Jochen Stutz

Tropical forests play a major role in the global terrestrial carbon cycle; however, substantial uncertainties remain regarding their capacity to continue acting as a carbon sink under changing climate conditions. This uncertainty is driven largely by the limited number of carbon flux observations in tropical regions. Using eddy covariance (EC) measurements over an 11-month period at La Selva, Costa Rica, we quantified net ecosystem exchange (NEE) and evaluated partitioning methods to estimate gross primary production (GPP) and ecosystem respiration. Daytime partitioning using morning flux data introduced substantial variability in respiration estimates due to rapidly changing micrometeorological conditions. In contrast, daytime afternoon, nighttime, and sundown partitioning methods produced strong agreement and physiologically realistic results. We therefore use the afternoon daytime partitioning method for subsequent light use efficiency (LUE) analysis. The La Selva rainforest functioned as a strong temporally stable carbon sink with no detectable seasonal trend, averaging 2.5 µmol CO₂ m^-2 s^-1, corresponding to an annual carbon uptake of 947 g C m^-2 yr^-1. GPP averaged 11.8 µmol CO₂ m^-2 s^-1. Random forest modeling revealed timescale-dependent environmental controls on LUE, with photosynthetically active radiation dominating short-term variability, while temperature and atmospheric moisture increasingly constrained productivity at weekly timescales. VPD and temperature exerted a strong negative effect on half-hourly LUE, indicating vulnerability to increasing atmospheric dryness. Our findings demonstrate that partitioning method choice and temporal scale strongly shape inferred GPP magnitude and drivers in this wet tropical forest, with important implications for interpreting carbon flux estimates from EC measurements.

Tropical forests play a major role in the global terrestrial carbon cycle; however, substantial uncertainties remain regarding their capacity to continue acting as a carbon sink under changing climate conditions. This uncertainty is driven largely by the limited number of carbon flux observations in tropical regions. Using eddy covariance (EC) measurements over an 11-month period at La Selva, Costa Rica, we quantified net ecosystem exchange (NEE) and evaluated partitioning methods to estimate gross primary production (GPP) and ecosystem respiration. Daytime partitioning using morning flux data introduced substantial variability in respiration estimates due to rapidly changing micrometeorological conditions. In contrast, daytime afternoon, nighttime, and sundown partitioning methods produced strong agreement and physiologically realistic results. We therefore use the afternoon daytime partitioning method for subsequent light use efficiency (LUE) analysis. The La Selva rainforest functioned as a strong temporally stable carbon sink with no detectable seasonal trend, averaging 2.5 µmol CO₂ m^-2 s^-1, corresponding to an annual carbon uptake of 947 g C m^-2 yr^-1. GPP averaged 11.8 µmol CO₂ m^-2 s^-1. Random forest modeling revealed timescale-dependent environmental controls on LUE, with photosynthetically active radiation dominating short-term variability, while temperature and atmospheric moisture increasingly constrained productivity at weekly timescales. VPD and temperature exerted a strong negative effect on half-hourly LUE, indicating vulnerability to increasing atmospheric dryness. Our findings demonstrate that partitioning method choice and temporal scale strongly shape inferred GPP magnitude and drivers in this wet tropical forest, with important implications for interpreting carbon flux estimates from EC measurements.

Aerosols play an important role for atmospheric radiative transfer in biomass burning (BB) plumes, where they control photochemistry, direct radiative forcing, and radiation-induced atmospheric dynamics. The optical properties of BB aerosol, however, remain poorly constrained with respect to their absorptive properties at ultraviolet and visible wavelengths. Aircraft-based observations show considerable variability due to heterogeneity in BB plumes, and different measurement methods do not agree with each other. To overcome this challenge, we have developed an algorithm based on the VLIDORT for photochemistry (VPC) radiative transfer model to retrieve the imaginary refractive index k ( λ ) from airborne actinic flux observations at wavelengths λ of 308–440 nm. Using three flights from NASA/NOAA’s Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) airborne experiment, we obtain values k (387 nm) between 0.02 and 0.03, and absorption Angstrom exponents α _k between 3 and 5 for different transects. Volume absorption cross section (VAC) and single scattering albedo generally agree with aircraft-based observations, but show less variability, most likely because of the inherent spatial averaging of our observations. k ( λ ), VAC and single-scattering co-albedo decrease with physical plume age, with half-lives τ1/2 of 13±3, 16±4 and 17±4 h, respectively. Based on our observations, we present a parameterization of the absorptive properties of BB aerosol from western US wildfires as a function of wavelength and plume age, which will help to improve the representation of BB aerosol in models.

