Supplemental Information: Chemically resolved particle fluxes over tropical and temperate forests
Delphine K. Farmer1*, Qi Chen2**, Joel R. Kimmel1,3,4, Kenneth S. Docherty1***, Eiko Nemitz5, Paulo A.
Artaxo6, Christopher D. Cappa7, Scot T. Martin2, Jose L. Jimenez1
1. CIRES and Dept. of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
2. School of Engineering and Applied Sciences and Dept. of Earth and Planetary Sciences, Harvard University,
Cambridge, MA, USA
3. Tofwerk AG, Thun, Switzerland
4. Aerodyne Research, Inc., Billerica, MA, USA
5. Center for Ecology and Hydrology, Penicuik, UK
6. Institute of Physics, University of São Paulo, São Paulo, Brazil
7. Dept. Civil and Environmental Engineering, University of California, Davis, CA, USA
* Now at Dept. of Chemistry, Colorado State University, Fort Collins, CO, USA
** Now at Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA,
USA
***Now at Alion Science & Technology, US EPA Office of Research & Development, Research Triangle Park, NC,
USA
Corresponding Authors: Jose L. Jimenez (jose.jimenez@colorado.edu) and Delphine K. Farmer
(Delphine.Farmer@colostate.edu)
S1. BEARPEX-07 Calculation details
S1.1 Partitioning flux due to thermal gradients
In order to determine the organic aerosol flux due to gas-particle partitioning along the vertical thermal
gradient, we assume a volatility distribution for total organic aerosol similar to that observed during the
MILAGRO and SOAR campaigns (Cappa and Jimenez 2010). We also assume constant concentrations of
SVOCs through the canopy profile; while these concentrations are subject to dry deposition, biogenic
emission, and in-canopy formation from rapid oxidation, no observations of fluxes or gradients of semivolatile organic compounds are available to refine this assumption. The typical residence time for
particles within the canopy and below the sensor plane is related to the measurement height (25 m) and
aerodynamic resistance (Martens et al. 2004). The average mid-day aerodynamic resistance (Gallagher
et al. 1997) was 3.5 s m-1, resulting in a canopy residence time of 90 s. Assuming a linear temperature
shift from 28°C (ground level) to 26°C (canopy top) for 4 μg m-3 over 90 s, we estimate that particles
moving from the bottom to the top of the canopy gained ~0.7%, or 0.029 μg m-3 of additional mass due
to gas-particle partitioning (Cappa 2010). This mass change corresponds to a flux of -8 ng m-2 s-1, on the
order of the estimated maximum dry deposition OA flux determined from sulfate deposition (10-12 ng
m-2 s-1). However, we note that deposition and production of SVOCs and organic aerosol shifts the
equilibrium partitioning and thus affects the organic aerosol concentration gradient. Predicting the
resulting partitioning requires unavailable observational constraints on deposition and production rates
of semi-volatile organic compounds (which are predominately produced from oxidation of VOCs and not
from direct emission), which may vary with volatility and water solubility. We speculate that the net
result of these effects would be an observed downward flux, but smaller in magnitude than calculated
here. Finally we also note that some recent results suggest that evaporation of organic aerosol in this
region may be kinetically limited and thus slower than assumed in our model (Vaden et al. 2011); in that
case the influence of repartitioning within the organic aerosol residence time within the canopy (~90 s)
may be smaller than estimated here.
S1.2. In-canopy SOA yields from VOC emissions
Using SOA yields for the organic aerosol mass concentrations observed in our study, we provide new
estimates for the potential SOA flux from Blodgett Forest from typical VOC fluxes and SOA yields. The
isoprene flux at Blodgett Forest is near-zero, but the daytime monoterpene and sesquiterpene fluxes are
on the order of +4.1 and +0.41 µmol m-2 hr-1 (Holzinger et al. 2005; Holzinger et al. 2006). Based on
estimates of rate constants and oxidant concentrations (Holzinger et al. 2005), we estimate that 10% of
the monoterpenes react in the canopy to form organic aerosol with a 10% yield and a molecular weight
of 180-250 g mol-1, resulting in an estimated monoterpene-derived organic aerosol upward flux of +(22.8) ng m-2 s-1 given a 10% SOA yield (Ng et al. 2007; Presto and Donahue 2006; Shilling et al. 2008). We
also estimate that 70% of the sesquiterpenes react in the canopy with a 10% SOA yield (Chen et al. 2011;
Ng et al. 2007), giving a sesquiterpene-derived SOA upward flux of +(1.4-2.0) ng m-2 s-1. Thus the total
estimated upward flux from these SOA precursors is 3.4-4.8 ng m-2 s-1. Additionally, we estimate 10-50%
of the 13-66 µmol m-2 hr-1 flux of “very reactive BVOC” (Holzinger et al. 2005) react with a 10-20% SOA
yield. Given the observed sesquiterpene emissions at Blodgett Forest are relatively small (<0.5 μmol m-2
h-1), we ignore its contributions to ‘very reactive BVOC’. Assuming molecular weights of 180-250 g mol-1
(King et al. 2009; Li et al. 2011), the very reactive BVOC would cause a 7 to 229 ng m-2 s-1 flux of SOA.
