Land surface modellers need measurable proxies to
constrain the quantity of carbon dioxide (CO2) assimilated by
continental plants through photosynthesis, known as gross primary production
(GPP). ...Carbonyl sulfide (COS), which is taken up by leaves through their
stomates and then hydrolysed by photosynthetic enzymes, is a candidate GPP
proxy. A former study with the ORCHIDEE land surface model used a fixed
ratio of COS uptake to CO2 uptake normalised to respective ambient
concentrations for each vegetation type (leaf relative uptake, LRU) to
compute vegetation COS fluxes from GPP. The LRU approach is known to have
limited accuracy since the LRU ratio changes with variables such as
photosynthetically active radiation (PAR): while CO2 uptake slows under
low light, COS uptake is not light limited. However, the LRU approach has
been popular for COS–GPP proxy studies because of its ease of application
and apparent low contribution to uncertainty for regional-scale
applications. In this study we refined the COS–GPP relationship and
implemented in ORCHIDEE a mechanistic model that describes COS uptake by
continental vegetation. We compared the simulated COS fluxes against
measured hourly COS fluxes at two sites and studied the model behaviour and
links with environmental drivers. We performed simulations at a global scale,
and we estimated the global COS uptake by vegetation to be −756 Gg S yr−1,
in the middle range of former studies (−490 to −1335 Gg S yr−1). Based
on monthly mean fluxes simulated by the mechanistic approach in ORCHIDEE, we
derived new LRU values for the different vegetation types, ranging between
0.92 and 1.72, close to recently published averages for observed values of
1.21 for C4 and 1.68 for C3 plants. We transported the COS using the monthly
vegetation COS fluxes derived from both the mechanistic and the LRU
approaches, and we evaluated the simulated COS concentrations at NOAA sites.
Although the mechanistic approach was more appropriate when comparing to
high-temporal-resolution COS flux measurements, both approaches gave similar
results when transporting with monthly COS fluxes and evaluating COS
concentrations at stations. In our study, uncertainties between these two
approaches are of secondary importance compared to the uncertainties in the
COS global budget, which are currently a limiting factor to the potential of
COS concentrations to constrain GPP simulated by land surface models on the
global scale.
Understanding climate controls on gross primary productivity (GPP) is crucial for accurate projections of the future land carbon cycle. Major uncertainties exist due to the challenge in separating ...GPP and respiration from observations of the carbon dioxide (CO₂) flux. Carbonyl sulfide (COS) has a dominant vegetative sink, and plant COS uptake is used to infer GPP through the leaf relative uptake (LRU) ratio of COS to CO₂ fluxes. However, little is known about variations of LRU under changing environmental conditions and in different phenological stages. We present COS and CO₂ fluxes and LRU of Scots pine branches measured in a boreal forest in Finland during the spring recovery and summer. We find that the diurnal dynamics of COS uptake is mainly controlled by stomatal conductance, but the leaf internal conductance could significantly limit the COS uptake during the daytime and early in the season. LRU varies with light due to the differential light responses of COS and CO₂ uptake, and with vapor pressure deficit (VPD) in the peak growing season, indicating a humidity-induced stomatal control. Our COS-based GPP estimates show that it is essential to incorporate the variability of LRU with environmental variables for accurate estimation of GPP on ecosystem, regional, and global scales.
The uptake of carbonyl sulfide (COS) by terrestrial plants is linked to
photosynthetic uptake of CO2 as these gases partly share the same
uptake pathway. Applying COS as a photosynthesis tracer in ...models requires an
accurate representation of biosphere COS fluxes, but these models have not
been extensively evaluated against field observations of COS fluxes. In this
paper, the COS flux as simulated by the Simple Biosphere Model, version 4
(SiB4), is updated with the latest mechanistic insights and evaluated with site
observations from different biomes: one evergreen needleleaf forest, two
deciduous broadleaf forests, three grasslands, and two crop fields spread over
Europe and North America. We improved SiB4 in several ways to improve its
representation of COS. To account for the effect of atmospheric COS mole
fractions on COS biosphere uptake, we replaced the fixed atmospheric COS mole
fraction boundary condition originally used in SiB4 with spatially and
temporally varying COS mole fraction fields. Seasonal amplitudes of COS mole
fractions are ∼50–200 ppt at the investigated sites with a
minimum mole fraction in the late growing season. Incorporating seasonal
variability into the model reduces COS uptake rates in the late growing
season, allowing better agreement with observations. We also replaced the
empirical soil COS uptake model in SiB4 with a mechanistic model that
represents both uptake and production of COS in soils, which improves the
match with observations over agricultural fields and fertilized grassland
soils. The improved version of SiB4 was capable of simulating the diurnal and
seasonal variation in COS fluxes in the boreal, temperate, and Mediterranean
region. Nonetheless, the daytime vegetation COS flux is underestimated on
average by 8±27 %, albeit with large variability across sites. On a
global scale, our model modifications decreased the modeled COS terrestrial
biosphere sink from 922 Gg S yr−1 in the original SiB4 to
753 Gg S yr−1 in the updated version. The largest decrease in
fluxes was driven by lower atmospheric COS mole fractions over regions with
high productivity, which highlights the importance of accounting for
variations in atmospheric COS mole fractions. The change to a different soil
model, on the other hand, had a relatively small effect on the global
biosphere COS sink. The secondary role of the modeled soil component in the
global COS budget supports the use of COS as a global photosynthesis tracer. A
more accurate representation of COS uptake in SiB4 should allow for improved
application of atmospheric COS as a tracer of local- to global-scale
terrestrial photosynthesis.
