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1. Identifying the Sources and Drivers of Nitrous Oxide Accumulation in the Eddy‐Influenced Eastern Tropical North Pacific Oxygen‐Deficient Zone

   https://doi.org/10.1029/2022GB007310  

   Monreal, Patrick J. ; Kelly, Colette L. ; Travis, Nicole M. ; Casciotti, Karen L. ( June 2022 , Global Biogeochemical Cycles)

   Nitrous oxide (N₂O) is a powerful greenhouse gas, and oceanic sources account for up to one third of the total natural flux to the atmosphere. In oxygen‐deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N₂O can be produced and consumed by several biological processes. In this study, the concentration and isotopocule ratios of N₂O from a 2016 cruise in the ETNP were analyzed to examine sources of and controls on N₂O cycling across this region. Along the north‐south transect, three distinct biogeochemical regimes were identified: background, core‐ODZ, and high‐N₂O stations. Background stations were characterized by smaller variations in N₂O concentration and isotopic profiles relative to the other regimes. Core‐ODZ stations were characterized by co‐occurring N₂O production and consumption at anoxic depths, indicated by high δ¹⁸O‐N₂O (>90‰) and low δ¹⁵N₂O^β(<−10‰) values, and confirmed by a time‐dependent model, which indicated that N₂O production via denitrification was significant and may occur with a nonzero site preference. High‐N₂O stations, located at the periphery of a mesoscale eddy, were defined by N₂O reaching 126.07 ± 12.6 nM and low oxygen concentrations expanding into near‐surface isopycnals. At these stations, model results indicated significant N₂O production from ammonia‐oxidizing archaea and denitrification from nitrate at the N₂O maximummore »within the oxycline, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N₂O production mechanisms to a mesoscale eddy through isotopocule measurements, suggesting the importance of eddies to spatiotemporal variability in N₂O cycling and emissions from this region.

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2. Sources of Uncertainty in Regional and Global Terrestrial CO ₂ Exchange Estimates

   https://doi.org/10.1029/2019GB006393  

   Bastos, A. ; O'Sullivan, M. ; Ciais, P. ; Makowski, D. ; Sitch, S. ; Friedlingstein, P. ; Chevallier, F. ; Rödenbeck, C. ; Pongratz, J. ; Luijkx, I. T. ; et al ( February 2020 , Global Biogeochemical Cycles)

   The Global Carbon Budget 2018 (GCB2018) estimated by the atmospheric COgrowth rate, fossil fuel emissions, and modeled (bottom‐up) land and ocean fluxes cannot be fully closed, leading to a “budget imbalance,” highlighting uncertainties in GCB components. However, no systematic analysis has been performed on which regions or processes contribute to this term. To obtain deeper insight on the sources of uncertainty in global and regional carbon budgets, we analyzed differences in Net Biome Productivity (NBP) for all possible combinations of bottom‐up and top‐down data sets in GCB2018: (i) 16 dynamic global vegetation models (DGVMs), and (ii) 5 atmospheric inversions that match the atmospheric COgrowth rate. We find that the global mismatch between the two ensembles matches well the GCB2018 budget imbalance, with Brazil, Southeast Asia, and Oceania as the largest contributors. Differences between DGVMs dominate global mismatches, while at regional scale differences between inversions contribute the most to uncertainty. At both global and regional scales, disagreement on NBP interannual variability between the two approaches explains a large fraction of differences. We attribute this mismatch to distinct responses to El Niño–Southern Oscillation variability between DGVMs and inversions and to uncertainties in land use change emissions, especially in Southmore »America and Southeast Asia. We identify key needs to reduce uncertainty in carbon budgets: reducing uncertainty in atmospheric inversions (e.g., through more observations in the tropics) and in land use change fluxes, including more land use processes and evaluating land use transitions (e.g., using high‐resolution remote‐sensing), and, finally, improving tropical hydroecological processes and fire representation within DGVMs.

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3. Multi‐hypothesis comparison of Farquhar and Collatz photosynthesis models reveals the unexpected influence of empirical assumptions at leaf and global scales

   https://doi.org/10.1111/gcb.15366  

   Walker, Anthony P. ; Johnson, Abbey L. ; Rogers, Alistair ; Anderson, Jeremiah ; Bridges, Robert A. ; Fisher, Rosie A. ; Lu, Dan ; Ricciuto, Daniel M. ; Serbin, Shawn P. ; Ye, Ming ( October 2020 , Global Change Biology)

