Natural methane from biomass burning can have an effect

 

            Natural
and anthropogenic fires play a crucial role in the ecology of many terrestrial
ecosystems, such as boreal forests, temperate forests, grasslands, and the
chaparral of Southern California, with significant effects on the carbon cycle,
ecological succession, and atmospheric chemistry/aerosols of these ecosystems
and the global climate system.1 For example, according to Nobel
Prize winning atmospheric chemist Paul Crutzen, the emission of carbon monoxide
and methane from biomass burning can have an effect on the concentration of the
OH radical and thus the oxidative capacity of the atmosphere.2 Moreover,
aerosols ejected by biomass burning can have radiative forcing effects, and the
burning itself can change the albedo of the land, causing a further climate
forcing.3 Forest fires in boreal regions may have particularly
important implications for climate forcing as they can lead to the thawing of
subterranean permafrost and the release of methane, an extremely powerful greenhouse
gas.4 Given the large and diverse consequences that fire can have
for the Earth System, it is important that researchers understand the causes
and effects of biomass burning in the context of the variability of the past on
both a global and regional scale. Unfortunately, there is little historical
data that documents biomass burning prior to the 20th century, so
researchers have instead used proxies, such as sedimentary charcoal records, to
determine global patterns in biomass burning over the past two millennia.1
These charcoal records indicate that over the last 150 years, there has been a
significant reduction in biomass burning, probably due to an increase in human
intervention.1 However, the decrease in moisture associated with
climate change has begun to predispose many forests to wildfires, and researchers
project that this will lead to an increase in biomass burning far beyond the
variability observed over the past 2000 years.1,6 In fact, this
sharp rise may have already begun as early as the mid 1980’s, with a
particularly severe increase in the Western United States7.8. Unfortunately,
the charcoal proxies traditionally used to quantify historical biomass burning
are spatially constrained with some large geographic gaps, for example, in
central Eurasia an Siberia1,5. Moreover, while the chronologies of some
charcoal records are well resolved, the chronologies of many others remain
highly uncertain.9 Thus, in addition to charcoal sediments,
researchers have turned to examining the aerosolized organic combustion
products of biomass burning which have been deposited and preserved in a more
easily dated record, ice sheets.

             Two of the most promising biomass burning
tracers are the phenolic acids, vanillic acid (VA) and para-hydrobenzoic acid
(p-HBA). These acids are produced by the pyrolysis of the polymer, lignin, an
important structural component of plants composed of cross linked phenols, and
they offer several advantages over other burning proxies found in ice cores,
particularly for the determination of burning in high latitude boreal forests.5,10
Namely, their intermediate atmospheric lifetime of several days (when protected
in aerosols) offers information unique from more reactive species such as
levoglucosan, a product of cellulose combustion, and longer lived species
associated with biomass burning such as methane, ethane, and carbon monoxide11-15.
Moreover, unlike some inorganic compounds used as burning proxies such as black
carbon, potassium, and nitrate, they do not have other known significant
sources. Lastly, the ratio of VA to p-HBA produced depends largely on the type
of vegetation that is burning. This enables researchers to draw conclusions
about the fuel sources that generate these phenolic acids with a relatively
high degree of confidence16,17. The concentrations of VA and p-HBA
found in ice cores are affected by not only emissions from biomass burning but
also complex transport, deposition, and possibly post-depositional processes
that lend some complications; however, these complications may be overcome with
future studies. While these species had been examined previously in 2007 and
2012, the first high resolution multi-millennial study of these compounds as
biomass burning proxies in ice cores was published in 2017 by Grieman et al. of
UC Irvine.4,5,18