This suggests a total organic aerosol flux of 10-234 ng m-2 s-1 from in-canopy chemistry at Blodgett
Forest.
We note that this analysis assumes that the SOA yields are valid for the first few minutes of chemistry
following biogenic VOC emission, and that oxidant concentrations and light availability are not limiting
factors in VOC oxidation and SOA formation.
S1.3. Difficulties measuring aerosol fluxes from gradient measurements
We note that the small organic aerosol fluxes described in this paper correspond to small vertical
gradients in concentration. For example, the upward organic aerosol flux of +(10-24) ng m-2 s-1
corresponds to production of ~(0.036-0.086) µg m-3 between the ground and sensor (25 m). Such a small
gradient is difficult to detect with current aerosol instrumentation, consistent with the results of
previous attempts at measuring chemically-resolved aerosol mass fluxes using the gradient method
(Delia 2004). For example, uncertainty in losses of aerosol particles in sampling lines would cause
substantial difficulties in interpreting observed gradients. In contrast, the eddy covariance method
provides a direct measurement of the flux, and is substantially more sensitive to vertical fluxes than the
indirect gradient approach.
S1.4. Comparison between NR-PM1 mass fluxes and number fluxes (BEARPEX-07)
The NR-PM1 mass flux at BEARPEX-07 shows deposition throughout the day (Farmer et al. 2011), and is
the sum of the fluxes of all non-refractory organic and inorganic components, including local organic
aerosol deposition, transported organic aerosol deposition, in-canopy SOA production, and deposition
of transported inorganic aerosol components, such as ammonium sulfate. Vong et al. (2010) derived a
parameterization for particle number fluxes during BEARPEX-07, finding that they are non-linearly
dependent on friction velocity. Comparing the observed NR-PM1 mass flux to the parameterization
developed for particle number fluxes highlights the differences between fluxes of particle number and
particle mass. No significant correlation (r2 = 0.00) is found between the parameters derived by Vong et
al. (2010) for 400 nm particles with the submicron NR-PM1 flux. This may be due to the limited signal-tonoise ratios in the HR-AMS flux measurements, or the nature of the measurement: the NR-PM1 mass
flux is a bulk measurement of mass, integrated over all submicron size-ranges, while the Vong fluxes are
a size-segregated particle number measurement.
S2. AMAZE-08
S2.1. Partitioning flux due to thermal gradients
We use the observed aerosol thermograms (Martin et al. 2010) to determine a volatility distribution for
Amazonian organic aerosol (Cappa and Jimenez 2010). Average mid-day aerodynamic resistance is
calculated (Gallagher et al. 1997) to be 10.6 s m-1, resulting in a canopy residence time of 5-6 minutes.
Assuming that aerosol particles are subject to a linear thermal shift from 25 to 30°C within five minutes,
the kinetic-equilibrium model shows a 12.8% loss in aerosol mass as the particle is warmed. Thermal
gas-particle partitioning thus results in an upward flux of +8 ng m-2 s-1. As described above, this approach
does not account for vertical concentration gradients in SVOC due to SVOC deposition and in-canopy
BVOC oxidation, which will alter the calculated partitioning.
The tower used in AMAZE-08 was surrounded by a gap of ~50 m2 cleared area for the instrument shed
and work area, and thus the temperature gradient (-2°C) observed between the top and bottom of the
tower is unlikely representative of the fetch (Miller et al. 2007). Thus we consider the +5°C gradient
observed in multiple, similar field sites (Kruijt et al. 2000).