   Mechanistic photosynthesis models are at the heart of terrestrial biosphere models (TBMs) simulating the daily, monthly, annual and decadal rhythms of carbon assimilation (A). These models are founded on robust mathematical hypotheses that describe howAresponds to changes in light and atmospheric CO₂concentration. Two predominant photosynthesis models are in common usage: Farquhar (FvCB) and Collatz (CBGB). However, a detailed quantitative comparison of these two models has never been undertaken. In this study, we unify the FvCB and CBGB models to a common parameter set and use novel multi‐hypothesis methods (that account for both hypothesis and parameter variability) for process‐level sensitivity analysis. These models represent three key biological processes: carboxylation, electron transport, triose phosphate use (TPU) and an additional model process: limiting‐rate selection. Each of the four processes comprises 1–3 alternative hypotheses giving 12 possible individual models with a total of 14 parameters. To broaden inference, TBM simulations were run and novel, high‐resolution photosynthesis measurements were made. We show that parameters associated with carboxylation are the most influentialparametersbut also reveal the surprising and marked dominance of the limiting‐rate selectionprocess(accounting for 57% of the variation inAvs. 22% for carboxylation). The limiting‐rate selection assumption proposed by CBGB smooths the transition between limiting ratesmore »and always reducesAbelow the minimum of all potentially limiting rates, by up to 25%, effectively imposing a fourth limitation onA. Evaluation of the CBGB smoothing function in three TBMs demonstrated a reduction in globalAby 4%–10%, equivalent to 50%–160% of current annual fossil fuel emissions. This analysis reveals a surprising and previously unquantified influence of a process that has been integral to many TBMs for decades, highlighting the value of multi‐hypothesis methods.

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4. Measurements to determine the mixing state of black carbon emitted from the 2017–2018 California wildfires and urban Los Angeles

   https://doi.org/10.5194/acp-20-15635-2020  

   Ko, Joseph ; Krasowsky, Trevor ; Ban-Weiss, George ( January 2020 , Atmospheric Chemistry and Physics)

   Abstract. The effects of atmospheric black carbon (BC) on climate and public health have been well established, but large uncertainties remain regarding the extent of the impacts of BC at different temporal and spatial scales. These uncertainties are largely due to the heterogeneous nature of BC in terms of its spatiotemporal distribution, mixing state, and coating composition. Here, we seek to further understand the size and mixing state of BC emitted from various sources and aged over different timescales using field measurements in the Los Angeles region. We measured refractory black carbon (rBC) with a single-particle soot photometer (SP2) on Catalina Island, California (∼70 km southwest of downtown Los Angeles) during three different time periods. During the first campaign (September 2017), westerly winds were dominant and measured air masses were representative of well-aged background over the Pacific Ocean. In the second and third campaigns (December 2017 and November 2018, respectively), atypical Santa Ana wind conditions allowed us to measure biomass burning rBC (BCbb) from air masses dominated by large biomass burning events in California and fossil fuel rBC (BCff) from the Los Angeles Basin. We observed that the emissions source type heavily influenced both the size distribution of the rBC cores and the rBC mixing state. BCbb had thicker coatings and larger coremore »diameters than BBff. We observed a mean coating thickness (CTBC) of ∼40–70 nm and a count mean diameter (CMD) of ∼120 nm for BCbb. For BCff, we observed a CTBC of ∼5–15 nm and a CMD of ∼100 nm. Our observations also provided evidence that aging led to an increased CTBC for both BCbb and BCff. Aging timescales < ∼1 d were insufficient to thickly coat freshly emitted BCff. However, CTBC for aged BCff within aged background plumes was ∼35 nm thicker than CTBC for fresh BCff. Likewise, we found that CTBC for aged BCbb was ∼18 nm thicker than CTBC for fresh BCbb. The results presented in this study highlight the wide variability in the BC mixing state and provide additional evidence that the emissions source type and aging influence rBC microphysical properties.« less
5. Fixing a snag in carbon emissions estimates from wildfires

   https://doi.org/10.1111/gcb.14716  

   Stenzel, Jeffrey E. ; Bartowitz, Kristina J. ; Hartman, Melannie D. ; Lutz, James A. ; Kolden, Crystal A. ; Smith, Alistair M. S. ; Law, Beverly E. ; Swanson, Mark E. ; Larson, Andrew J. ; Parton, William J. ; et al ( July 2019 , Global Change Biology)

   Wildfire is an essential earth‐system process, impacting ecosystem processes and the carbon cycle. Forest fires are becoming more frequent and severe, yet gaps exist in the modeling of fire on vegetation and carbon dynamics. Strategies for reducing carbon dioxide (CO₂) emissions from wildfires include increasing tree harvest, largely based on the public assumption that fires burn live forests to the ground, despite observations indicating that less than 5% of mature tree biomass is actually consumed. This misconception is also reflected though excessive combustion of live trees in models. Here, we show that regional emissions estimates using widely implemented combustion coefficients are 59%–83% higher than emissions based on field observations. Using unique field datasets from before and after wildfires and an improved ecosystem model, we provide strong evidence that these large overestimates can be reduced by using realistic biomass combustion factors and by accurately quantifying biomass in standing dead trees that decompose over decades to centuries after fire (“snags”). Most model development focuses on area burned; our results reveal that accurately representing combustion is also essential for quantifying fire impacts on ecosystems. Using our improvements, we find that western US forest fires have emitted 851 ± 228 Tg CO₂(~half ofmore »alternative estimates) over the last 17 years, which is minor compared to 16,200 Tg CO₂from fossil fuels across the region.

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