Twentieth Century Black Carbon and Dust Deposition on South Cascade Glacier, Washington State, USA, as Reconstructed From a 158‐m‐Long Ice Core

Light absorbing particles (LAPs) include black carbon (BC) and mineral dust and are of interest due to their positive radiative forcing and contribution to albedo reductions and snow and glacier melt. This study documents historic BC and dust deposition as well as their effect on albedo on South Cascade Glacier (SCG) in Washington State (USA) through the analysis of a 158‐m (139.5‐m water equivalent [w.e.]) ice core extracted in 1994 and spanning the period 1840–1991. Peak BC deposition occurred between 1940 and 1960, when median BC concentrations were 16 times higher than background, likely dominated by domestic coal and forest fire emissions. Post 1960 BC concentrations decrease, followed by an increase from 1977 to 1991 due to melt consolidation and higher emissions. Differences between the SCG record and BC emission inventories, as well as ice core records from other regions, highlight regional differences in the timing of anthropogenic and biomass BC emissions. Dust deposition on SCG is dominated by local sources and is variable throughout the record. Albedo reductions from LAP are dominated by dust deposition, except during high BC deposition events from forest fires and during 1940–1960 when BC and dust similarly contribute to albedo reductions. This study furthers understanding of the factors contributing to historical snowmelt and glacier retreat in the Cascades and demonstrates that ice cores retrieved from temperate glaciers have the potential to provide valuable records of LAP deposition

Determining water sources in the boundary layer from tall tower profiles of water vapor and surface water isotope ratios after a snowstorm in Colorado

The D/H isotope ratio is used to attribute boundary layer humidity changes to the set of contributing fluxes for a case following a snowstorm in which a snow pack of about 10 cm vanished. Profiles of H<sub>2</sub>O and CO<sub>2</sub> mixing ratio, D/H isotope ratio, and several thermodynamic properties were measured from the surface to 300 m every 15 min during four winter days near Boulder, Colorado. Coeval analysis of the D/H ratios and CO<sub>2</sub> concentrations find these two variables to be complementary with the former being sensitive to daytime surface fluxes and the latter particularly indicative of nocturnal surface sources. Together they capture evidence for strong vertical mixing during the day, weaker mixing by turbulent bursts and low level jets within the nocturnal stable boundary layer during the night, and frost formation in the morning. The profiles are generally not well described with a gradient mixing line analysis because D/H ratios of the end members (i.e., surface fluxes and the free troposphere) evolve throughout the day which leads to large uncertainties in the estimate of the D/H ratio of surface water flux. A mass balance model is constructed for the snow pack, and constrained with observations to provide an optimal estimate of the partitioning of the surface water flux into contributions from sublimation, evaporation of melt water in the snow and evaporation from ponds. Results show that while vapor measurements are important in constraining surface fluxes, measurements of the source reservoirs (soil water, snow pack and standing liquid) offer stronger constraint on the surface water balance. Measurements of surface water are therefore essential in developing observational programs that seek to use isotopic data for flux attribution

Influence of clouds and diffuse radiation on ecosystem-atmosphere CO2 and CO18O exchanges

This study evaluates the potential impact of clouds on ecosystem CO 2 and CO2 isotope fluxes ("isofluxes") in two' contrasting ecosystems (a broadleaf deciduous forest and a C4 grassland) in a region for which cloud cover, meteorological, and isotope data are available for driving the isotope-enabled land surface model (ISOLSM). Our model results indicate a large impact of clouds on ecosystem CO2 fluxes and isofluxes. Despite lower irradiance on partly cloudy and cloudy days, predicted forest canopy photosynthesis was substantially higher than on clear, sunny days, and the highest carbon uptake was achieved on the cloudiest day. This effect was driven by a large increase in light-limited shade leaf photosynthesis following an increase in the diffuse fraction of irradiance. Photosynthetic isofluxes, by contrast, were largest on partly cloudy days, as leaf water isotopic composition was only slightly depleted and photosynthesis was enhanced, as compared to adjacent clear-sky days. On the cloudiest day, the forest exhibited intermediate isofluxes: although photosynthesis was highest on this day, leaf-to-atmosphere isofluxes were reduced from a feedback of transpiration on canopy relative humidity and leaf water. Photosynthesis and isofluxes were both reduced in the C4 grass canopy with increasing cloud cover and diffuse fraction as a result of near-constant light limitation of photosynthesis. These results suggest that some of the unexplained variation in global mean δ18O of CO2 may be driven by large-scale changes in clouds and aerosols and their impacts on diffuse radiation, photosynthesis, and relative humidity