Tropical forests are critical carbon sinks, yet the sub-daily mechanisms controlling their carbon and water exchange remain poorly understood. Using an 11-month eddy covariance (EC) record from La Selva Biological Station, a mature lowland wet tropical rainforest in Costa Rica, we investigated diurnal hysteresis in gross primary productivity (GPP) and the dynamics of carbon-water decoupling at the half-hourly timescale. GPP exhibited a pronounced morning bias that could not be explained by light availability, as photosynthetically active radiation (PAR) was nearly symmetric around solar noon. Instead, clockwise hysteresis loops in GPP versus vapor pressure deficit (VPD) and temperature, indicated that progressive stomatal closure under rising afternoon atmospheric demand was the primary driver of asymmetric carbon uptake. Carbon and water fluxes also diverged in diurnal timing, reflecting the greater sensitivity of CO ₂ diffusion to partial stomatal closure relative to water vapor. Ecosystem water use efficiency (WUE) and underlying WUE (uWUE) both peaked in the early morning before VPD-driven stomatal restriction developed and declined sharply through midday, confirming that the morning-afternoon asymmetry reflects a genuine reduction in carbon-water exchange efficiency rather than an artifact of atmospheric demand. Improving the representation of stomatal regulation and boundary layer VPD feedbacks in land surface models is critical for accurately projecting tropical forest carbon sink strength under continued atmospheric warming and rising VPD.

Tropical forests play a major role in the global terrestrial carbon cycle; however, substantial uncertainties remain regarding their capacity to continue acting as a carbon sink under changing climate conditions. This uncertainty is driven largely by the limited number of carbon flux observations in tropical regions. Using eddy covariance (EC) measurements over an 11-month period at La Selva, Costa Rica, we quantified net ecosystem exchange (NEE) and evaluated partitioning methods to estimate gross primary production (GPP) and ecosystem respiration. Daytime partitioning using morning flux data introduced substantial variability in respiration estimates due to rapidly changing micrometeorological conditions. In contrast, daytime afternoon, nighttime, and sundown partitioning methods produced strong agreement and physiologically realistic results. We therefore use the afternoon daytime partitioning method for subsequent light use efficiency (LUE) analysis. The La Selva rainforest functioned as a strong temporally stable carbon sink with no detectable seasonal trend, averaging 2.5 µmol CO₂ m^-2 s^-1, corresponding to an annual carbon uptake of 947 g C m^-2 yr^-1. GPP averaged 11.8 µmol CO₂ m^-2 s^-1. Random forest modeling revealed timescale-dependent environmental controls on LUE, with photosynthetically active radiation dominating short-term variability, while temperature and atmospheric moisture increasingly constrained productivity at weekly timescales. VPD and temperature exerted a strong negative effect on half-hourly LUE, indicating vulnerability to increasing atmospheric dryness. Our findings demonstrate that partitioning method choice and temporal scale strongly shape inferred GPP magnitude and drivers in this wet tropical forest, with important implications for interpreting carbon flux estimates from EC measurements.

Air pollution in cold wintertime urban areas is a ubiquitous problem. Surface temperature inversions under low wind conditions and surface emissions lead to the formation of a shallow polluted surface layer (PSL). The presence of this PSL is well documented, but its height, the rates of exchange with the background atmosphere, and the interplay between vertical mixing and chemistry remain poorly quantified. Here we provide quantitative insights into this coupled chemistry-transport system using observations and modeling of vertical pollutant profiles. Long-path Differential Optical Absorption Spectroscopy of vertical trace gas profiles during the 2022 Alaskan Layered Pollution and Chemical Analysis (ALPACA) experiment in Fairbanks, Alaska, quantitatively track the frequent establishment of PSLs with heights of 20–40 m, which sometimes persist for several days. PSL trace gas mixing ratios often plateau at levels of up to 35 ppb SO ₂ , 60 ppb NO ₂ , 2.5 ppb HONO, 7 ppb HCHO, and low ozone as a result of surface emissions, gas-phase chemistry, and ineffective mixing with the background atmosphere. Parameterizing vertical mixing with observed surface temperature gradients and wind speed in the PACT-1D chemistry and transport model leads to excellent agreement of modeled and measured trace gas profiles. The PACT-1D transport scheme determines the residence time of pollutants within the PSL to be 1–4 h, confirming the strong influence of atmospheric mixing on the composition of sustained PSL events. Altitude-dependent ozone and NO _x chemistry highlight the strong coupling between mixing and chemistry, which must be considered to quantify pollutant concentrations accurately in shallow PSLs.

Tropical forests play a major role in the global terrestrial carbon cycle; however, substantial uncertainties remain regarding their capacity to continue acting as a carbon sink under changing climate conditions. This uncertainty is driven largely by the limited number of carbon flux observations in tropical regions. Using eddy covariance (EC) measurements over an 11-month period at La Selva, Costa Rica, we quantified net ecosystem exchange (NEE) and evaluated partitioning methods to estimate gross primary production (GPP) and ecosystem respiration. Daytime partitioning using morning flux data introduced substantial variability in respiration estimates due to rapidly changing micrometeorological conditions. In contrast, daytime afternoon, nighttime, and sundown partitioning methods produced strong agreement and physiologically realistic results. We therefore use the afternoon daytime partitioning method for subsequent light use efficiency (LUE) analysis. The La Selva rainforest functioned as a strong temporally stable carbon sink with no detectable seasonal trend, averaging 2.5 µmol CO₂ m^-2 s^-1, corresponding to an annual carbon uptake of 947 g C m^-2 yr^-1. GPP averaged 11.8 µmol CO₂ m^-2 s^-1. Random forest modeling revealed timescale-dependent environmental controls on LUE, with photosynthetically active radiation dominating short-term variability, while temperature and atmospheric moisture increasingly constrained productivity at weekly timescales. VPD and temperature exerted a strong negative effect on half-hourly LUE, indicating vulnerability to increasing atmospheric dryness. Our findings demonstrate that partitioning method choice and temporal scale strongly shape inferred GPP magnitude and drivers in this wet tropical forest, with important implications for interpreting carbon flux estimates from EC measurements.

