Ralf M. Staebler

Energy balance and canopy conductance of a boreal aspen forest: Partitioning overstory and understory components

The energy balance components were measured throughout most of 1994 in and above a southern boreal aspen (Populus tremuloides Michx.) forest (53.629°N 106.200°W) with a hazelnut (Corylus cornuta Marsh.) understory as part of the Boreal Ecosystem-Atmosphere Study. The turbulent fluxes were measured at both levels using the eddy-covariance technique. After rejection of suspect data due to instationarity or inhomogeneity, occasional erratic behavior in turbulent fluxes and lack of energy balance closure led to a recalculation of the fluxes of sensible and latent heat using their ratio and the available energy. The seasonal development in leaf area was reflected in a strong seasonal pattern of the energy balance. Leaf growth began during the third week of May with a maximum forest leaf area index of 5.6 m 2 m -2 reached by mid-July. During the full-leaf period, aspen and hazelnut accounted for approximately 40 and 60% of the forest leaf area, respectively. Sensible heat was the dominant consumer of forest net radiation during the preleaf period, while latent heat accounted for the majority of forest net radiation during the leafed period. Hazelnut transpiration accounted for 25% of the forest transpiration during the summer months. During the full-leaf period (June 1 to September 7) daytime dry-canopy mean aspen and hazelnut canopy conductances were 330 mmol m -2 s -1 (8.4 mm s -1 ) (70% of the total forest conductance) and 113 mmol m -2 s -1 (2.9 mm s -1 ) (24% of the total forest conductance), respectively. Maximum aspen and hazelnut canopy conductances were 1200 mmol m -2 s -1 (30 mm s -1 ) and 910 mmol m -2 s -1 (23 mm s -1 ), respectively, and maximum stomatal conductances were 490 mmol m -2 s -1 (12.5 mm s -1 ) and 280 mmol m -2 s -1 (7 mm s -1 ), aspen and hazelnut, respectively. Both species showed a decrease in canopy conductance as the saturation deficit increased and both showed an increase in canopy conductance as the photosynthetic active radiation increased. There was a linear relationship between forest leaf area index and forest canopy conductance. The timing, duration, and maximum leaf area of this deciduous boreal forest was found to be an important control on transpiration at both levels of the canopy. The full-leaf hazelnut daytime mean Priestley and Taylor [1972] α coefficient of 1.22 indicated transpiration was largely energy controlled and the quantity of energy received at the hazelnut surface was a function of aspen leaf area. The full-leaf aspen daytime mean α of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship between forest leaf area and forest canopy conductance, varying α during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary laver feedbacks.

Observed increase in local cooling effect of deforestation at higher latitudes

Deforestation in mid- to high latitudes is hypothesized to have the potential to cool the Earth’s surface by altering biophysical processes. In climate models of continental-scale land clearing, the cooling is triggered by increases in surface albedo and is reinforced by a land albedo–sea ice feedback. This feedback is crucial in the model predictions; without it other biophysical processes may overwhelm the albedo effect to generate warming instead. Ongoing land-use activities, such as land management for climate mitigation, are occurring at local scales (hectares) presumably too small to generate the feedback, and it is not known whether the intrinsic biophysical mechanism on its own can change the surface temperature in a consistent manner. Nor has the effect of deforestation on climate been demonstrated over large areas from direct observations. Here we show that surface air temperature is lower in open land than in nearby forested land. The effect is 0.85 ± 0.44 K (mean ± one standard deviation) northwards of 45° N and 0.21 ± 0.53 K southwards. Below 35° N there is weak evidence that deforestation leads to warming. Results are based on comparisons of temperature at forested eddy covariance towers in the USA and Canada and, as a proxy for small areas of cleared land, nearby surface weather stations. Night-time temperature changes unrelated to changes in surface albedo are an important contributor to the overall cooling effect. The observed latitudinal dependence is consistent with theoretical expectation of changes in energy loss from convection and radiation across latitudes in both the daytime and night-time phase of the diurnal cycle, the latter of which remains uncertain in climate models.

