extension of a former version [Lucas and Prinn, 2003, 2005] used to calculate BN rates from DMS-derived H2SO4(g). These simulations use the same spatial resolution (2.8 2.8, 28 vertical s-levels) and the 6-hourly NCEP meteorology for the year 1995. Major relevant updates are described below. [7] H2SO4(g) is simulated using a sulfur cycle that includes the previous DMS modules (the PAR scheme from Lucas and Prinn [2003]) and new modules for its production from anthropogenic and volcanic SO2. An aqueous chemistry package [Barth et al., 2000] was added and modified to account for the influence of ammonia on cloud and rainwater pH. The EDGAR V3.2 inventory [Olivier and Berdowski, 2001] for the base year 1995 defines the anthropogenic surface emissions of SO2. Emissions of SO2 from continuously erupting (‘slow leakage’) volcanoes are also included [Andres and Kasgnoc, 1998]. Aircraft-
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Figure 1. Annual zonal averages of the six input variables in the binary, ternary and ion-induced nucleation parameterizations as a function of latitude and altitude (s = pressure/surface pressure). The thin black line is the approximate tropopause height and the black squares denote the locations of the slow leakage volcanoes. based SO2 is a minor portion of the global sulfur burden, but may be important to nucleation at cruise altitudes. Aircraft SO2 emissions are estimated from the EDGAR aircraft CO2 inventory using emission factors (3.1 kg CO2/ kg fuel, 0.001 kg SO2/kg fuel). Stratospheric carbonyl sulfide (OCS) oxidation can also play a role in aerosol nucleation. This source is estimated from altitude-varying OCS loss rates [Chin and Davis, 1995]. [8] The model includes a new ammonia cycle that transports NH3(g) and aerosol NH+4 separately. Terrestrial and oceanic sources of ammonia are considered [Bouwman et al., 1997], where the terrestrial emissions are given as annual surface fluxes. Oceanic fluxes are calculated from prescribed monthly sea surface concentrations and a sea-air transfer velocity. The ammonia flux can be into or out of the ocean [Asman et al., 1994], so it is calculated interactively as a function of the sea-air concentration difference and Henry’s law solubility at preset sea surface temperatures. The original NH3 sea surface concentrations were derived assuming negligible atmospheric NH3(g), so they were reduced by a factor of two as suggested [Bouwman et al., 1997]. The oxidation of NH3(g) by OH, wet and dry deposition of NH3(g) and NH+4 , and the scavenging of NH3 by acidic aerosols are also included. [9] The ion-pair production rate, Q, used in IN is calculated from GCRs and 222Rn decay. GCRs are specified as a function of the 11-year solar cycle, latitude and altitude. In the top 8 levels of MATCH we use a pressure-dependent GCR formulation [Heaps, 1978], at the surface we use Hensen and van der Hage [1994], and between these levels we linearly interpolate log Q. The 222Rn source is from Rasch et al. [2000] and each radon decay yields 6.4 105 ion pairs [Martell, 1985]. [10] BN, TN and IN rates are calculated using the parameterizations of Vehkama¨ki et al. [2002], Napari et al. [2002], and Modgil et al. [2005], respectively. A stiff solver is used to resolve conditions of rapid nucleation, which can quickly deplete H2SO4(g). The schemes are evaluated sequentially, alternating first between BN and IN, but always solving TN last. The schemes are only applied within their defined ranges of validity (e.g., RH = 0.05 –0.95 for TN). For
BN and TN, the ranges are mainly obeyed, but the occasional violations are handled as follows: T outside of its range is set to its nearest limit; RH, H2SO4 or NH3 are set to their upper limits if they are too high; BN or TN rates are set to their lower limits if RH, H2SO4 or NH3 are too low; and, the BN rate is set to its lower limit if the critical cluster contains less than four molecules. The IN parameterization is only applied for T 260 K to prevent the scheme from diverging at high temperatures. This restriction does not affect our results because IN is inefficient at high T. The IN fit is also sometimes poor at very low H2SO4, so values between 1 105 to 3.2 105 cm 3 are set to the upper limit of this range. Other IN adjustments are similar as before (e.g., values of RH, Q and ASA outside of their ranges are set to their limiting values). Also, TN and IN should coincide with BN in the limit of low NH3 and ions, but different assumptions in the parameterizations prevent this from occurring.