Carbonyl sulfide (COS) has the potential to be used as a climate diagnostic due to its close coupling to the biospheric uptake of CO2 and its role in the formation of stratospheric aerosol. The ...current understanding of the COS budget, however, lacks COS sources, which have previously been allocated to the tropical ocean. This paper presents a first attempt at global inverse modelling of COS within the 4-dimensional variational data-assimilation system of the TM5 chemistry transport model (TM5-4DVAR) and a comparison of the results with various COS observations. We focus on the global COS budget, including COS production from its precursors carbon disulfide (CS2) and dimethyl sulfide (DMS). To this end, we implemented COS uptake by soil and vegetation from an updated biosphere model (Simple Biosphere Model – SiB4). In the calculation of these fluxes, a fixed atmospheric mole fraction of 500 pmol mol−1 was assumed. We also used new inventories for anthropogenic and biomass burning emissions. The model framework is capable of closing the COS budget by optimizing for missing emissions using NOAA observations in the period 2000–2012. The addition of 432 Gg a−1 (as S equivalents) of COS is required to obtain a good fit with NOAA observations. This missing source shows few year-to-year variations but considerable seasonal variations. We found that the missing sources are likely located in the tropical regions, and an overestimated biospheric sink in the tropics cannot be ruled out due to missing observations in the tropical continental boundary layer. Moreover, high latitudes in the Northern Hemisphere require extra COS uptake or reduced emissions. HIPPO (HIAPER Pole-to-Pole Observations) aircraft observations, NOAA airborne profiles from an ongoing monitoring programme and several satellite data sources are used to evaluate the optimized model results. This evaluation indicates that COS mole fractions in the free troposphere remain underestimated after optimization. Assimilation of HIPPO observations slightly improves this model bias, which implies that additional observations are urgently required to constrain sources and sinks of COS. We finally find that the biosphere flux dependency on the surface COS mole fraction (which was not accounted for in this study) may substantially lower the fluxes of the SiB4 biosphere model over strong-uptake regions. Using COS mole fractions from our inversion, the prior biosphere flux reduces from 1053 to 851 Gg a−1, which is closer to 738 Gg a−1 as was found by Berry et al. (2013). In planned further studies we will implement this biosphere dependency and additionally assimilate satellite data with the aim of better separating the role of the oceans and the biosphere in the global COS budget.
Nighttime vegetative uptake of carbonyl sulfide (COS) can exist due to the incomplete closure of stomata and the light independence of the enzyme carbonic anhydrase, which complicates the use of COS ...as a tracer for gross primary productivity (GPP). In this study we derived nighttime COS fluxes in a boreal forest (the SMEAR II station in Hyytiälä, Finland; 61°51′ N, 24°17′ E; 181 m a.s.l.) from June to November 2015 using two different methods: eddy-covariance (EC) measurements (FCOS-EC) and the radon-tracer method (FCOS-Rn). The total nighttime COS fluxes averaged over the whole measurement period were −6.8 ± 2.2 and −7.9 ± 3.8 pmol m−2 s−1 for FCOS-Rn and FCOS-EC, respectively, which is 33–38 % of the average daytime fluxes and 21 % of the total daily COS uptake. The correlation of 222Rn (of which the source is the soil) with COS (average R2 = 0.58) was lower than with CO2 (0.70), suggesting that the main sink of COS is not located at the ground. These observations are supported by soil chamber measurements that show that soil contributes to only 34–40 % of the total nighttime COS uptake. We found a decrease in COS uptake with decreasing nighttime stomatal conductance and increasing vapor-pressure deficit and air temperature, driven by stomatal closure in response to a warm and dry period in August. We also discuss the effect that canopy layer mixing can have on the radon-tracer method and the sensitivity of (FCOS-EC) to atmospheric turbulence. Our results suggest that the nighttime uptake of COS is mainly driven by the tree foliage and is significant in a boreal forest, such that it needs to be taken into account when using COS as a tracer for GPP.
