analysis suggest that BVOC oxidation products
contributed to the majority of SOA even in urban
regions [32]; however, it should be noted that
even BSOA production is somewhat linked to
anthropogenic activity as the main VOC oxidants
are associated with anthropogenic emissions.
For BVOC emissions, terpenes and sesquiterpenes from terrestrial vegetation emissions, are
thought to be the dominant precursors of BSOA,
and to a lesser extent isoprene (particularly under
high NOx conditions [33]), while aromatic compounds such as toluene, xylene, and trimethyl
benzene are thought to account for the majority
of ASOA formation. There has been considerable
effort to identify the composition of SOA in recent
years; however, the vast majority of compounds
remain unidentified due to complex mixtures of
compounds often with few distinguishing features. SOA is generally oxygenated and water
soluble, in contrast to hydrocarbon-like waterinsoluble POA. Various approaches have been
taken to speciate the organic mass found in
SOA. For example,
1 H nuclear magnetic resonance (HNMR) techniques have been applied to
evaluate functional groups in water-soluble OA
for different aerosol source regimes [34]. The
technique readily detects four main functional
groups in atmospheric water-soluble OA:
unfunctionalized alkyls (H–C); aliphatic carbons
bound to an unsaturated carbon atom (H–C–C¼);
aliphatic alcohols, ethers, and esters (H–C–O; and
aromatic rings (H–Ar). Additionally, formylic
(H–C¼O), acetalic (O–CH–O), and sulfonic
(H–C–S¼O) groups are detected. Water-soluble
OA samples from the boreal forest in Finland
(terrestrial biogenic), biomass burning events in
Brazil, rural UK, and the Northeast Atlantic
(marine) were analyzed for functional groups
using this approach. Figure 5 illustrates, on
the vertical axis, the aliphatic carbon fraction
(carbonylic/carboxylic, HC–C¼O groups) and,
on the horizontal axis, the fraction of aliphatic carbon accounted for by hydroxyl groups
for a range of standard SOA compounds
(levoglucosan, succinic acid, adipic acid, pinonic
acid, glutaric acid, and nonanoic acid) alongside
the diverse atmospheric samples. The analysis
show that marine and terrestrial biogenics are
characterized by relatively high H–C and low
H–C–O groups which partly overlap; however,
the marine samples have a significantly lower
aromatic content. The functional composition of
biogenic OA and that of rural anthropogenic OA
are relatively close with the rural samples having a
higher H–C–O content. Both also contain high
fractions of aliphatic carbonyls. The biomass
burning water-soluble OA contained the highest
content of hydroxyls and the lowest amount of
alkylic groups.
The formation of SOA is not always via the gas
phase formation of condensable vapors. In the
case of isoprene-related SOA production, its relatively high vapor pressure oxidation products suggest little contribution to SOA; however,
alternative routes have been suggested, which
produce low-volatility aerosol phase products.
For example, tetrols with the same backbone as
isoprene have been observed in atmospheric aerosols [35] possibly formed by the heterogeneous
acid-catalyzed oxidation of isoprene in the presence of peroxides [36]. Additionally, heterogeneous reaction of isoprene under highly acidic
conditions was also observed to lead to the formation of polymeric, humic-like substances
[37]. Other studies showed that some highvolatility carbonylic oxidation products can contribute to SOA through the interaction with highly
acidic inorganic preexisting aerosols [38], while
polymerization of second-generation products has
been suggested to contribute to SOA growth
[39]. Cloud modeling studies [40] also predict
that low-volatility water-soluble oxidation products of isoprene can be oxidized in the aqueous
phase in clouds to produce dicarboxylic acids.
Once SOA is formed, it contributes to a
dynamic thermodynamic and chemical evolution
system. SOA species can be repartitioned to the
gas phase and undergo polymerization and oxidation leading to significant changes in the chemical
nature of SOA. The evolution of SOA is well
illustrated through the examination of SOA mass
spectral properties moving from strong urban
source regions to remote continental regions.