3. Aerosol Nucleation Variables [11] The six variables used as inputs in the nucleation parameterizations are displayed in Figure 1. These variables vary strongly with altitude and latitude, so they are shown as annual zonal average vertical profiles. Of the variables, T is from NCEP, RH is from the hydrological cycle in MATCH, ASA used in the IN scheme is prescribed [see Lucas and Prinn, 2005], Q is from the 222Rn and GCRs, and H2SO4(g) and NH3(g) are detailed more below. [12] In a former study [Lucas and Prinn, 2003] the simulated H2SO4(g) agreed reasonably well with remote marine observations, and the uncertainties in DMS oxidation were shown to have a small impact on the nucleation rate conclusions. Relative to that study, the annual H2SO4(g) levels in Figure 1 are larger and located in the Northern Hemisphere due to anthropogenic SO2 emissions and at the sites of SO2-emitting volcanoes. The maximum levels (>107 cm 3) are associated with the slow leakage volcanoes, which provides a critical test of their influence on aerosol nucleation. Unlike the volcanic emissions, the high altitude aircraft SO2 emissions are not readily discernible. Aircraft SO2 is prescribed at cruise altitudes near the
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Figure 2. Annual zonal averages of aerosol nucleation rates using the binary, ion-induced and ternary nucleation parameterizations. Nucleation rates are in units of log10(cm 3 s 1). Note that a different scale is used for ternary nucleation. The black lines and squares are described in Figure 1. tropopause of the Northern Hemisphere, but there is only a small H2SO4(g) enhancement at that region. Also, the simulated H2SO4(g) levels near the tropopause in the northern midlatitudes are comparable to values (1 106 cm 3) derived in a previous IN study in the same region [Lee et al., 2003]. [13] For the general assessment purposes of this study, the modeled NH3(g) mole fractions in Figure 1 broadly agree with observations [e.g., Asman et al., 1994]. The levels over background and polluted continental surface areas are about 103 parts per trillion (ppt) and 104 ppt, respectively. The levels in the tropical and extratropical marine boundary layer are about 100– 500 ppt and 10– 100 ppt, respectively. With height, NH3(g) decays to upper tropospheric values of about 1 – 10 ppt in the tropics and northern extratropics and sub-ppt levels in the southern extratropics. The NH3(g) spatial trends are also similar to those given by Dentener and Crutzen [1994], but the magnitudes in MATCH are typically lower everywhere except the northern extratropical upper troposphere. This implies that, all other factors being equal, the NH3(g) from Dentener and Crutzen [1994] would give higher TN rates than those in MATCH nearly everywhere.
4. Aerosol Nucleation Rates [14] The annual zonal vertical profiles of the parameterized BN, IN and TN rates are shown in Figure 2. The BN rates are similar to those given by Lucas and Prinn [2003]. Both studies have high BN rates near the tropopause and very low BN rates in the warm regions of the lower troposphere. The BN rates in this study are larger, however, due to the anthropogenic and volcanic SO2. The largest annual BN rates (300 cm 3 s 1) are associated with volcanic emissions, which suggests that even slow leaking volcanoes may promote aerosol nucleation in the troposphere. Aircraft-based SO2 may also impact BN near the tropopause in the Northern Hemisphere, but this impact seems to be smaller than from other SO2 sources and will require further analysis. Figure 2 also agrees with previous studies showing that BN is not sufficient for explaining aerosol nucleation in the lower troposphere because the annual BN rates there are less than 10 5 cm 3 s 1 in most locations. [15] The annual IN rates have a similar pattern as the BN rates, with large values near the tropopause and very low rates in the lower troposphere. This overall pattern follows primarily from the altitude dependencies of T and Q. IN rates drop to very low values above the tropopause, how-
ever, due to H2SO4(g) levels that are below the parameterization’s lower limit of 105 cm 3. Moreover, the critical H2SO4(g) values required for significant nucleation by this specific IN parameterization do not occur in our simulations. At the conditions most conducive to IN (i.e., low ASA and T, and high RH and Q), this IN scheme requires H2SO4(g) values greater than about 3 106 cm 3 for IN rates greater than 1 cm 3 s 1. These critical sulfuric acid levels only occur at lower altitudes where it is too warm and Q is too small. Figure 2 also shows a systematic discrepancy between the BN and IN rates. Ions should enhance nucleation relative to BN, but the rates in the figure show the opposite trend. The relatively lower IN rates, which are likely caused by thermodynamical differences, exposes a very large uncertainty in this IN parameterization. Also, the negligible IN rates in the lower troposphere in Figure 2 are supported by recent measurements [Eisele et al., 2006], but note that other ion models [e.g., Yu and Turco, 2000] suggest that ions may contribute to lower tropospheric nucleation. [16] In contrast to BN and IN, the annual zonal average TN rates are very high throughout the troposphere. TN rates larger than 105 cm 3 s 1 are calculated over most continental regions emitting ammonia, and rates as large as 103 cm 3 s 1 are even predicted over some remote areas (e.g., the central tropical Pacific). If the TN mechanism is valid, our simulations would suggest that TN is the most likely aerosol formation path in the troposphere. This TN parameterization, however, yields much higher rates than required by observations of ultrafine particles. Inferred nucleation rates of detectable particles (>3 nm) in the boundary layer are typically 10 2 –101 cm 3 s 1, but can be up to 102 cm 3 s 1 in urban regions and 104 –105 cm 3 s 1 in highly-polluted plumes [Kulmala et al., 2004]. Relative to these values, our annual simulated TN rates in the nonpolluted boundary layer are about 2–4 orders of magnitude too large. These excessive rates have been characterized in new [Yu, 2006] and revised [Anttila et al., 2005] TN models, which reduce the rates by many orders of magnitude and indicate that TN is probably not important in the lower troposphere. TN may still be important at higher altitudes, but a new parameterization is not yet available for testing.
