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Most terrestrial carbon sequestration at mid-latitudes in the Northern Hemisphere occurs in seasonal, montane forest ecosystems. Winter respiratory carbon dioxide losses from these ecosystems are high, and over half of the carbon assimilated by photosynthesis in the summer can be lost the following winter. The amount of winter carbon dioxide loss is potentially susceptible to changes in the depth of the snowpack; a shallower snowpack has less insulation potential, causing colder soil temperatures and potentially lower soil respiration rates. Recent climate analyses have shown widespread declines in the winter snowpack of mountain ecosystems in the western USA and Europe that are coupled to positive temperature anomalies. Here we study the effect of changes in snow cover on soil carbon cycling within the context of natural climate variation. We use a six-year record of net ecosystem carbon dioxide exchange in a subalpine forest to show that years with a reduced winter snowpack are accompanied by significantly lower rates of soil respiration. Furthermore, we show that the cause of the high sensitivity of soil respiration rate to changes in snow depth is a unique soil microbial community that exhibits exponential growth and high rates of substrate utilization at the cold temperatures that exist beneath the snow. Our observations suggest that a warmer climate may change soil carbon sequestration rates in forest ecosystems owing to changes in the depth of the insulating snow cover.

Alkenes are reactive hydrocarbons that influence local and regional atmospheric chemistry by playing important roles in the photochemical production of tropospheric ozone and in the formation of secondary organic aerosols. The simplest alkene, ethene (ethylene), is a major plant hormone and ripening agent for agricultural commodities. The group of light alkenes (C2-C4) originates from both biogenic and anthropogenic sources, but their biogenic sources are poorly characterized, with limited field-based flux observations. Here we report net ecosystem fluxes of light alkenes and isoprene from a semiarid ponderosa pine forest in the Rocky Mountains of Colorado, USA using the relaxed eddy accumulation (REA) technique during the summer of 2014. Ethene, propene, butene and isoprene emissions have strong diurnal cycles, with median daytime fluxes of 123, 95, 39 and 17 µg m−2 h−1, respectively. The fluxes were correlated with each other, followed general ecosystem trends of CO2 and water vapor, and showed similar sunlight and temperature response curves as other biogenic VOCs. The May through October flux, based on measurements and modeling, averaged 62, 52, 24 and 18 µg m−2 h−1 for ethene, propene, butene and isoprene, respectively. The light alkenes contribute significantly to the overall biogenic source of reactive hydrocarbons: roughly 18 % of the dominant biogenic VOC, 2-methyl-3-buten-2-ol. The measured ecosystem scale fluxes are 40–80 % larger than estimates used for global emissions models for this type of ecosystem.

We describe and characterize a modular folded tubular photometer for making direct measurements of the concentrations of nitrogen dioxide (NO2) and specify how this method could be extended to measure other pollutants such as sulfur dioxide (SO2), ozone (O3), and black carbon particulate matter. Direct absorbance measurements using this photometer can be made across the spectral range from the ultraviolet (UV) to the near infrared. The absorbance cell makes use of modular components (tubular detection cells and mirror cubes) that allow construction of path lengths of up to 2 m or more while maintaining low cell volumes. The long path lengths and low cell volumes enable sensitive detection of ambient air pollutants down to low part-per-billion levels for gas species and aerosol extinctions down to 1 Mm-1, corresponding to∼ 0.1 µg m-3 for black carbon particulates. Pressure equalization throughout the stages of the absorbance measurement is shown to be critical to accurate measurements of analyte concentrations. The present paper describes the application of this photometer to direct measurements of nitrogen dioxide (NO2) and the incorporation of design features that also enable measurement of nitric oxide (NO) in the same instrument. Excellent agreement for ambient measurements along an urban roadside was found for both NO2 and NO measured by the folded tubular photometer compared to existing standard techniques. Compared to commonly used methods for measurements of NOx species, the advantages of this approach include (1) an absolute quantification for NO2 based on the Beer–Lambert law, thereby greatly reducing the frequency at which calibrations are required; (2) the direct measurement of NO2 concentration without prior conversion to NO as is required for the commonly used chemiluminescence method; (3) the use of modular components that allow construction of absorbance detection cells of varying lengths for extending the dynamic range of concentrations that can be measured; (4) a more economical instrument than other currently available direct measurement techniques for NO2; and (5) the potential for simultaneous detection of additional species such as SO2, O3, and black carbon in the same instrument. In contrast to other commercially available direct NO2 measurements, such as cavity-attenuated phase-shift spectroscopy (CAPS), the folded tubular photometer also measures NO simultaneously in the same apparatus by quantitatively converting NO to NO2 with ozone, which is then detected by direct absorbance.

