| Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements
of the hydrological cycle and climate. We have measured and characterized CCN at
water vapor supersaturations in the range of S = 0.10-0.82% in pristine tropical
rainforest air during the AMAZE-08 campaign in central Amazonia. The effective
hygroscopicity parameters describing the influence of chemical composition on the CCN
activity of aerosol particles varied in the range of κ = 0.05-0.45. The overall median
value of κ - 0.15 was only half of the value typically observed for continental
aerosols in other regions of the world. Aitken mode particles were less hygroscopic
than accumulation mode particles (κ - 0.1 at D - 50 nm; κ - 0.2 at D - 200
nm).
The CCN measurement results were fully consistent with aerosol mass spectrometry
(AMS) data, which showed that the organic mass fraction (Xm,org) was on average as
high as ~90% in the Aitken mode (D -¤ 100 nm) and decreased with increasing
particle diameter in the accumulation mode (~80% at D - 200 nm). The κ values
exhibited a close linear correlation with Xm,org and extrapolation yielded the following
effective hygroscopicity parameters for organic and inorganic particle components:
κorg - 0.1 which is consistent with laboratory measurements of secondary organic
aerosols and κinorg - 0.6 which is characteristic for ammonium sulfate and related
salts. Both the size-dependence and the temporal variability of effective particle
hygroscopicity could be parameterized as a function of AMS-based organic and
inorganic mass fractions (κp = 0.1 Xm,org + 0.6Xm,inorg), and the CCN number
concentrations predicted with κp were in fair agreement with the measurement results. The
median CCN number concentrations at S = 0.1-0.82% ranged from NCCN,0.10 - 30
cm-3to NCCN,0.82 - 150 cm-3, the median concentration of aerosol particles
larger than 30 nm was NCN,30 - 180 cm-3, and the corresponding integral CCN
efficiencies were in the range of NCCN,0.10-NCN,30 - 0.1 to NCCN,0.82-NCN,30 -
0.8.
Although the number concentrations and hygroscopicity parameters were much lower, the
integral CCN efficiencies observed in pristine rainforest air were similar to those in highly
polluted mega-city air. Moreover, model calculations of NCCN,S with a global average value
of κ = 0.3 led to systematic overpredictions, but the relative deviations exceeded ~50% only
at low water vapor supersaturation (0.1%) and low particle number concentrations (-¤ 100
cm-3). These findings confirm earlier studies suggesting that aerosol particle number and
size are the major predictors for the variability of the CCN concentration in continental
boundary layer air, followed by particle composition and hygroscopicity as relatively minor
modulators.
Depending on the required and applicable level of detail, the information and
parameterizations presented in this paper should enable efficient description of the CCN
properties of pristine tropical rainforest aerosols in detailed process models as well as in
large-scale atmospheric and climate models.
References
Andreae, M. O.: Aerosols before pollution, Science, 315, (5808), 50-51, 2007.
Andreae, M. O.: Correlation between cloud condensation nuclei concentration and
aerosol optical thickness in remote and polluted regions, Atmos. Chem. Phys. Discuss., 8,
11293-11320, 2008.
Andreae, M. O. and Rosenfeld, D.: Aerosol-cloud-precipitation interactions. Part 1. The
nature and sources of cloud-active aerosols, Earth Science Reviews, 89, 13-41,
2008.
Martin, S. T., et al.: Amazonian Aerosol Characterization Experiment 2008
(AMAZE-08), Atmospheric Chemistry and Physics, in preparation, 2009.
Petters, M. D. and Kreidenweis, S. M.: A single parameter representation of hygroscopic
growth and cloud condensation nucleus activity, Atmospheric Chemistry and Physics, 7, (8),
1961-1971, 2007.
Rose, D., Gunthe, S. S., Mikhailov, E., Frank, G. P., Dusek, U., Andreae, M. O. and
Pöschl, U.: Calibration and measurement uncertainties of a continuous-flow cloud
condensation nuclei counter (DMT-CCNC): CCN activation of ammonium sulfate and
sodium chloride aerosol particles in theory and experiment, Atmospheric Chemistry and
Physics, 8, (5), 1153-1179, 2008a.
Rose, D., Nowak, A., Achtert, P., Wiedensohler, A., Hu, M., Shao, M., Zhang, Y.,
Andreae, M. O. and Pöschl, U.: Cloud condensation nuclei in polluted air and biomass
burning smoke near the mega-city Guangzhou, China - Part 1: Size-resolved measurements
and implications for the modeling of aerosol particle hygroscopicity and CCN activity,
Atmos. Chem. Phys. Discuss., 8, (5), 17343-17392, 2008b |