Responses of net ecosystem exchanges of carbon dioxide to changes in cloudiness: Results from two North American deciduous forests

We analyzed half-hourly tower-based flux measurements of carbon dioxide (CO 2 ) from a boreal aspen forest and a temperate mixed deciduous forest in Canada to examine the influences of clouds on forest carbon uptake. We showed that the presence of clouds consistently and significantly increased the net ecosystem exchanges (NEE) of CO 2 of both forests from the level under clear skies. The enhancement varied with cloudiness, solar elevation angles, and differed between the two forests. For the aspen forest the enhancement at the peak ranged from about 30% for the 20°-25° interval of solar elevation angles to about 55% for the 55°-60° interval. For the mixed forest the enhancement at the peak ranged from more than 60% for the 30°-35° interval of solar elevation angles to about 30% for the 65°-70° interval. Averaged over solar elevation angles >20°, the aspen and mixed forests had the maximal NEE at the irradiance equivalent to 78 and 71% of the clear-sky radiation, respectively. The general patterns of current sky conditions at both sites permit further increases in cloudiness to enhance their carbon uptake. We found that both forests can tolerate exceedingly large reductions of solar radiation (53% for the aspen forest and 46% for the mixed forest) caused by increases in cloudiness without lowering their capacities of carbon uptake. We suggest that the enhancement of carbon uptake under cloudy conditions results from the interactions of multiple environmental factors associated with the presence of clouds.

Chemical processing of biogenic hydrocarbons within and above a temperate deciduous forest

A one-dimensional canopy model was developed to study photochemical processes inside and above a mixed deciduous forest in southern Ontario. The Eulerian model made use of Lagrangian dispersion principles with a correction factor to incorporate the average ensemble time since emission to calculate atmospheric mixing; traditional diffusion methods were found to provide insufficient mixing to match the measurements. Neglecting chemical losses while making isoprene emission estimates was found to underestimate emission rates up to 40%. The ozone oxidation of biogenic and anthropogenic alkenes was found to be a potential source of hydroxyl (OH) and hydroperoxy (HO2) radicals during the morning and night. Reactions of HO2 with organic peroxy radicals formed by the OH oxidation of isoprene during the day were shown to be a significant source of organic peroxide formation above the canopy. The primary pathways of methacrolein and methylvinylketone formation were shown to be ozone oxidation of isoprene and hydroxyl radical oxidation of isoprene, respectively. Local ozone formation was shown to be limited by low mixing ratios of nitrogen oxides, despite high levels of isoprene present at the site.

Nocturnal mixing in a forest subcanopy

The vertical structure of the flow in the old aspen canopy in BOREAS is examined in terms of thermocouple profiles and sonic anemometers above, within, and below the aspen canopy. The data are composited for different periods in order to isolate seasonal changes of the canopy and sun angle. On clear nights, a strong surface inversion develops in the lower part of the subcanopy in contrast to more closed canopies where strong stratification does not develop in the subcanopy. On clear nights with weak winds, a second weaker inversion develops at the top of the aspen canopy. On average, the subcanopy is very stable in the early evening and becomes less stable later in the evening. This appears to be due to a general increase in wind speed above the canopy during the night. On some of the nights, the stability of the flow in and above the canopy suddenly decreases in association with cold air advection. The characteristics of these events are examined. The vertical structure of the heat and momentum flux below and above the canopy are examined. The drag coefficient for the subcanopy stress exhibits a maximum at neutral stability and systematically decreases with increasing subcanopy stability and also decreases slowly with increasing instability. Possible explanations for this unexpected decrease with instability are examined. ©2000 Elsevier Science B.V. All rights reserved.