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VA
and p-HBA enter the atmosphere primarily through the combustion of lignin.
Lignin is a type of crosslinked phenolic polymer that is an important
constituent of the cell walls in the vascular tissue of plants. It is composed
of three major monomeric building blocks (see figure 1). They are H-type
(p-hydroxyphenyl), G-type (3-methoxy 4-hydroxyphenyl) and S-type (3,5-dimethoxy
4-hydroxyphenyl); these monomers are linked by ether (C-O) or C-C bonds.19
The exact composition of lignin by monomer varies according to plant species; (figure
2 shows a macrostructure of lignin along with the structures of VA, and p-HBA.)
thus, the products of the pyrolysis of lignin also change according to what
plant is burning. The lignin in conifer trees, the principal component of high
latitude boreal forests, has a disproportionately high amount of G monomers,
making VA a particularly good tracer of conifer burning. The products of lignin
pyrolysis are also affected by the temperature of burning.19 Upon
combustion at relatively low temperatures (200oC-400oC),
the polymeric linkages are dissolved, liberating the phenolic monomers. The
side chains of these monomers may then be converted to a carboxyl group in a
secondary reaction to form VA from G monomers or p-HBA from H monomers (or
syringic acid from S monomers). Figure 3 shows a mechanism proposed by Kawamoto
for the low temperature combustion of G monomers.19 However, at
higher temperatures (400oC-600oC) the cleaving of methoxy
groups to form catechols, cresols, and coke is favored, and at still higher
temperatures(>600oC) the formation of inorganic black carbon and
carbon monoxide is more likely.5,19 The temperature dependence of
lignin pyrolysis products has interesting implications for the utility of VA
and p-HBA as biomass burning tracers as different regions often have different
types and intensities of wildland fires. There are no other documented sources
for these compounds in the atmosphere, and as such, their highest observed
atmospheric concentrations are in biomass burning derived aerosols near burning
sources (the potential for unknown sources will be addressed later). However,
once these compounds have reached the atmosphere, the picture becomes more
complicated.                                                                                                                     

VA and p-HBA are
semi volatile organics, so, in the atmosphere, according to Grieman, they can
exist in both the gas phase and in aerosols “depending on temperature, aerosol
water content, pH, and cation concentrations.”5 In the gas phase,
like most aromatics and other unsaturated hydrocarbons, these molecules are
highly susceptible to oxidation by the OH radical, and will have an atmospheric
lifetime of approximately 1 day.5,11 If these molecules only existed
in the gas phase then, the potential for long distance transport would be
minimal, and, unless the biomass burning was occurring directly adjacent to the
ice core site (unlikely), their usefulness as a biomass burning tracer would be
severely limited. Fortunately, however, in the atmosphere, VA and p-HBA mostly
exist inside of aerosols, where they are protected from such rapid oxidation by
the OH radical. In 2013, Donahue used models to show that heterogeneous phase
oxidation by the OH radical can take over ten times longer than gas phase
oxidation.11 This means that the atmospheric lifetime of VA and
p-HBA is greatly affected by the fraction dissolved in aerosols versus the gas
phase, and, when the aerosol phase predominates, the atmospheric lifetime for
these compounds can easily reach five days or more allowing for long range
transport. Observations of these molecules in aerosol particles derived from
burning in both coarse and fine modes over the ocean (away from any burning) at
a variety of latitudes offer evidence that these compounds do in fact have
longer lifetimes than gas phase oxidation would permit and that long distance
transport is possible.20,21 This intermediate atmospheric lifetime
distinguishes VA and p-HBA from other biomass burning tracers in that it
constrains the possible source location of these combustion products as zonal
transport is likely (on the order of 1000 to 2000 km from the site of the fire),
but significant meridional transport is not. Grieman’s study illustrates the
utility of this lifetime by using the HYSPLIT model to show that the most
likely source for VA and p-HBA in the Akademii Nauk ice core is Siberian boreal
forest (see figure 4).5 A similar exercise with longer lived biomass
burning tracers like ethane or carbon monoxide, both of which have an
atmospheric lifetime of about two months, would be futile as the potential
exists for these compounds to be transported much further. In fact, in 2016
Nicewonger et al. demonstrated that the levels of these compounds found in high
latitude ice cores is mostly affected by burning in distant tropical latitudes.15
On the other end of the spectrum are species such as levoglucosan, a product of
cellulose combustion which has also been used as a biomass burning proxy in ice
core analyses. However, laboratory experiments have shown that the lifetime of
levoglucosan can be as little as two days, even when protected in aerosols;
moreover, Donahue suggests that because of its chemical composition, a large
fraction of levoglucosan resides in the gas phase where it is more susceptible
to OH radical oxidation.11-13 This limits the potential for
transport, and while levoglucosan has also been observed in long-distance
transported aerosols derived from biomass burning, further studies to reconcile
these conflicts are warranted. Like all atmospheric hydrocarbons, VA and p-HBA
are either removed from the atmosphere by wet or dry deposition or ultimately
oxidized to carbon dioxide through a complex series of reactions with the OH
radical.5