The seasonality and interannual variability of
terrestrial carbonyl sulfide (COS) fluxes are poorly constrained. We present
the first easy-to-use parameterization for the net COS forest sink based on ...the
longest existing eddy covariance record from a boreal pine forest, covering 32 months
over 5 years. Fluxes from hourly to yearly scales are reported, with the aim
of revealing controlling factors and the level of interannual variability.
The parameterization is based on the photosynthetically active radiation,
vapor pressure deficit, air temperature, and leaf area index. Wavelet
analysis of the ecosystem fluxes confirmed earlier findings from
branch-level fluxes at the same site and revealed a 3 h lag between COS
uptake and air temperature maxima at the daily scale, whereas no lag between
radiation and COS flux was found. The spring recovery of the flux after the
winter dormancy period was mostly governed by air temperature, and the onset
of the uptake varied by 2 weeks. For the first time, we report a significant
reduction in ecosystem-scale COS uptake under a large water vapor pressure
deficit in summer. The maximum monthly and weekly median COS uptake varied by
26 % and 20 % between years, respectively. The timing of the latter varied
by 6 weeks. The fraction of the nocturnal uptake remained below 21 % of
the total COS uptake. We observed the growing season (April–August) average
net flux of COS totaling −58.0 g S ha−1 with 37 % interannual
variability. The long-term flux observations were scaled up to evergreen
needleleaf forests (ENFs) in the whole boreal region using the Simple Biosphere
Model Version 4 (SiB4). The observations were closely simulated using
SiB4 meteorological drivers and phenology. The total COS uptake by boreal
ENFs was in line with a missing COS sink at high latitudes pointed out in
earlier studies.
Carbonyl sulfide (COS) is a promising tracer for the estimation of
terrestrial ecosystem gross primary production (GPP). However, understanding
its non-GPP-related sources and sinks, e.g., ...anthropogenic sources and soil
sources and sinks, is also critical to the success of the approach. Here we
infer the regional sources and sinks of COS using continuous in situ mole fraction
profile measurements of COS along the 60 m tall Lutjewad tower (1 m a.s.l.;
53∘24′ N, 6∘21′ E) in the Netherlands. To identify
potential sources that caused the observed enhancements of COS mole
fractions at Lutjewad, both discrete flask samples and in situ measurements
in the province of Groningen were made from a mobile van using a quantum
cascade laser spectrometer (QCLS). We also simulated the COS mole fractions
at Lutjewad using the Stochastic Time-Inverted Lagrangian Transport (STILT)
model combined with emission inventories and plant uptake fluxes. We
determined the nighttime COS fluxes to be -3.0±2.6 pmol m−2 s−1 using the radon-tracer correlation approach and Lutjewad
observations. Furthermore, we identified and quantified several COS sources,
including biodigesters, sugar production facilities and silicon carbide
production facilities in the province of Groningen. Moreover, the simulation
results show that the observed COS enhancements can be partially explained
by known industrial sources of COS and CS2, in particular from the Ruhr
Valley (51.5∘ N, 7.2∘ E) and Antwerp (51.2∘ N,
4.4∘ E) areas. The contribution of likely missing anthropogenic
sources of COS and CS2 in the inventory may be significant. The impact
of the identified sources in the province of Groningen is estimated to be
negligible in terms of the observed COS enhancements. However, in specific
conditions, these sources may influence the measurements in Lutjewad. These
results are valuable for improving our understanding of the sources and
sinks of COS, contributing to the use of COS as a tracer for GPP.
In the literature, no consensus can be found on the exact form of the universal funtions of Monin-Obukhov similarity theory (MOST) for the structure parameters of temperature,
C
T
2
, and humidity,
C
...q
2
, and the dissipation rate of turbulent kinetic energy,
ε
. By combining 11 datasets and applying data treatment with spectral data filtering and error-weighted curve-fitting we first derived robust MOST functions of
C
T
2
,
C
q
2
and
ε
that cover a large stability range for both unstable and stable conditions. Second, as all data were gathered with the same instrumentation and were processed in the same way—in contrast to earlier studies—we were able to investigate the similarity of MOST functions across different datasets by defining MOST functions for all datasets individually. For
C
T
2
and
ε
we found no substantial differences in MOST functions for datasets over different surface types or moisture regimes. MOST functions of
C
q
2
differ from that of
C
T
2
, but we could not relate these differences to turbulence parameters often associated with non-local effects. Furthermore, we showed that limited stability ranges and a limited number of data points are plausible reasons for variations of MOST functions in the literature. Last, we investigated the sensitivity of fluxes to the uncertainty of MOST functions. We provide an overview of the uncertainty range for MOST functions of
C
T
2
,
C
q
2
and
ε
, and suggest their use in determining the uncertainty in surface fluxes.