Tropical forests play a major role in the global terrestrial carbon cycle; however, substantial uncertainties remain regarding their capacity to continue acting as a carbon sink under changing climate conditions. This uncertainty is driven largely by the limited number of carbon flux observations in tropical regions. Using eddy covariance (EC) measurements over an 11-month period at La Selva, Costa Rica, we quantified net ecosystem exchange (NEE) and evaluated partitioning methods to estimate gross primary production (GPP) and ecosystem respiration. Daytime partitioning using morning flux data introduced substantial variability in respiration estimates due to rapidly changing micrometeorological conditions. In contrast, daytime afternoon, nighttime, and sundown partitioning methods produced strong agreement and physiologically realistic results. We therefore use the afternoon daytime partitioning method for subsequent light use efficiency (LUE) analysis. The La Selva rainforest functioned as a strong temporally stable carbon sink with no detectable seasonal trend, averaging 2.5 µmol CO₂ m^-2 s^-1, corresponding to an annual carbon uptake of 947 g C m^-2 yr^-1. GPP averaged 11.8 µmol CO₂ m^-2 s^-1. Random forest modeling revealed timescale-dependent environmental controls on LUE, with photosynthetically active radiation dominating short-term variability, while temperature and atmospheric moisture increasingly constrained productivity at weekly timescales. VPD and temperature exerted a strong negative effect on half-hourly LUE, indicating vulnerability to increasing atmospheric dryness. Our findings demonstrate that partitioning method choice and temporal scale strongly shape inferred GPP magnitude and drivers in this wet tropical forest, with important implications for interpreting carbon flux estimates from EC measurements.

Air pollution in cold wintertime urban areas is a ubiquitous problem. Surface temperature inversions under low wind conditions and surface emissions lead to the formation of a shallow polluted surface layer (PSL). The presence of this PSL is well documented, but its height, the rates of exchange with the background atmosphere, and the interplay between vertical mixing and chemistry remain poorly quantified. Here we provide quantitative insights into this coupled chemistry-transport system using observations and modeling of vertical pollutant profiles. Long-path Differential Optical Absorption Spectroscopy of vertical trace gas profiles during the 2022 Alaskan Layered Pollution and Chemical Analysis (ALPACA) experiment in Fairbanks, Alaska, quantitatively track the frequent establishment of PSLs with heights of 20–40m, which sometimes persist for several days. PSL trace gas mixing ratios often plateau at levels of up to 35 ppb SO₂, 60 ppb NO₂, 2.5 ppb HONO, 7 ppb HCHO, and low ozone as a result of surface emissions, gas-phase chemistry, and ineffective mixing with the background atmosphere. Parameterizing vertical mixing with observed surface temperature gradients and wind speed in the PACT-1D chemistry and transport model leads to excellent agreement of modeled and measured trace gas profiles. The PACT-1D transport scheme determines the residence time of pollutants within the PSL to be 1–4 h, confirming the strong influence of atmospheric mixing on the composition of sustained PSL events. Altitude-dependent ozone and NOx chemistry highlight the strong coupling between mixing and chemistry, which must be considered to quantify pollutant concentrations accurately in shallow PSLs.

Air pollution in cold wintertime urban areas is a ubiquitous problem. Surface temperature inversions under low wind conditions and surface emissions lead to the formation of a shallow polluted surface layer (PSL). The presence of this PSL is well documented, but its height, the rates of exchange with the background atmosphere, and the interplay between vertical mixing and chemistry remain poorly quantified. Here we provide quantitative insights into this coupled chemistry-transport system using observations and modeling of vertical pollutant profiles. Long-path Differential Optical Absorption Spectroscopy of vertical trace gas profiles during the 2022 Alaskan Layered Pollution and Chemical Analysis (ALPACA) experiment in Fairbanks, Alaska, quantitatively track the frequent establishment of PSLs with heights of 20–40m, which sometimes persist for several days. PSL trace gas mixing ratios often plateau at levels of up to 35 ppb SO₂, 60 ppb NO₂, 2.5 ppb HONO, 7 ppb HCHO, and low ozone as a result of surface emissions, gas-phase chemistry, and ineffective mixing with the background atmosphere. Parameterizing vertical mixing with observed surface temperature gradients and wind speed in the PACT-1D chemistry and transport model leads to excellent agreement of modeled and measured trace gas profiles. The PACT-1D transport scheme determines the residence time of pollutants within the PSL to be 1–4 h, confirming the strong influence of atmospheric mixing on the composition of sustained PSL events. Altitude-dependent ozone and NOx chemistry highlight the strong coupling between mixing and chemistry, which must be considered to quantify pollutant concentrations accurately in shallow PSLs.