5. Uncertainties [17] Many sources of uncertainty need to be considered when using these nucleation parameterizations. Indetermi-
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nate H2SO4(g) levels can introduce enormous uncertainties into the nucleation rate calculations. There are also numerical errors in using parameterized fits instead of actual nucleation models. These numerical errors were estimated in the parameterization papers by comparing the fits to the actual values over relevant ranges of conditions. The BN, TN and IN fits agreed with theory to within factors of about 10, 10 and 10– 100, respectively. Our simulated TN and BN rate differences in Figure 2 are therefore robust because their fitting errors are comparatively small. The relatively larger error range for the IN fit may also partially explain the IN and BN rate discrepancy. [18] The uncertainties in the underlying nucleation theories are also very critical. Our simulated TN rates, for example, are too high by several orders of magnitude because the parameterization is based on an old TN theory. New and revised TN theories [Yu, 2006; Anttila et al., 2005] instead indicate that TN is not likely in the lower troposphere. Even though the old TN scheme seems to have some predictive capability [Jung et al., 2006], we do not advocate its use in its current form. The widely-studied BN theory is also susceptible to large theoretical errors. Improvements by Vehkama¨ki et al. [2002] increased the BN rates by 1 – 4 orders of magnitude over their previous BN theory. The IN parameterization also clearly has large uncertainties because its rates are systematically lower than the BN rates. Moreover, unlike the IN scheme tested here, other ion nucleation models [e.g., Yu and Turco, 2000] can give efficient nucleation in the lower troposphere. Due to these many large uncertainties, we recommend caution when using these nucleation parameterizations. [19] Acknowledgments We thank M. Modgil and S. Tripathi for discussions about IN. D.D.L. also thanks the FRCGC for their support.
References Andres, R. J., and A. D. Kasgnoc (1998), A time-averaged inventory of subaerial volcanic sulfur emissions, J. Geophys. Res., 103(D19), 25,251 – 25,261. Anttila, T., H. Vehkama¨ki, I. Napari, and M. Kulmala (2005), Effect of ammonium bisulphate formation on atmospheric water-sulphuric acidammonia nucleation, Boreal Environ. Res., 10, 511 – 523. Asman, W., R. Harrison, and C. Ottley (1994), Estimation of the net air-sea flux of ammonia over the southern bight of the North Sea, Atmos. Environ., 28(22), 3647 – 3654. Barth, M. C., P. J. Rasch, J. T. Kiehl, C. M. Benkovitz, and S. E. Schwartz (2000), Sulfur chemistry in the National Center for Atmospheric Research Community Climate Model: Description, evaluation, features, and sensitivity to aqueous chemistry, J. Geophys. Res., 105(D1), 1387 – 1415. Berndt, T., et al. (2005), Rapid formation of sulfuric acid particles at nearatmospheric conditions, Science, 309, 698 – 700. Bouwman, A. F., D. S. Lee, W. A. H. Asman, F. J. Dentener, K. W. Van Der Hoek, and J. G. J. Olivier (1997), A global high-resolution emission inventory for ammonia, Global Biogeochem. Cycles, 11(4), 561 – 587. Carslaw, K., R. Harrison, and J. Kirkby (2002), Cosmic rays, clouds, and climate, Science, 298, 1732 – 1737.
