| Aerosol particle number size distributions and hygroscopic properties were
measured at a pasture site in the southwestern Amazon region (Rondonia). The
measurements were performed 11 September-14 November 2002 as part of
LBA-SMOCC (Large scale Biosphere atmosphere experiment in Amazonia - SMOke
aerosols, Clouds, rainfall and Climate), and cover the later part of the dry
season (with heavy biomass burning), a transition period, and the onset of
the wet period.
Particle number size distributions were measured with a DMPS (Differential
Mobility Particle Sizer, 3-850nm) and an APS (Aerodynamic Particle Sizer),
extending the distributions up to 3.3 µm in diameter. An H-TDMA (Hygroscopic
Tandem Differential Mobility Analyzer) measured the hygroscopic diameter
growth factors (Gf) at 90% relative humidity (RH), for particles with dry
diameters (dp) between 20-440 nm, and at several occasions RH scans
(30-90% RH) were performed for 165nm particles. These data provide the
most extensive characterization of Amazonian biomass burning aerosol, with
respect to particle number size distributions and hygroscopic properties,
presented until now. The evolution of the convective boundary layer over the
course of the day causes a distinct diel variation in the aerosol physical
properties, which was used to get information about the properties of the
aerosol at higher altitudes.
The number size distributions averaged over the three defined time periods
showed three modes; a nucleation mode with geometrical median diameters
(GMD) of ~12 nm, an Aitken mode (GMD=61-92 nm) and an accumulation
mode (GMD=128-190 nm). The two larger modes were shifted towards larger
GMD with increasing influence from biomass burning.
The hygroscopic growth at 90% RH revealed a somewhat external mixture
with two groups of particles; here denoted nearly hydrophobic (Gf~1.09
for 100 nm particles) and moderately hygroscopic (Gf~1.26). While the
hygroscopic growth factors were surprisingly similar over the periods, the
number fraction of particles belonging to each hygroscopic group varied
more, with the dry period aerosol being more dominated by nearly hydrophobic
particles. As a result the total particle water uptake rose going into the
cleaner period. The fraction of moderately hygroscopic particles was
consistently larger for particles in the accumulation mode compared to the
Aitken mode for all periods. Scanning the H-TDMA over RH (30-90% RH)
showed no deliquescence behavior. A parameterization of both Gf(RH) and
Gf(dp), is given. |