A highly portable calibration source of nitric oxide (NO) based on the photolysis of nitrous oxide (N2O) supplied by 8 or 16 g disposable cartridges is demonstrated to serve as an accurate and reliable transfer standard for the calibration of NO monitors in the field. The instrument provides output mixing ratios in the range 0–1000 ppb with a precision and accuracy better than the greater of 3 ppb or 3 % of the target NO mixing ratio over a wide range of environmental conditions of ambient temperature (8.5–35.0 ∘C), pressure (745–1015 mbar corresponding to 2.7–0.0 km of elevation), and relative humidity (0 %–100 % RH). The combination of the NO calibration source with a previously described ozone calibration source based on the photolysis of oxygen in air provides a new instrument capable of outputting calibrated mixing ratios of NO, ozone (O3), and nitrogen dioxide (NO2), where the NO2 is produced by the stoichiometric gas-phase reaction of NO with O3. The portableNO2/NO/O3 calibration source requires no external gas cylinders and can be used for calibrations of NO, NO2, and O3 instruments for mixing ratios up to 1000, 500, and 1000 ppb, respectively. This portable calibrator may serve as a convenient transfer standard for field calibrations of ozone and NOx air pollution monitors.

Dry deposition velocities (Vd) for peroxyacetyl nitrate (PAN) calculated using two community dry deposition models with different treatments of both stomatal and nonstomatal uptakes were evaluated using measurements of PAN eddy covariance fluxes over a Loblolly pine forest in July 2003. The observed daytime maximum of Vd(PAN) was ∼1.0 cm s−1on average, while the estimates by the WRF‐Chem dry deposition module (WDDM) and the Noah land surface model coupled with a photosynthesis‐based Gas Exchange Model (Noah‐GEM) were only 0.2 cm s−1 and 0.6 cm s−1, respectively. The observations also showed considerable PAN deposition at night with typical Vd values of 0.2–0.6 cm s−1, while the estimated values from both models were less than 0.1 cm s−1. Noah‐GEM modeled more realistic stomatal resistance (Rs) than WDDM, as compared with observations of water vapor exchange fluxes. The poor performance of WDDM for stomatal uptake is mainly due to its lack of dependence on leaf area index. Thermal decomposition was found to be relatively unimportant for measured PAN fluxes as shown by the lack of a relationship between measured total surface conductance and temperature. Thus, a large part of the underprediction in Vd from both models should be caused by the underestimation of nonstomatal uptake, in particular, the cuticle uptake. Sensitivity tests on both stomatal and nonstomatal resistances terms were conducted and some recommendations were provided. Key Points The primary uptake of PAN by the vegetative surface is through the stomata Noah‐GEM had a smaller bias of Vd(PAN) due to its more realistic simulated Rs Cuticle resistance is another key parameter for PAN deposition

The Canopy Horizontal Array Turbulence Study (CHATS) took place in spring 2007 and is the third in the series of Horizontal Array Turbulence Study (HATS) experiments. The HATS experiments have been instrumental in testing and developing subfilterscale (SFS) models for large-eddy simulation (LES) of planetary boundary layer (PBL) turbulence. The CHATS campaign took place in a deciduous walnut orchard near Dixon, California, and was designed to examine the impacts of vegetation on SFS turbulence. Measurements were collected both prior to and following leafout to capture the impact of leaves on the turbulence, stratification, and scalar source/sink distribution. CHATS utilized crosswind arrays of fast-response instrumentation to investigate the impact of the canopy-imposed distribution of momentum extraction and scalar sources on SFS transport of momentum, energy, and three scalars. To directly test and link with PBL parameterizations of canopy-modified turbulent exchange, CHATS also included a 30-m profile tower instrumented with turbulence instrumentation, fast and slow chemical sensors, aerosol samplers, and radiation instrumentation. A highresolution scanning backscatter lidar characterized the turbulence structure above and within the canopy; a scanning Doppler lidar, mini sodar/radio acoustic sounding system (RASS), and a new helicopter-observing platform provided details of the PBL-scale flow. Ultimately, the CHATS dataset will lead to improved parameterizations of energy and scalar transport to and from vegetation, which are a critical component of global and regional land, atmosphere, and chemical models. This manuscript presents an overview of the experiment, documents the regime sampled, and highlights some preliminary key findings.