            Perhaps
the most interesting and one of the most useful facets of using VA and p-HBA as
a biomass burning tracers is that the ratio of the two compounds may be used to
determine what type of vegetation was the fuel source. While the emission of other
biomass burning tracers such as ethane, carbon monoxide, and levoglucosan is
affected only by burning conditions and not vegetation type, the ratio of
lignin pyrolysis products is unique to a given type of vegetation because each
type of plant has a different specific lignin composition, and these ratios
have been studied extensively in laboratory settings. For example, laboratory
studies have shown that the combustion of conifers in North America generates
more VA than p-HBA while grass fires in similar regions produce exclusively
p-HBA.16,17 Grieman’s study identified three pre-industrial periods
and one post-industrial period of elevated concentrations of VA and p-HBA in
the Akademii Nauk ice core. The ratio of the two compounds was similar for all
three pre-industrial periods and consistent with conifer forest and woodland
burning, that is there is significantly more VA than p-HBA. However, in the
post-industrial peak, there is more p-HBA than VA. This is evidence that the
source of this peak is more likely to be tundra grass burning or peat burning.
This is an example of one type of information that is missed by using
levoglucosan or other proxies as biomass burning tracers.

            Because
of the relative novelty of using VA and p-HBA in ice cores as biomass burning
tracers and the complexity of the many factors that determine their
concentrations in ice, there is a great deal of room for further research in
the field before this method can be used as a quantitative biomass burning
tracer (Grieman suggests that it should be used qualitatively in conjunction
with other tracers).5 As I mentioned previously, there are no other
known sources for these phenolic acids in the atmosphere. However, to my
knowledge, nobody has examined the possibility that plants cannot emit VA or
p-HBA or their precursors nor that these compounds cannot enter the atmosphere
directly from the soil. Many plants are known to emit aromatic compounds (i.e.
the chemicals that give aromatic plants their scent), and in fact, the aldehyde
of VA, vanillin, derives its name from the vanilla genus of orchids which emit
vanillin in large quantities. So, it is possible that some chemistry may occur
in the atmosphere that forms VA or p-HBA from these aromatic emissions. In
fact, in a recent study of aromatic Antarctic aerosol constituents, Zangrando
et al found that in aerosols that had been transported long distances and were
associated with biomass burning, there was a relatively high concentration of
VA; however, in coastal, marine-derived aerosols, the researchers found
significant amounts of Vanillin and trace amounts of VA. This suggests that the
ocean may generate Vanillin containing aerosols which may then be oxidized to
VA in the atmosphere.20 Until these questions of other potential
sources have been examined, we cannot say with certainty that the level of VA
and p-HBA found in ice cores is correlated only to biomass burning.

            Similarly,
atmospheric transport processes for VA and p-HBA should be studied further
before they can be used as quantitative biomass burning tracers. While models
have suggested that long distance transport is possible, and these compounds
have been observed in aerosols away from the site of any biomass burning, it is
difficult to quantify the proportion of these chemicals generated by burning
that will be oxidized by the OH radical or deposited quickly. In fact, the same
studies that observed the presence of these compounds in aerosols away from
biomass burning also noted that they are present in concentrations 1-2 orders
of magnitude higher nearer to fires.20,21 Therefore, if we wish to
take a more quantitative approach to biomass burning, we must determine what
proportion of these chemicals travel a given distance in the atmosphere. One of
the central questions is what proportion of these acids will be in the aerosol
phase (somewhat shielded from the OH radical) versus the gas phase where the
acids are easily susceptible to oxidation, and, as Donahue’s study demonstrated,
small perturbations can have large effects on gas/aerosol partitioning for
organics and therefore the atmospheric lifetimes of these species.11
In 1994, Subramanyam et al. used an annular denuder sampling system followed by
concentration and HPLC-UV analysis to determine the gas/aerosol phase
partitioning for polycyclic aromatic hydrocarbons and phenols, compounds that
are chemically similar to phenolic acids, in a polluted atmosphere in
Louisiana.22 It would be interesting and informative to perform a
similar experiment to quantify the partitioning of VA and p-HBA at varying
distances from a fire site. In short, open questions remain regarding the speed
of degradation and distance of transport of these compounds.

            Furthermore,
in order to relate the concentrations of VA and p-HBA in ice to those in the
atmosphere, the deposition of VA and p-HBA on ice sheets should be examined.
The importance of depositional processes has been demonstrated by other studies
examining biomass burning tracers in ice cores. For example, McConnel et al.
were able to further constrain the source of black carbon found in Greenland
ice cores using models that showed black carbon is primarily deposited by wet
processes.18 Similarly, Legrand explains differences in ammonium
concentrations in two Greenland ice cores by citing differences in
precipitation. That is, burning derived ammonium is more likely to be deposited
by rain in the summer than snow in winter, so even though net accumulation
rates at the two sites are similar, summer precipitation rates are significantly
higher at one of the ice core cites causing he discrepancy.23 In the
same vein, Fischer et al. attempted to measure the ammonium in the NGRIP and
GRIP ice cores from Greenland to examine North American biomass burning. They
used a simple method developed from the wet deposition scavenging ratio of
sulfate and from (admittedly sparse) observations of dry deposition velocity to
model the deposition of ammonium both during transit and at the ice core site.24
This enabled the researchers to relate concentrations of ammonium in the ice
core to those in the atmosphere both directly above the ice and at the source
of combustion. This is an important step in moving towards the quantification
of biomass burning, and Grieman suggests that a similar technique may be used
to evaluate VA and p-HBA concentrations in the atmosphere in the future.