Arctic aerosol size‐segregated chemical observations in relation to ozone depletion during Polar Sunrise Experiment 1992

During Polar Sunrise Experiment 1992 at Alert in the Canadian high Arctic, size-fractionated observations of aerosol constituents (halogens, Na, V, As, Sb, Zn, Al, Ca, SO4=, Sm, K, Mn, and Mg) were made using a low-pressure cascade impactor and a high-volume virtual impactor (HVVI). Over 80% of the mass of V, Br, I, As, Sb, Zn, and SO4= was in particles <2.5 μm diameter. In contrast to SO4=, NO3− has more mass in larger particles. Similarly, Cl peaked in larger particles (1.72 to 6.0 μm) than Na (0.49 and 1.72 μr). Both of these effects are likely caused by reactions of sulphate acids with salts of nitrates and chloride in submicrometer particles and the volatilization of HNO3 and HCl. For fine particles (diameter <2.5 μm), observations of relatively, nonvolatile primary particulate elements (Na, V, Mn) with the two measurement devices agreed within analytical uncertainty. In contrast, they did not agree well for the more volatile halogens. The HVVI always recorded higher. Sea-salt enrichment factors of Cl (EFs s) referenced to Na for the HVVI data showed enrichment (1.2 to 1.8) in both fine and coarse aerosols during January. Thereafter, EFss averaged 0.92 and 0.57 for coarse and fine fractions, respectively, and were well correlated with each other. In contrast, EFss on fine particles collected with the cascade impactor were considerably lower (0.18 to 0.44), indicating volatilization of Cl within the device. A principal component analysis of fine particle (<2.5 μm) constituents from the HVVI identified six aerosol components consisting of anthropogenic (Mn, As, Sb, V, Zn, Al), sea salt (Cl, Na), iodine-bromine (I, Br), photochemical (Br, −O3), soil (Ca, Al), and one loaded solely by Sm. For the whole period, there was a strong anticorrelation between O3 and aerosol Br but not with the other halogens. In this study, iodine was significantly linearly correlated with the square root of the Br concentration. It showed a weak anticorrelation with O3 only after polar sunrise but not before.

Micrometeorological measurements of methane and nitrous oxide exchange above a boreal aspen forest

This paper presents tower-based measurements of methane (CH 4 ) and nitrous oxide (N 2 O) exchange between a boreal aspen stand and the atmosphere. Boreal ecosystems are a priority trace gas research area, and the work was conducted as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Methane and nitrous oxide fluxes were measured continuously between April 16 and September 16, 1994, in the Prince Albert National Park, Saskatchewan. The fluxes were determined using a high-resolution tunable diode laser Trace Gas Analysis System (TGAS) together with micrometeorological techniques. Both the CH 4 and the N 2 O fluxes were small and required long averaging times to be resolved. Over the full experiment, small emissions of both CH 4 and N 2 O were measured above the aspen stand. The mean flux of N 2 O was 1.4 ± 0.7 ng m -2 s -1 , or 1.9-2.5 ng m -2 s -1 when an enhancement factor to compensate for the breakdown of similarity theory just above forest canopies is included. Low rates of nitrification and denitrification throughout the growing season may explain the consistently small N 2 O fluxes. The CH 4 flux averaged 15.7 ± 2.8 ng m -2 s -1 , or 21-28 ng m -2 s -1 , including the similarity theory enhancement factor. The CH 4 emissions were highest between late July and mid-September, and there was a strong correlation between the CH 4 flux and the soil temperature. Whereas CH 4 emission was measured from the above-canopy footprint, uptake was recorded close to the tower base. Overall, it appears that CH 4 emissions from anoxic wet patches located throughout the above-canopy footprint overwhelmed uptake from drier areas to produce a net emission of CH 4 from the aspen site.

Are Emissions of Black Carbon from Gasoline Vehicles Underestimated? Insights from Near and On-Road Measurements

Measurements of black carbon (BC) with a high-sensitivity laser-induced incandescence (HS-LII) instrument and a single particle soot photometer (SP2) were conducted upwind, downwind, and while driving on a highway dominated by gasoline vehicles. The results are used with concurrent CO(2) measurements to derive fuel-based BC emission factors for real-world average fleet and heavy-duty diesel vehicles separately. The derived emission factors from both instruments are compared, and a low SP2 bias (relative to the HS-LII) is found to be caused by a BC mass mode diameter less than 75 nm, that is most prominent with the gasoline fleet but is not present in the heavy-duty diesel vehicle exhaust on the highway. Results from both the LII and the SP2 demonstrate that the BC emission factors from gasoline vehicles are at least a factor of 2 higher than previous North American measurements, and a factor of 9 higher than currently used emission inventories in Canada, derived with the MOBILE 6.2C model. Conversely, the measured BC emission factor for heavy-duty diesel vehicles is in reasonable agreement with previous measurements. The results suggest that greater attention must be paid to black carbon from gasoline engines to obtain a full understanding of the impact of black carbon on air quality and climate and to devise appropriate mitigation strategies.