The transition between wintertime net carbon loss and springtime net carbon assimilation has an important role in controlling the annual rate of carbon uptake in coniferous forest ecosystems. We studied the contributions of springtime carbon assimilation to the total annual rate of carbon uptake and the processes involved in the winter-to-spring transition across a range of scales from ecosystem CO₂ fluxes to chloroplast photochemistry in a coniferous, subalpine forest. We observed numerous initiations and reversals in the recovery of photosynthetic CO₂ uptake during the initial phase of springtime recovery in response to the passage of alternating warm- and cold-weather systems. Full recovery of ecosystem carbon uptake, whereby the 24-h cumulative sum of NEE ($\\text{NEE}_{\\text{daily}}$) was consistently negative, did not occur until 3-4 weeks after the first signs of photosynthetic recovery. A key event that preceded full recovery was the occurrence of isothermality in the vertical profile of snow temperature across the snow pack; thus, providing consistent daytime percolation of melted snow water through the snow pack. Interannual variation in the cumulative annual NEE ($\\text{NEE}_{\\text{annual}}$) was mostly explained by variation in NEE during the snow-melt period ($\\text{NEE}_{\\text{snow-melt}}$), not variation in NEE during the snow-free part of the growing season ($\\text{NEE}_{\\text{snow-free}}$).$\\text{NEE}_{\\text{snow-melt}}$was highest in those years when the snow melt occurred later in the spring, leading us to conclude that in this ecosystem, years with earlier springs are characterized by lower rates of$\\text{NEE}_{\\text{annual}}$, a conclusion that contrasts with those from past studies in deciduous forest ecosystems. Using studies on isolated branches we showed that the recovery of photosynthesis occurred through a series of coordinated physiological and biochemical events. Increasing air temperatures initiated recovery through the upregulation of PSII electron transport caused in part by disengagement of thermal energy dissipation by the carotenoid, zeaxanthin. The availability of liquid water permitted a slightly slower recovery phase involving increased stomatal conductance. The most rate-limiting step in the recovery process was an increase in the capacity for the needles to use intercellular CO₂, presumably due to slow recovery of Rubisco activity. Interspecific differences were observed in the timing of photosynthetic recovery for the dominant tree species. The results of our study provide (1) a context for springtime CO₂ uptake within the broader perspective of the annual carbon budget in this subalpine forest, and (2) a mechanistic explanation across a range of scales for the coupling between springtime climate and the carbon cycle of high-elevation coniferous forest ecosystems.

A new solid-phase scrubber for use in conventional ozone (O3) photometers was investigated as a means of reducing interferences from other UV-absorbing species and water vapor. It was found that when heated to 100–130 °C, a tubular graphite scrubber efficiently removed up to 500 ppb ozone and ozone monitors using the heated graphite scrubber were found to be less susceptible to interferences from water vapor, mercury vapor, and aromatic volatile organic compounds (VOCs) compared to conventional metal oxide scrubbers. Ambient measurements from a graphite scrubber-equipped photometer and a co-located Federal equivalent method (FEM) ozone analyzer showed excellent agreement over 38 days of measurements and indicated no loss in the scrubber's ability to remove ozone when operated at 130 °C. The use of a heated graphite scrubber was found to reduce the interference from mercury vapor to ≤ 3 % of that obtained using a packed-bed Hopcalite scrubber. For a series of substituted aromatic compounds (ranging in volatility and absorption cross section at 253.7 nm), the graphite scrubber was observed to consistently exhibit reduced levels of interference, typically by factors of 2.5 to 20 less than with Hopcalite. Conventional solid-phase scrubbers also exhibited complex VOC adsorption and desorption characteristics that were dependent upon the relative humidity (RH), volatility of the VOC, and the available surface area of the scrubber. This complex behavior involving humidity is avoided by use of a heated graphite scrubber. These results suggest that heated graphite scrubbers could be substituted in most ozone photometers as a means of reducing interferences from other UV-absorbing species found in the atmosphere. This could be particularly important in ozone monitoring for compliance with the United States (U.S.) Clean Air Act or for use in VOC-rich environments such as in smog chambers and monitoring indoor air quality.