            Of
the four major known factors that determine VA and p-HBA levels in ice cores
(emissions, transport, depositional processes, and post-deposition processes)
we have addressed uncertainties and potential future research directions for
all except post-depositional processes including photochemistry, meltwater
contamination, and re-volatilization of the deposited phenolic acids. To my
knowledge, these processes as they relate to VA and p-HBA have not been
studied. However, it has been shown conclusively that other organic acids such
as glycolate disappear from ice cores over time, limiting its use as a tracer
to more recent times.23 Similarly, Grannas et al assert that
post-depositional chemistry in snow is not trivial for organic species and
their study found that the concentration of benzopyrene decreased by more than
90% from the surface to the bottom of a 3 m deep snow pit in Greenland.25
Benzopyrene is a polycyclic aromatic hydrocarbon that is also a product of
organic combustion and is fairly chemically similar to VA and p-HBA. Additionally,
semi-volatile organic compounds like VA and p-HBA, while not as likely to be
subject to re-volatilization as more highly volatile species, could well
dissolve in any meltwater, causing changes in the chemical stratigraphy within
the ice core, presenting obvious problems for the purposes of biomass burning
determination. These post-depositional processes must be understood as well in
order to determine the quantitative relationship between the levels of VA and
p-HBA in ice cores and biomass burning.

            In
addition to these four major uncertainties, I would like to address a couple
more minor ones. The first relates to method validation. Thus far, to my
knowledge, Grieman’s study is the only literature that uses VA and p-HBA found
in ice cores as biomass burning tracers (over a period of longer than a few
centuries). The need for doing so largely rose out of the paucity of charcoal
sediments in Central Eurasia and Siberia. Despite Siberia being the largest
forested area in the Northern Hemisphere and contributing significantly to
burning emissions, the Global Charcoal Database contains only 11 records from
Siberia. While this makes the utility of this new method obvious in this
context, it is still important to compare the results of this method with more
established methods. The only Siberian charcoal record that is resolved well
enough to compare with the Akademii Nauk Ice Core is the Bolshoe Bog record
which does exhibit similarly elevated levels of these phenolic acids.5
Moreover, the 2007 McConnel study of a Greenland ice core showed that black
carbon and VA levels were well correlated between 1790 and 1850 after which
anthropogenic sources of black carbon became too prevalent to determine biomass
burning.18 Interestingly, in 2012, Kawamura found decent agreement
between VA, p-HBA and levoglucosan peaks in a 300 year ice core taken from the
Kamchatka peninsula. While these early results are promising, further
validation should be undertaken by performing a similar analysis on an ice core
taken from a region where charcoal records are more numerous. High latitude
regions of both North America and South America would nicely meet this
requirement.

            The
final uncertainty that I wish to address is how changes in burning conditions
may affect the lignin combustion products. In addition to temperature, humidity
and available oxygen may also change how lignin burns. Modeling of the past
millennia and contemporary observations indicate that most burning in Eurasian boreal
forests consists of relatively low intensity ground fires.26
Meanwhile, in North American conifer forests, fires tend to burn with high temperature,
above the ground in the tree foliage. It would be interesting to investigate
the effects these different types of fires could have on which phenolic acids
are present in ice cores and their relative abundance. Currently, I am analyzing
the levels of these phenolic acids in an ice core taken from the Eclipse
Icefield in the Alaska Range, and I am hopeful that this work may answer some
of the questions outlined above.

            In
conclusion, VA and p-HBA are promising biomass burning tracers found in ice cores,
and they may prove particularly useful for analyzing burning in high latitude
boreal forests. Their atmospheric lifetime and lack of other documented sources
enables researchers to derive information unique from the information gleaned
from other biomass burning tracers. However, there is ample room to improve the
scientific understanding of several processes that affect the concentrations of
these phenolic acids in ice cores, namely the potential for other emission
sources, complex atmospheric transport processes including aerosol/gas phase partitioning,
depositional processes, and post-depositional processes. Currently, research is
under way to examine some of these questions and to improve the understanding
of the quantitative relationship between biomass burning and the levels of VA and
p-HBA found in ice cores.