S2.2 In-canopy SOA formation (AMAZE-08)
In the tropical forest canopy, typical mid-day isoprene, monoterpene and sesquiterpene fluxes are 130,
10 and 1 µmol m-2 hr-1 (Karl et al. 2009). Based on oxidant and organic aerosol concentrations, we
estimate that 10% of the isoprene and monoterpenes, and 70% of the sesquiterpenes, react in the
canopy with SOA yields of 1% (Kroll et al. 2006), 10% (Ng et al. 2007; Presto and Donahue 2006) and 10%
(Chen et al. 2011; Chen et al. 2012) to produce SOA (MW of 180-250 g mol-1). We note that the
estimates for the proportion of sesquiterpenes that oxidize within the forest canopy are consistent with
VOC observations in Amazonia (Jardine et al. 2011). We derive SOA flux contributions of (6.5-9.0), (5.06.9) and (3.5-4.9) ng m-2 s-1 for a total flux of 15.0-20.8 ng m-2 s-1. Considering instead the SOA yield
derived from the BEARPEX-07 observations of 0.5-4.3%, we estimate an SOA flux from in-canopy
chemistry of 3.7-43.9 ng m-2 s-1. We consider the lower SOA yields for the very-reactive BVOC, including
sesquiterpenes, are more reasonable for the short residence times of air in the forest canopy. Further,
considering that the canopy was closed during AMAZE-08 and thus darker, resulting in likely lower incanopy OH concentrations, and that O3 concentrations were an order of magnitude smaller than during
BEAERPEX-07, we consider this calculation to provide an upper estimate of canopy SOA formation in the
Amazon.
S2.3. Comparison between NR-PM1 mass fluxes and number fluxes (AMAZE-08).
The particle number fluxes correlate with friction velocity (r2=0.88) (Ahlm et al. 2009; Ahlm et al. 2010),
but the NR-PM1 mass fluxes described herein do not (r2 < 0.1). We hypothesize that the correlation may
be undetectable due to the low signal-to-noise ratios of the HR-AMS flux data.
References
Ahlm, L., Nilsson, E. D., Krejci, R., Martensson, E. M., Vogt, M. and Artaxo, P. (2009). Aerosol number
fluxes over the Amazon rain forest during the wet season. Atmos Chem Phys 9:9381-9400.
Ahlm, L., Krejci, R., Nilsson, E. D., Martensson, E. M., Vogt, M. and Artaxo, P. (2010). Emission and dry
deposition of accumulation mode particles in the Amazon Basin. Atmos Chem Phys 10:10237-10253.
Cappa, C. D. (2010). A Model of Aerosol Evaporation Kinetics in a Thermodenuder. Atmos Meas Tech
3:579-592.
Cappa, C. D. and Jimenez, J. L. (2010). Quantitative estimates of the volatility of ambient organic aerosol.
Atmos Chem Phys 10:5409-5424.
Chen, Q., Liu, Y., Donahue, N. M., Shilling, J. E. and Martin, S. T. (2011). Particle-phase chemistry of
secondary organic material: Modeled compared to measured O:C and H:C elemental ratios provide
constraints. Environ Sci Technol:dx.doi.org/10.1021/es104398s.
Chen, Q., Li, Y. L., McKinney, K. A., Kuwata, M. and Martin, S. T. (2012). Particle mass yield from betacaryophyllene ozonolysis. Atmos Chem Phys 12:3165-3179.
Delia, A. E. (2004). Real-Time Measurements of Non-Refractory Particle Composition and Interactions at
Forested Sites. Ph.D. Thesis at the University of Colorado.
Farmer, D. K., Kimmel, J. R., Phillips, G., Docherty, K. S., Worsnop, D. R., Sueper, D., Nemitz, E. and
Jimenez, J. L. (2011). Eddy Covariance Measurements with High-Resolution Time-of-Flight Aerosol Mass
Spectrometry: A New Approach to Chemically Resolved Aerosol Fluxes. Atmos Meas Tech 4:1275-1289,
doi:1210.5194/amt-1274-1275-2011.
Gallagher, M. W., Beswick, K. M., Duyzer, J., Westrate, H., Choularton, T. W. and Hummelshoj, P. (1997).
Measurements of aerosol fluxes to Speulder forest using a micrometeorological technique. Atmos
Environ 31:359-373.
Holzinger, R., Lee, A., Paw, K. T. and Goldstein, A. H. (2005). Observations of oxidation products above a
forest imply biogenic emissions of very reactive compounds. Atmos Chem Phys 5:67-75.
Holzinger, R., Lee, A., McKay, M. and Goldstein, A. H. (2006). Seasonal variability of monoterpene
emission factors for a Ponderosa pine plantation in California. Atmos Chem Phys 6:1267-1274.
