Modified Bowen Ratio Technique Research Articles

Influence of local air pollution on the deposition of peroxyacetyl nitrate to a nutrient-poor natural grassland ecosystem

Abstract. Dry deposition of peroxyacetyl nitrate (PAN) is known to have a phytotoxic impact on plants under photochemical smog conditions, but it may also lead to higher productivity and threaten species richness of vulnerable ecosystems in remote regions. However, underlying mechanisms or controlling factors for PAN deposition are not well understood and studies on dry deposition of PAN are limited. In this study, we investigate the impact of PAN deposition on a nutrient-poor natural grassland ecosystem situated at the edge of an urban and industrialized region in Germany. PAN mixing ratios were measured within a 3.5 months summer to early autumn period. In addition, PAN fluxes were determined with the modified Bowen ratio technique for a selected period. The evaluation of both stomatal and non-stomatal deposition pathways was used to model PAN deposition over the entire summer–autumn period. We found that air masses at the site were influenced by two contrasting pollution regimes, which led to median diurnal PAN mixing ratios ranging between 50 and 300 ppt during unpolluted and between 200 and 600 ppt during polluted episodes. The measured PAN fluxes showed a clear diurnal cycle with maximal deposition fluxes of ~−0.1 nmol m−2 s−1 (corresponding to a deposition velocity of 0.3 cm s−1) during daytime and a significant non-stomatal contribution was found. The ratio of PAN to ozone deposition velocities was found to be ~0.1, which is much larger than assumed by current deposition models. The modelled PAN flux over the entire period revealed that PAN deposition over an entire day was 333 μg m−2 d−1 under unpolluted and 518 μg m−2 d−1 under polluted episodes. Additionally, thermochemical decomposition PAN deposition accounted for 32% under unpolluted episodes and 22% under polluted episodes of the total atmospheric PAN loss. However, the impact of PAN deposition as a nitrogen source to the nutrient-poor grassland was estimated to be only minor, under both unpolluted and polluted episodes.

The influence of plants on atmospheric methane in an agriculture-dominated landscape

The primary objective of this study was to clarify the influence of crop plants on atmospheric methane (CH4) in an agriculture-dominated landscape in the Upper Midwest of the United States. Measurements were carried out at two contrasting scales. At the plant scale, CH4 fluxes from soybean and corn plants were measured with a laser-based plant chamber system. At the landscape scale, the land surface flux was estimated with a modified Bowen ratio technique using measurements made on a tall tower. The chamber data revealed a diurnal pattern for the plant CH4 flux: it was positive (an emission rate of 0.4±0.1 nmol m(-2) s(-1), average of soybean and corn, in reference to the unit ground area) during the day, and negative (an uptake rate of -0.8±0.8 nmol m(-2) s(-1)) during the night. At the landscape scale, the flux was estimated to be 14.8 nmol m(-2) s(-1) at night and highly uncertain during the day, but the available references and the flux estimates from the equilibrium methods suggested that the CH4 flux during the entire observation period was similar to the estimated nighttime flux. Thus, soybean and corn plants have a negligible role in the landscape-scale CH4 budget.

Distributed Modified Bowen Ratio method for surface layer fluxes of reactive and non-reactive trace gases

Modified Bowen ratio technique was used in a horizontally distributed form to determine turbulent fluxes of CO 2, H 2O, O 3, NO and NO 2 over a semi-natural grassland site in North-Eastern Germany. The applicability of the distributed variation of the Modified Bowen Ratio technique was proven prior to the calculation of trace gas fluxes. Turbulent NO fluxes were compared to fluxes up-scaled from laboratory measurements of biogenic NO emission from soil samples, which have been taken at the field site. The NO fluxes up-scaled from laboratory measurements were slightly larger than the fluxes observed in the field. However, both NO fluxes agreed within a factor of two. Under suitable night time conditions, we performed a detailed comparison of turbulent fluxes of CO 2 and O 3 with fluxes derived by the boundary layer budget technique. While there was agreement between these fluxes in a general sense, specific deviations were observed. They could be attributed to different footprint sizes of both methods and to in situ chemistry within the nocturnal boundary layer.

Effects of different eddy covariance correction schemes on energy balance closure and comparisons with the modified Bowen ratio system

Eddy covariance (EC) and modified Bowen ratio (MBR) systems have been shown to yield subtly different estimates of sensible heat ( H), latent heat (LE), and CO 2 fluxes (Fc). Our study analyzed the discrepancies between these two systems by first considering the role of the data processing algorithm used to estimate fluxes using EC and later examining the effects of atmospheric stability on discrepancies between EC and MBR. We found that EC correction algorithms disproportionately increase the magnitude of LE and Fc, and consequently have a strong effect on the Bowen ratio measured by EC. Two corrections not universally employed were each found to account for up to 20% of LE and Fc: the correction for signal asynchrony by adjusting the lag between the IRGA and the sonic anemometer, and the frequency domain correction for path-length averaging in both instruments. Comparison of fluxes between two EC systems 10 m apart showed that LE and Fc are inherently more variable than H, highlighting the role of field heterogeneity in determining canopy gas exchange at very small spatial scales. When all relevant corrections were applied, there was no bias for H and LE between EC and MBR. An examination of discrepancies between EC and MBR showed that the discrepancies were highest during neutral periods when shear drove vertical mixing much more than buoyancy. During these neutral periods, the correlation between T, H 2O, and CO 2 was much lower on average, which violates the similarity assumption exploited in the MBR technique. The largest discrepancies in Fc in both systems were clearly visible when plotted against a light–response curve, which creates the possibility of using a screening technique to exclude data that depart markedly from other recently collected data. The EC system tended to have the largest departures from the expected value at low light and during the night, while the MBR system tended to show occasionally spikes of high photosynthetic uptake.