Influence of clouds and diffuse radiation on ecosystem-atmosphere CO 2 and CO 18 O exchanges

This study evaluates the potential impact of clouds on ecosystem CO2 and CO2 isotope fluxes (“isofluxes”) in two contrasting ecosystems (a broadleaf deciduous forest and a C4 grassland) in a region for which cloud cover, meteorological, and isotope data are available for driving the isotope-enabled land surface model (ISOLSM). Our model results indicate a large impact of clouds on ecosystem CO2 fluxes and isofluxes. Despite lower irradiance on partly cloudy and cloudy days, predicted forest canopy photosynthesis was substantially higher than on clear, sunny days, and the highest carbon uptake was achieved on the cloudiest day. This effect was driven by a large increase in light-limited shade leaf photosynthesis following an increase in the diffuse fraction of irradiance. Photosynthetic isofluxes, by contrast, were largest on partly cloudy days, as leaf water isotopic composition was only slightly depleted and photosynthesis was enhanced, as compared to adjacent clear-sky days. On the cloudiest day, the forest exhibited intermediate isofluxes: although photosynthesis was highest on this day, leaf-to-atmosphere isofluxes were reduced from a feedback of transpiration on canopy relative humidity and leaf water. Photosynthesis and isofluxes were both reduced in the C4 grass canopy with increasing cloud cover and diffuse fraction as a result of near-constant light limitation of photosynthesis. These results suggest that some of the unexplained variation in global mean δ 18O of CO2 may be driven by large-scale changes in clouds and aerosols and their impacts on diffuse radiation, photosynthesis, and relative humidity

Influence of clouds and diffuse radiation on ecosystem-atmosphere CO 2 and CO 18 O exchanges

This study evaluates the potential impact of clouds on ecosystem CO2 and CO2 isotope fluxes (“isofluxes”) in two contrasting ecosystems (a broadleaf deciduous forest and a C4 grassland) in a region for which cloud cover, meteorological, and isotope data are available for driving the isotope-enabled land surface model (ISOLSM). Our model results indicate a large impact of clouds on ecosystem CO2 fluxes and isofluxes. Despite lower irradiance on partly cloudy and cloudy days, predicted forest canopy photosynthesis was substantially higher than on clear, sunny days, and the highest carbon uptake was achieved on the cloudiest day. This effect was driven by a large increase in light-limited shade leaf photosynthesis following an increase in the diffuse fraction of irradiance. Photosynthetic isofluxes, by contrast, were largest on partly cloudy days, as leaf water isotopic composition was only slightly depleted and photosynthesis was enhanced, as compared to adjacent clear-sky days. On the cloudiest day, the forest exhibited intermediate isofluxes: although photosynthesis was highest on this day, leaf-to-atmosphere isofluxes were reduced from a feedback of transpiration on canopy relative humidity and leaf water. Photosynthesis and isofluxes were both reduced in the C4 grass canopy with increasing cloud cover and diffuse fraction as a result of near-constant light limitation of photosynthesis. These results suggest that some of the unexplained variation in global mean δ 18O of CO2 may be driven by large-scale changes in clouds and aerosols and their impacts on diffuse radiation, photosynthesis, and relative humidity