Jardine, K., Yanez Serrano, A., Arneth, A., Abrell, L., Jardine, A., van Haren, J., Artaxo, P., Rizzo, L. V.,
Ishida, F. Y., Karl, T., Kesselmeier, J., Saleska, S. and Huxman, T. (2011). Within-canopy sesquiterpene
ozonolysis in Amazonia. Geophys Res Lett 116:doi:10.1029/2011JD016243
Karl, T., Guenther, A., Turnipseed, A., Tyndall, G., Artaxo, P. and Martin, S. (2009). Rapid formation of
isoprene photo-oxidation products observed in Amazonia. Atmos Chem Phys 9:7753-7767.
King, S. M., Rosenoern, T., Shilling, J. E., Chen, Q. and Martin, S. T. (2009). Increased cloud activation
potential of secondary organic aerosol for atmospheric mass loadings. Atmospheric Chemistry & Physics
9:2959-2971; DOI 2910.5194/acp-2959-2959-2009.
Kroll, J. H., Ng, N. L., Murphy, S. M., Flagan, R. C. and Seinfeld, J. H. (2006). Secondary organic aerosol
formation from isoprene photooxidation. Environ Sci Technol 40:1869-1877.
Kruijt, B., Malhi, Y., Lloyd, J., Norbre, A. D., Miranda, A. C., Pereira, M. G. P., Culf, A. and Grace, J. (2000).
Turbulence statistics above and within two Amazon rain forest canopies. Boundary-Layer Meteorology
94:297-331.
Li, Y. J., Chen, Q., Guzman, M. I., Chan, C. K. and Martin, S. T. (2011). Second-generation products
contribute substantially to the particle-phase organic material produced by β-caryophyllene ozonolysis.
Atmospheric Chemistry & Physics 11:121-132; DOI 110.5194/acp-5110-5191-2010.
Martens, C. S., Shay, T. J., Mendlovitz, H. P., Matross, D. M., Saleska, S. R., Wofsy, S. C., Woodward, W.
S., Menton, M. C., De Moura, J. M. S., Crill, P. M., De Moraes, O. L. L. and Lima, R. L. (2004). Radon fluxes
in tropical forest ecosystems of Brazilian Amazonia: night-time CO2 net ecosystem exchange derived
from radon and eddy covariance methods. Global Change Biol 10:618-629.
Martin, S. T., Andreae, M. O., Althausen, D., Artaxo, P., Baars, H., Borrmann, S., Chen, Q., Farmer, D. K.,
Guenther, A., Gunthe, S. S., Jimenez, J. L., Karl, T., Longo, K., Manzi, A., Muller, T., Pauliquevis, T.,
Petters, M. D., Prenni, A. J., Poschl, U., Rizzo, L. V., Schneider, J., Smith, J. N., Swietlicki, E., Tota, J., Wang,
J., Wiedensohler, A. and Zorn, S. R. (2010). An overview of the Amazonian Aerosol Characterization
Experiment 2008 (AMAZE-08) (vol 10, pg 11415, 2010). Atmos Chem Phys 10:11565-11565.
Miller, S. D., Goulden, M. L. and da Rocha, H. R. (2007). The effect of canopy gaps on subcanopy
ventilation and scalar fluxes in a tropical forest. Agricultural and Forest Meteorology 142:25-34.
Ng, N. L., Chhabra, P. S., Chan, A. W. H., Surratt, J. D., Kroll, J. H., Kwan, A. J., McCabe, D. C., Wennberg,
P. O., Sorooshian, A., Murphy, S. M., Dalleska, N. F., Flagan, R. C. and Seinfeld, J. H. (2007). Effect of NOx
level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes. Atmospheric
Chemistry & Physics 7:5159-5174, doi:5110.5194/acp-5157-5159-2007.
Presto, A. A. and Donahue, N. M. (2006). Investigation of -pinene + ozone secondary oganic aerosol
formation at low total aerosol mass. Environ Sci Technol 40:3536-3543.
Shilling, J. E., Chen, Q., King, S. M., Rosenoern, T., Kroll, J. H., Worsnop, D. R., McKinney, K. A. and
Martin, S. T. (2008). Particle mass yield in secondary organic aerosol formed by the dark ozonolysis of αpinene. Atmos Chem Phys 8:2073-2088, doi:2010.5194/acp-2078-2073-2008.
Vaden, T. D., Imre, D., Beranek, J., Shrivastava, M. and Zelenyuk, A. (2011). Evaporation kinetics and
phase of laboratory and ambient secondary organic aerosol. P Natl Acad Sci USA 108:2190-2195.
Vong, R. J., Vong, I. J., Vickers, D. and Covert, D. S. (2010). Size-dependent aerosol deposition velocities
during BEARPEX' 07. Atmos Chem Phys 10:5749-5758.