Aerosol mass spectrometry (particularly using
the Aerodyne Aerosol Mass Spectrometer –
AMS) delivers increased speciation capability on
Aerosol in Global Atmosphere
249
contributed to the majority of SOA even in urban
regions [32]; however, it should be noted that
even BSOA production is somewhat linked to
anthropogenic activity as the main VOC oxidants
are associated with anthropogenic emissions.
For BVOC emissions, terpenes and sesquiterpenes from terrestrial vegetation emissions, are
thought to be the dominant precursors of BSOA,
and to a lesser extent isoprene (particularly under
high NOx conditions [33]), while aromatic compounds such as toluene, xylene, and trimethyl
benzene are thought to account for the majority
of ASOA formation. There has been considerable
effort to identify the composition of SOA in recent
years; however, the vast majority of compounds
remain unidentified due to complex mixtures of
compounds often with few distinguishing features. SOA is generally oxygenated and water
soluble, in contrast to hydrocarbon-like waterinsoluble POA. Various approaches have been
taken to speciate the organic mass found in
SOA. For example,
1 H nuclear magnetic resonance (HNMR) techniques have been applied to
evaluate functional groups in water-soluble OA
for different aerosol source regimes [34]. The
technique readily detects four main functional
groups in atmospheric water-soluble OA:
unfunctionalized alkyls (H–C); aliphatic carbons
bound to an unsaturated carbon atom (H–C–C¼);
aliphatic alcohols, ethers, and esters (H–C–O; and
aromatic rings (H–Ar). Additionally, formylic
(H–C¼O), acetalic (O–CH–O), and sulfonic
(H–C–S¼O) groups are detected. Water-soluble
OA samples from the boreal forest in Finland
(terrestrial biogenic), biomass burning events in
Brazil, rural UK, and the Northeast Atlantic
(marine) were analyzed for functional groups
using this approach. Figure 5 illustrates, on
the vertical axis, the aliphatic carbon fraction
(carbonylic/carboxylic, HC–C¼O groups) and,
on the horizontal axis, the fraction of aliphatic carbon accounted for by hydroxyl groups
for a range of standard SOA compounds
(levoglucosan, succinic acid, adipic acid, pinonic
acid, glutaric acid, and nonanoic acid) alongside
the diverse atmospheric samples. The analysis
show that marine and terrestrial biogenics are
characterized by relatively high H–C and low
H–C–O groups which partly overlap; however,
the marine samples have a significantly lower
aromatic content. The functional composition of
biogenic OA and that of rural anthropogenic OA
are relatively close with the rural samples having a
higher H–C–O content. Both also contain high
fractions of aliphatic carbonyls. The biomass
burning water-soluble OA contained the highest
content of hydroxyls and the lowest amount of
alkylic groups.
The formation of SOA is not always via the gas
phase formation of condensable vapors. In the
case of isoprene-related SOA production, its relatively high vapor pressure oxidation products suggest little contribution to SOA; however,
alternative routes have been suggested, which
produce low-volatility aerosol phase products.
For example, tetrols with the same backbone as
isoprene have been observed in atmospheric aerosols [35] possibly formed by the heterogeneous
acid-catalyzed oxidation of isoprene in the presence of peroxides [36]. Additionally, heterogeneous reaction of isoprene under highly acidic
conditions was also observed to lead to the formation of polymeric, humic-like substances
[37]. Other studies showed that some highvolatility carbonylic oxidation products can contribute to SOA through the interaction with highly
acidic inorganic preexisting aerosols [38], while
polymerization of second-generation products has
been suggested to contribute to SOA growth
[39]. Cloud modeling studies [40] also predict
that low-volatility water-soluble oxidation products of isoprene can be oxidized in the aqueous
phase in clouds to produce dicarboxylic acids.
Once SOA is formed, it contributes to a
dynamic thermodynamic and chemical evolution
system. SOA species can be repartitioned to the
gas phase and undergo polymerization and oxidation leading to significant changes in the chemical
nature of SOA. The evolution of SOA is well
illustrated through the examination of SOA mass
spectral properties moving from strong urban
source regions to remote continental regions.
Aerosol mass spectrometry (particularly using
the Aerodyne Aerosol Mass Spectrometer –
AMS) delivers increased speciation capability on
Aerosol in Global Atmosphere
249