The eddy covariance technique, which is used in the determination of net ecosystem CO₂ exchange (NEE), is subject to significant errors when advection that carries CO₂ in the mean flow is ignored. We measured horizontal and vertical advective CO₂ fluxes at the Niwot Ridge AmeriFlux site (Colorado, USA) using a measurement approach consisting of multiple towers. We observed relatively high rates of both horizontal $(F_{{\\rm{hadv}}} )$ and vertical $(F_{{\\rm{vadv}}} )$ advective fluxes at low surface friction velocities ${\\rm{(u}}_* )$ which were associated with downslope katabatic flows. We observed that $F_{{\\rm{hadv}}} $ was confined to a relatively thin layer (0-6 m thick) of subcanopy air that flowed beneath the eddy covariance sensors principally at night, carrying with it respired CO₂ from the soil and lower parts of the canopy. The observed $F_{{\\rm{vadv}}}$ came from above the canopy and was presumably due to the convergence of drainage flows at the tower site. The magnitudes of both $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ were similar, of opposite sign, and increased with decreasing ${\\rm{u}}_*$, meaning that they most affected estimates of the total CO₂ flux on calm nights with low wind speeds. The mathematical sign, temporal variation and dependence on ${\\rm{u}}_*$ of both $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ were determined by the unique terrain of the Niwot Ridge site. Therefore, the patterns we observed may not be broadly applicable to other sites. We evaluated the influence of advection on the cumulative annual and monthly estimates of the total CO₂ flux $(F_c )$, which is often used as an estimate of NEE, over six years using the dependence of $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ on ${\\rm{u}}_*$. When the sum of $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ was used to correct monthly $F_c$, we observed values that were different from the monthly $F_c$ calculated using the traditional ${\\rm{u}}_*$-filter correction by -16 to 20 g C.m‾².mo‾¹; the mean percentage difference in monthly $F_c$ for these two methods over the six-year period was 10%. When the sum of $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ was used to correct annual $F_c$ we observed a 65% difference compared to the traditional ${\\rm{u}}_*$-filter approach. Thus, the errors to the local CO₂ budget, when $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$ are ignored, can become large when compounded in cumulative fashion over long time intervals. We conclude that the \"micrometeorological\" (using observations of $F_{{\\rm{hadv}}}$ and $F_{{\\rm{vadv}}}$) and \"biological\" (using the ${\\rm{u}}_*$ filter and temperature vs. $F_c$ relationship) corrections differ on the basis of fundamental mechanistic grounds. The micrometeorological correction is based on aerodynamic mechanisms and shows no correlation to drivers of biological activity. Conversely, the biological correction is based on climatic responses of organisms and has no physical connection to aerodynamic processes. In those cases where they impose corrections of similar magnitude on the cumulative $F_c$ sum, the result is due to a serendipitous similarity in scale but has no clear mechanistic explanation.

A highly portable ozone (O3) calibration source that can serve as a U.S. EPA level 4 transfer standard for the calibration of ozone analyzers is described and evaluated with respect to analytical figures of merit and effects of ambient pressure and humidity. Reproducible mixing ratios of ozone are produced by the photolysis of oxygen in O3-scrubbed ambient air by UV light at 184.9 nm light from a low-pressure mercury lamp. By maintaining a constant volumetric flow rate (thus constant residence time within the photolysis chamber), the mixing ratio produced is independent of both pressure and temperature and can be varied by varying the lamp intensity. Pulse width modulation of the lamp with feedback from a photodiode monitoring the 253.7 nm emission line is used to maintain target ozone mixing ratios in the range 30–1000 ppb. In order to provide a constant ratio of intensities at 253.7 and 184.9 nm, the photolysis chamber containing the lamp is regulated at a temperature of 40 ∘C. The resulting O3 calibrator has a response time for step changes in output ozone mixing ratio of < 30 s and precision (σp) of 0.4 % of the output mixing ratio for 10 s measurements (e.g., σp=±0.4 ppb for 100 ppb of O3). Ambient humidity was found to affect the output mixing ratio of ozone primarily by dilution of the oxygen precursor. This potential humidity interference could be up to a few percent in extreme cases but is effectively removed by varying the lamp intensity to compensate for the reduced oxygen concentration based on feedback from a humidity sensor.