Halogen-induced organic aerosol (XOA) Decarboxylation of … · Halogen-induced organic aerosol...
Transcript of Halogen-induced organic aerosol (XOA) Decarboxylation of … · Halogen-induced organic aerosol...
Halogen-induced organic aerosol (XOA)
formation and decarboxylation of carboxylic
acids by reactive halogen species –
a time-resolved aerosol flow-reactor study
JOHANNES OFNER 1 AND CORNELIUS ZETZSCH2
1 INSTITUTE OF CHEMICAL TECHNOLOGIES AND ANALYTICS, VIENNA UNIVERSITY OF TECHNOLOGIES, AUSTRIA 2 ATMOSPHERIC CHEMISTRY RESEARCH LABORATORY, UNIVERSITY OF BAYREUTH, GERMANY
Introduction
Reactive halogen species (RHS) released by heterogeneous processes from sea-salt aerosols or above salt lakes (Buxmann et al., 2012) are important for heterogeneous and homogeneous reactions in the
troposphere. The gaseous halogen species can interact with gaseous organic species (VOC) or organic aerosols in the boundary layer. Recent studies (Ofner et al., 2012) exhibit an aerosol formation-potential of RHS with selected terpenes (see also by Cai et al., 2006 and 2008), forming halogen-induced organic aerosol (XOA). Ofner et al. (2012) also propose an halogen-driven decarboxylation of oxidized sites of SOA (see also Aranda et al., 2003).
The present work studies the formation of XOA from various terpenes and the decarboxylation of different carboxylic acids by photolized molecular chlorine using infrared spectroscopy with an aerosol flow-reactor. The dataset was evaluated using 2D Correlation Spectroscopy (Noda, 1993).
Fig. 1 – Basic
setup of the aerosol flow-reac-
tor coupled to a
FTIR spectrometer:
A) Flow-reactor
with moveable
inlet to vary reaction times.
B) Solar simulator
for photolysis
of Cl2
C) Multi-reflect.
cell for FTIR
spectroscopy
D) reaction zone
1) Inlet for the or-
ganic precursor
2) Inlet for the mixture of RHS
and sheath air
3) Exhaust
4) Infrared beam of the FTIR
spectrometer
Decarboxylation of carboxylic acids
The decarboxylation process is demonstrated with the reaction of formic acid and photolized chlorine at a timescale of 10 to 40 seconds.
The time-depended transformation of vibrational species is indicated in fig. 2 but the transformation of single vibrational species is hardly visible.
The 2D synchronous correlation plot of the reaction of formic acid with chlorine (Fig. 3) indicates a decrease of the formic acid dimer with an simultaneous increase of CO2 and a coupled HCl formation. Thus, the formic acid dimer is stronger affected by the decarboxylation than the monomer. A general degradation of formic acid is visible.
Conclusions
Time-resolved FTIR spectroscopy using an aerosol flow-reactor is very useful to obtain mechanistic data from atmospheric reactions. 2D correlation spectroscopy allows analyzing the dataset in detail. The chlorine-induced decarboxylation of formic acid is strongly correlated with CO2 and HCl formation.
Further, carboxylic acid dimers seem to be more affected than the free monomers. While carboxylic acid degrade at the reaction with halogens, the XOA formation is coupled to a formation of them in the solid phase. The degradation takes place on a time-scale of 40 seconds, while the related formation in XOA takes only a few seconds.
Methods
An aerosol flow-reactor (Ofner et al., 2010A) was coupled to a high-resolution Bruker IFS113v FTIR spectrometer using a home-made multi-reflection cell (Ofner et al., 2010B). Photolysis of molecular chlorine is performed using a simple UV/VIS solar simulator. The basic setup is shown in figure 1. The reactor has been operated to vary reaction times of terpenes (α-pinene, limonene and Δ3-carene) by 1 - 8 seconds and of carboxylic acids (formic acid, acetic acid and arylic acid) by 5 - 40 seconds.
Formation of halogen-induced organic aerosol (XOA)
The formation of XOA from limonene is shown in figure 4. Is indicated in the 2D synchronous correlation plot, the XOA formation is coupled to a strong HCl and CO2 release. An additional coupled formation of carboxylic acids at about 1730 cm-1 is visible. The well defined C-H stretching region of limonene degrades to the known broad absorptions of organic aerosols.
Related posters:
B925 –BrO loss due secondary organic aerosols
B929 – Particle formation above natural and simulated salt lakes
B930 – Formation of halogen-induced secondary organic aerosol (XOA)
References:
Aranda et al., Atmos. Environ., 2003. Buxmann et al., Int. J. Chem. Kin., 2012.
Cai and Griffin, J. Geophys. Res., 2006. Cai et al., Atmos. Environ., 2008.
Noda, Appl. Spectrosc., 1993. Ofner et al., Z. Phys. Chem., 2010A.
Ofner et al., Appl. Opt., 2010B. Ofner et al., ACP, 2012.
Acknowledgement:
The authors thank the German research foundation for funding within
the research unit HALOPROC (DFG RU 763) and Prof. Bernhard
Lendl and Georg Ramer (Vienna University of Technology) for their
support for applying 2D correlation spectroscopy to the dataset.
A
B
C
D
1
2
3
4
Fig. 2 – Contour plot of the time-dependent decarboxylation of
formic acid.
Absorbance [a.u.]
v(H
-Cl)
v(C
-H)
of
fom
ric
aci
d
va
s(C
=O
) o
f C
O2
v(C
=O
) o
f fo
mri
c a
cid
dim
er
v(C
=O
) o
f fo
mri
c a
cid
mo
no
me
r
Fig. 3 – 2D synchronous correlation spectroscopy plot of the
decarboxylation of formic acid where A10 is the initial spectrum of
formic acid and A40 is the combined spectrum after 40 seconds of
reaction time (red areas – increasing; blue areas – decreasing).
v(C-H) v(C=O)
of FA
monomer
v(C=O)
of FA
dimer
v(C
-H)
an
d v
(H-C
l v(C
=O
)
of
FA
mo
no
me
r
v(C
=O
)
of
FA
dim
er
va
s(C
=O
) o
f C
O2
EGU General Assembly 2013
Poster B927
Fig. 4 – 2D synchronous correlation spectroscopy plot of the XOA
formation from limonene where A1 is the initial spectrum and A8 is
the combined spectrum after 8 seconds of reaction time (red areas – increasing; blue areas – decreasing).
v(C-H) of limonenewith
intial formation of HCl
v(C
=O
) v
as(
C=
O)
of
CO
2
v(C
-H)
an
d v
(H-C
l