Carbonyl sulfide (COS) is a useful tracer to estimate
gross primary production (GPP) because it shares part of the uptake pathway
with CO2. COS is taken up in plants through hydrolysis, catalyzed by
...the enzyme carbonic anhydrase (CA), but is not released. The Simple
Biosphere model version 4 (SiB4) simulates COS leaf uptake using a
conductance approach. SiB4 applies the temperature response of the RuBisCo
enzyme (used for photosynthesis) to simulate the COS leaf uptake, but the CA enzyme might respond differently to temperature. We introduce a new
temperature response function for CA in SiB4, based on enzyme kinetics with
an optimum temperature. Moreover, we determine Ball–Woodrow–Berry (BWB)
model parameters for stomatal conductance (gs) using observation-based estimates of COS flux, GPP, and gs along with meteorological measurements in an evergreen needleleaf forest (ENF) and deciduous broadleaf forest (DBF). We find that CA has optimum temperatures of 20 ∘C (ENF) and 36 ∘C (DBF), which is lower than that of RuBisCo (45 ∘C), suggesting that canopy temperature changes can critically affect CA's catalyzation activity. Optimized values for the BWB offset parameter are similar to the original value (0.010 ± 0.003 mol m−2 s−1), and optimized values for the BWB slope parameter (ENF: 16.4, DBF: 11.4) are higher than the original value (9.0) at both sites. The optimization reduces prior errors on all parameters by more than 50 % at both stations. We apply the optimized gi and gs parameters in
SiB4 site simulations, thereby improving the timing and peak of COS
assimilation. In addition, we show that SiB4 underestimates the leaf
humidity stress under conditions where high vapor pressure deficit (VPD) should limit gs in the afternoon, thereby overestimating gs. Furthermore, global COS biosphere sinks with optimized parameters show smaller COS uptake in regions where the air temperature is over 25 ∘C, mostly in the tropics, and larger uptake in regions where the temperature is below 25 ∘C. This change
corresponds with reported deficiencies in the global COS fluxes, such as
missing sinks at high latitudes and required sources in the tropics. Using
our optimization and additional observations of COS uptake over various
climate and plant types, we expect further improvements in global COS
biosphere flux estimates.
Carbonyl sulfide (COS), a trace gas in our atmosphere that leads to the formation of aerosols in the stratosphere, is largely taken up by terrestrial ecosystems. Quantifying the biosphere uptake of ...COS could provide a useful quantity to estimate gross primary productivity (GPP). Some COS sources and sinks still contain large uncertainties, and several top-down estimates of the COS budget point to an underestimation of sources, especially in the tropics. We extended the inverse model TM5-4DVAR to assimilate Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite data, in addition to National Oceanic and Atmospheric Administration (NOAA) surface data as used in a previous study. To resolve possible discrepancies among the two observational data sets, a bias correction scheme is necessary and implemented. A set of inversions is presented that explores the influence of the different measurement streams and the settings of the prior fluxes. To evaluate the performance of the inverse system, the HIAPER Pole-to-Pole Observations (HIPPO) aircraft observations and NOAA airborne profiles are used. All inversions reduce the COS biosphere uptake from a prior value of 1053 GgS a−1 to much smaller values, depending on the inversion settings. These large adjustments of the biosphere uptake often turn parts of Amazonia into a COS source. Only inversions that exclusively use MIPAS observations, or strongly reduce the prior errors on the biosphere flux, maintain the Amazon as a COS sink. Inclusion of MIPAS data in the inversion leads to a better separation of land and ocean fluxes. Over the Amazon, these inversions reduce the biosphere uptake from roughly 300 to 100 GgS a−1, indicating a strongly overestimated prior uptake in this region. Although a recent study also reported reduced COS uptake over the Amazon, we emphasise that a careful construction of prior fluxes and their associated errors remains important. For instance, an inversion that gives large freedom to adjust the anthropogenic and ocean fluxes of CS2, an important COS precursor, also closes the budget satisfactorily with much smaller adjustments to the biosphere. We achieved better characterisation of biosphere prior and uncertainty, better characterisation of combined ocean and land fluxes, and better constraint of both by combining surface and satellite observations. We recommend more COS observations to characterise biosphere and ocean fluxes, especially over the data-poor tropics.
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