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Charlson, R., J. Lovelock, M. Andreae, and S. Warren (1987), Oceanic phytoplankton, atmospheric sulphur, cloud albedo, and climate, Nature, 326, 655 – 661. Chin, M., and D. D. Davis (1995), A reanalysis of carbonyl sulfide as a source of stratospheric background sulfur aerosol, J. Geophys. Res., 100(D5), 8993 – 9005. Dentener, F., and P. Crutzen (1994), A three-dimensional model of the global ammonia cycle, J. Atmos. Chem., 19, 331 – 369. Eisele, F. L., E. R. Lovejoy, E. Kosciuch, K. F. Moore, R. L. Mauldin III, J. N. Smith, P. H. McMurry, and K. Iida (2006), Negative atmospheric ions and their potential role in ion-induced nucleation, J. Geophys. Res., 111, D04305, doi:10.1029/2005JD006568. Heaps, M. G. (1978), Parameterization of the cosmic ray ion-pair production rate above 18 km, Planet. Space Sci., 26, 513 – 517. Hensen, A., and J. C. H. van der Hage (1994), Parameterization of cosmic radiation at sea level, J. Geophys. Res., 99(D5), 10,693 – 10,695. Jung, J., P. Adams, and S. Pandis (2006), Simulating the size distribution and chemical composition of ultrafine particles during nucleation events, Atmos. Environ., 40, 2248 – 2259. Kulmala, M., et al. (2004), Formation and growth rates of ultrafine atmospheric particles: A review of observations, J. Aerosol Sci., 35, 143 – 176. Laakso, L., et al. (2004), Kinetic nucleation and ions in boreal forest particle formation events, Atmos. Chem. Phys., 4, 2353 – 2366. Lee, S.-H., et al. (2003), Particle formation by ion nucleation in the upper troposphere and lower stratosphere, Science, 301, 1886 – 1889. Lucas, D. D., and R. G. Prinn (2003), Tropospheric distributions of sulfuric acid-water vapor aerosol nucleation rates from dimethylsulfide oxidation, Geophys. Res. Lett., 30(22), 2136, doi:10.1029/2003GL018370. Lucas, D. D., and R. G. Prinn (2005), Sensitivities of gas-phase dimethylsulfide oxidation products to the assumed mechanisms in a chemical transport model, J. Geophys. Res., 110, D21312, doi:10.1029/ 2004JD005386. Martell, E. A. (1985), Enhanced ion production in convective storms by transpired radon isotopes and their decay products, J. Geophys. Res., 90(D4), 5909 – 5916. Modgil, M. S., S. Kumar, S. N. Tripathi, and E. R. Lovejoy (2005), A parameterization of ion-induced nucleation of sulphuric acid and water for atmospheric conditions, J. Geophys. Res., 110, D19205, doi:10.1029/ 2004JD005475. Napari, I., M. Noppel, H. Vehkama¨ki, and M. Kulmala (2002), Parameterization of ternary nucleation rates for H2SO4-NH3-H2O vapors, J. Geophys. Res., 107(D19), 4381, doi:10.1029/2002JD002132. Olivier, J., and J. Berdowski (2001), Global emissions sources and sinks, in The Climate System, edited by J. Berdowski, R. Guicherit, and B. Heij, pp. 33 – 77, A. A. Balkema, Brookfield, Vt. Pirjola, L., K. E. J. Lehtinen, H.-C. Hansson, and M. Kulmala (2004), How important is nucleation in regional/global modelling?, Geophys. Res. Lett., 31, L12109, doi:10.1029/2004GL019525. Rasch, P., et al. (2000), A comparison of scavenging and deposition processes in global models: Results from the WCRP Cambridge Workshop of 1995, Tellus, Ser. B, 52(4), 1025 – 1056. Vehkama¨ki, H., M. Kulmala, I. Napari, K. E. J. Lehtinen, C. Timmreck, M. Noppel, and A. Laaksonen (2002), An improved parameterization for sulfuric acid-water nucleation rates for tropospheric and stratospheric conditions, J. Geophys. Res., 107(D22), 4622, doi:10.1029/ 2002JD002184. Yu, F. (2006), Effect of ammonia on new particle formation: A kinetic H2SO4-H2O-NH3 nucleation model constrained by laboratory measurements, J. Geophys. Res., 111, D01204, doi:10.1029/2005JD005968. Yu, F., and R. P. Turco (2000), Ultrafine aerosol formation via ion-mediated nucleation, Geophys. Res. Lett., 27(6), 883 – 886. H. Akimoto and D. D. Lucas, Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showamachi, Yokohama 236-0001, Japan. (
[email protected];
[email protected])
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