dc.description.abstract | This paper investigates the physical and chemical
characteristics of aerosols at ground level at a site
heavily impacted by biomass burning. The site is located
near Porto Velho, Rondônia, in the southwestern part of the
Brazilian Amazon rainforest, and was selected for the deployment
of a large suite of instruments, among them an
Aerosol Chemical Speciation Monitor. Our measurements
were made during the South American Biomass Burning
Analysis (SAMBBA) field experiment, which consisted of
a combination of aircraft and ground-based measurements
over Brazil, aimed to investigate the impacts of biomass
burning emissions on climate, air quality, and numerical
weather prediction over South America. The campaign took
place during the dry season and the transition to the wet season
in September/October 2012.
During most of the campaign, the site was impacted by
regional biomass burning pollution (average CO mixing ratio
of 0.6 ppm), occasionally superimposed by intense (up
to 2 ppm of CO), freshly emitted biomass burning plumes.
Aerosol number concentrations ranged from ∼ 1000 cm−3
to
peaks of up to 35 000 cm−3
(during biomass burning (BB)
events, corresponding to an average submicron mass mean
concentrations of 13.7 µg m−3
and peak concentrations close
to 100 µg m−3
. Organic aerosol strongly dominated the submicron
non-refractory composition, with an average concentration
of 11.4 µg m−3
. The inorganic species, NH4, SO4,
NO3, and Cl, were observed, on average, at concentrations
of 0.44, 0.34, 0.19, and 0.01 µg m−3
, respectively. Equivalent
black carbon (BCe) ranged from 0.2 to 5.5 µg m−3
,
with an average concentration of 1.3 µg m−3
. During BB
peaks, organics accounted for over 90 % of total mass (submicron
non-refractory plus BCe), among the highest values
described in the literature.
We examined the ageing of biomass burning organic
aerosol (BBOA) using the changes in the H : C and O : C
ratios, and found that throughout most of the aerosol processing
(O : C ∼= 0.25 to O : C ∼= 0.6), no remarkable change
is observed in the H : C ratio (∼ 1.35). Such a result contrasts
strongly with previous observations of chemical ageing
of both urban and Amazonian biogenic aerosols. At higher
levels of processing (O : C > 0.6), the H : C ratio changes
with a H : C/O : C slope of −0.5, possibly due to the development
of a combination of BB (H : C/O : C slope =
0) and biogenic (H : C/O : C slope = −1) organic aerosol
(OA). An analysis of the 1OA/1CO mass ratios yields
very little enhancement in the OA loading with atmospheric
processing, consistent with previous observations. These
results indicate that negligible secondary organic aerosol
(SOA) formation occurs throughout the observed BB plume
Published by Copernicus Publications on behalf of the European Geosciences Union.
12070 J. Brito et al.: Ground-based aerosol characterization during SAMBBA
processing, or that SOA formation is almost entirely balanced
by OA volatilization.
Positive matrix factorization (PMF) of the organic aerosol
spectra resulted in three factors: fresh BBOA, aged BBOA,
and low-volatility oxygenated organic aerosol (LV-OOA).
Analysis of the diurnal patterns and correlation with external
markers indicates that during the first part of the campaign,
OA concentrations are impacted by local fire plumes
with some chemical processing occurring in the near-surface
layer. During the second part of the campaign, long-range
transport of BB plumes above the surface layer, as well as
potential SOAs formed aloft, dominates OA concentrations
at our ground-based sampling site.
This manuscript describes the first ground-based deployment
of the aerosol mass spectrometry at a site heavily impacted
by biomass burning in the Amazon region, allowing
a deeper understanding of aerosol life cycle in this important
ecosystem. | en_GB |
dc.description.sponsorship | This work was supported by the Foundation
for Research Support of the State of São Paulo (FAPESP,
projects 2012/14437-9 and 2013/05014-0), CNPq project 475735-
2012-9, INCT Amazonia, and Natural Environment Research
Council (NERC) project NE/J010073/1. We thank A. Ribeiro,
A. L. Loureiro, F. Morais, F. Jorge, and S. Morais for technical and
logistics support. We thank the National Institute of Meteorology
for providing valuable meteorological data. We gratefully acknowledge
S. Hacon, J. Silva, and W. Bastos for support in the successful
operation of the sampling site. | en_GB |