Air—Water Exchange of Anthropogenic and Natural Organohalogens on International Polar Year (IPY) Expeditions in the Canadian Arctic

Shipboard measurements of organohalogen compounds in air and surface seawater were conducted in the Canadian Arctic in 2007-2008. Study areas included the Labrador Sea, Hudson Bay, and the southern Beaufort Sea. High volume air samples were collected at deck level (6 m), while low volume samples were taken at 1 and 15 m above the water or ice surface. Water samples were taken within 7 m. Water concentration ranges (pg L(-1)) were as follows: α-hexachlorocyclohexane (α-HCH) 465-1013, γ-HCH 150-254, hexachlorobenzene (HCB) 4.0-6.4, 2,4-dibromoanisole (DBA) 8.5-38, and 2,4,6-tribromoanisole (TBA) 4.7-163. Air concentration ranges (pg m(-3)) were as follows: α-HCH 7.5-48, γ-HCH 2.1-7.7, HCB 48-71, DBA 4.8-25, and TBA 6.4 - 39. Fugacity gradients predicted net deposition of HCB in all areas, while exchange directions varied for the other chemicals by season and locations. Net evasion of α-HCH from Hudson Bay and the Beaufort Sea during open water conditions was shown by air concentrations that averaged 14% higher at 1 m than 15 m. No significant difference between the two heights was found over ice cover. The α-HCH in air over the Beaufort Sea was racemic in winter (mean enantiomer fraction, EF = 0.504 ± 0.008) and nonracemic in late spring-early summer (mean EF = 0.476 ± 0.010). This decrease in EF was accompanied by a rise in air concentrations due to volatilization of nonracemic α-HCH from surface water (EF = 0.457 ± 0.019). Fluxes of chemicals during the southern Beaufort Sea open water season (i.e., Leg 9) were estimated using the Whitman two-film model, where volatilization fluxes are positive and deposition fluxes are negative. The means ± SD (and ranges) of net fluxes (ng m(-2) d(-1)) were as follows: α-HCH 6.8 ± 3.2 (2.7-13), γ-HCH 0.76 ± 0.40 (0.26-1.4), HCB -9.6 ± 2.7 (-6.1 to -15), DBA 1.2 ± 0.69 (0.04-2.0), and TBA 0.46 ± 1.1 ng m(-2) d(-1) (-1.6 to 2.0).

Measuring Canopy Structure and the Kinematics of Subcanopy Flows in Two Forests

A better understanding of forest subcanopy flows is needed to evaluate their role in the horizontal movement of scalars, particularly in complex terrain. This paper describes detailed measurements of the canopy structure and its variability in both the horizontal and vertical directions at a deciduous forest in complex terrain (the Harvard Forest, Petersham, Massachusetts). The effects of the trunks and subcanopy shrubs on the flow field at each of six subcanopy array locations are quantified. The dynamics of the subcanopy flow are examined with pragmatic methods that can be implemented on a small scale with limited resources to estimate the stress divergence, buoyancy, and pressure gradient forces that drive the flow. The subcanopy flow at the Harvard Forest was driven by mechanisms other than vertical stress divergence 75% of the time. Nocturnal flows were driven predominantly by the negative buoyancy of a relatively cool layer near the forest floor. The direction of the resulting drainage flows followed the azimuth of the longest forest-floor slope. Similar results were found at a much flatter site at Borden, Ontario, Canada. There was no clear evidence of flow reversals in the subcanopy in the lee of ridges or hills at the Harvard Forest even in high wind conditions, contrary to some model predictions.