Evaluation of Novel Techniques for Measurement of Air−Water Exchange of Persistent Bioaccumulative Toxicants in Lake Superior

We report initial measurements of concentrations and net air-water exchange fluxes of target persistent bioaccumulative toxicants (PBTs) in Lake Superior utilizing techniques not previously applied for this purpose. Gaseous PBTs are collected in diffusion denuders containing sections of commercial chromatography columns and subsequently thermally extracted into the cooled injection inlet of a high-resolution gas chromatograph. The PBT sampling/analytical methods enable accurate determination of gas-phase PBT concentration and micrometeorological measurement of fluxes to be carried out. PBT fluxes are measured by the modified Bowen ratio technique in which sensible heat flux is related to PBT flux, with the assumption of identical transfer velocities of heat and PBTs between two heights in the atmospheric surface layer. Micrometeorological measurement of flux accounts for all sources of resistance to mass transfer, including atmospheric stability effects, surface films, waves, sea spray, and bubbles. The sensible heatflux, PBT concentration, and PBT flux measurements carried out in 14 2- or 3-h periods during seven sampling events in Lake Superior in summer and fall 2002 and spring 2003 demonstrate advantages under the constraints of the techniques. The uncertainty of the flux measurements was typically in the range from 1% to 160%. Gaseous concentrations of a-hexachlorocyclohexane (alpha-HCH) and hexachlorobenzene (HCB) over Lake Superior were in the range from 6 to 170 and 12-95 pg/m3, respectively. Fluxes out of Lake Superior were measurable in 75% of the cases in which a concentration gradient was measured, and were in the range from -0.17 to +0.064 ng/m2 x h for alpha-HCH and from -0.60 to -0.093 ng/m2 x h for HCB.

Observations of summertime NO fluxes and boundary-layer height at the South Pole during ISCAT 2000 using scalar similarity

Abstract Eddy-covariance heat flux as well as temperature and NO concentration gradients were measured during the ISCAT 2000 (Investigation of Sulfur Chemistry in the Antarctic Troposphere) field study at the South Pole (SP). These quantities allowed for the use of the modified Bowen ratio technique to estimate the surface flux of NO and, from photochemical considerations, the NO x flux. The meteorological measurement package employed in these experiments consisted of sonic anemometer/thermometers (ATI K-probes) and temperature/humidity sensors (NCAR). A chemiluminescent analyzer housed in an environmental enclosure was used to measure NO. All sampling took place on a 22 m meteorological tower. The time period over which flux measurements were recorded was 26 November through 30 November 2000. The average value of the NO flux was estimated to be 2.6 ± 0.3 × 10 8 molec cm - 2 s - 1 ; whereas, for NO x the average flux was 3.9 ± 0.4 × 10 8 molec cm - 2 s - 1 . To assist in the interpretation of these results, the height of the atmospheric boundary-layer at the SP from 23 November to 28 December 2000 was also estimated.

Air‐surface exchange of peroxyacetyl nitrate at a grassland site

Direct measurements of the dry deposition velocity of peroxyacetyl nitrate (PAN) were made during the daytime between the months of July and October above a grassland surface in northern Illinois by a modified Bowen ratio technique. Differences in the air temperature, water vapor content, and PAN concentration were measured between the heights of 3.0 m and 0.92 m. Although the measurement uncertainties were large, the cumulative data indicate a slight downward flux of PAN, with an average and standard error of 0.13 ± 0.13 cm s−1 for the dry deposition velocity. Theoretical calculations showed that thermochemical decomposition of PAN on leaf and soil surfaces heated to temperatures above the ambient air levels would contribute less than 15% of the total PAN flux at the elevations of the PAN measurements. A theoretical evaluation of the transfer of PAN through leaf stomata and the plant cuticular membrane indicated that uptake of PAN by vegetation during the daytime is controlled by transfer through the leaf stomata rather than the cuticular membrane. The stomatal resistance for PAN is greater by a factor of 1.6 than the value for O3. The mesophyll resistance for O3 is also expected to be less than the value for PAN, because O3 has more reaction sites within plant cells and reacts faster than PAN with protein thiols of the cell membranes. Measurements from other studies indicate that the dry deposition velocity for PAN above a vegetated surface during the daytime is lower by a factor of 0.5–0.3 than for O3. Our measurements of the PAN deposition velocity agree with the results of previous studies and with theoretical calculations based on the physicochemical properties of PAN and the grassland surface. These measurements imply that removal of PAN from the daytime atmospheric boundary layer by thermochemical decomposition is more rapid than dry deposition to a grassland surface.

Use of the modified bowen-ratio technique to measure fluxes of trace gases

The modified Bowen-ratio (MBR) technique is a micrometeorological method that can be used to estimate air-surface exchange rates, providing differences in concentrations between two heights can be resolved. Application of the MBR method requires that fluxes and gradients for at least one scalar entity be measured in order to directly compute the exchange coefficient k, which is assumed to be applicable for all scalars. Direct measurements of fluxes (eddy correlation) and gradients for CO 2 and H 2O were made during three experiments. Measurements were made at the forest floor of a boreal forest, over a lake surface, and at the forest floor of a leafless deciduous forest. Data from these experiments were used to examine the equality of H 2O and CO 2 exchange coefficients. The mean difference between the exchange coefficients for CO 2 and H 2O was not found to be statistically different from zero. Uncertainties in the determination of k associated with gradient measurements and sampling strategies are evaluated. The MBR technique is then used to assess the air-surface exchange rates of gas-phase mercury. Although the acceptably good agreement between the exchange coefficients for CO 2 and H 2O does not prove they are equal to k for Hg, greater confidence is assumed in the determination of Hg fluxes than just using k for heat.