David E. Schorran

Prolonged suppression of ecosystem carbon dioxide uptake after an anomalously warm year

Terrestrial ecosystems control carbon dioxide fluxes to and from the atmosphere through photosynthesis and respiration, a balance between net primary productivity and heterotrophic respiration, that determines whether an ecosystem is sequestering carbon or releasing it to the atmosphere. Global and site-specific data sets have demonstrated that climate and climate variability influence biogeochemical processes that determine net ecosystem carbon dioxide exchange (NEE) at multiple timescales. Experimental data necessary to quantify impacts of a single climate variable, such as temperature anomalies, on NEE and carbon sequestration of ecosystems at interannual timescales have been lacking. This derives from an inability of field studies to avoid the confounding effects of natural intra-annual and interannual variability in temperature and precipitation. Here we present results from a four-year study using replicate 12,000-kg intact tallgrass prairie monoliths located in four 184-m3 enclosed lysimeters. We exposed 6 of 12 monoliths to an anomalously warm year in the second year of the study and continuously quantified rates of ecosystem processes, including NEE. We find that warming decreases NEE in both the extreme year and the following year by inducing drought that suppresses net primary productivity in the extreme year and by stimulating heterotrophic respiration of soil biota in the subsequent year. Our data indicate that two years are required for NEE in the previously warmed experimental ecosystems to recover to levels measured in the control ecosystems. This time lag caused net ecosystem carbon sequestration in previously warmed ecosystems to be decreased threefold over the study period, compared with control ecosystems. Our findings suggest that more frequent anomalously warm years, a possible consequence of increasing anthropogenic carbon dioxide levels, may lead to a sustained decrease in carbon dioxide uptake by terrestrial ecosystems.

Size-resolved measurements of light scattering by ambient particles in the southwestern U.S.A.

Abstract Measurements are reported from integrating nephelometers modified sample alternately from a fine-particle ( D zero μ m) cyclone and an unrestricted inlet. These nephelometers were successfully operated at two sites in the southwest. U.S.A. as part of the SCENES program in the spring and summer of 1989. One of the nephelometers was collocated with a transmissometer monitoring total extinction. Fine-and total-particle samples were collected at each site for determination of m ass and light absorption. Intercomparisons of the collacated daytime scattering, absorption. and total extinction measurements indicate that the nephelometer reported somewhat less than half the actual scattering to coarse particles. When this under-response is corrected for, the nephelometer and transmissometer show good agreement. The corrected data indicate that corse particles were responsible from one-quarter to one-third of the total particle scattering. Predominatly coare-particle dusts are estimated to have contributed one-third to one-half of the total-particle scatterring.

Quantifying the effects of phenology on ecosystem evapotranspiration in planted grassland mesocosms using EcoCELL technology

Use of plant phenological variables in models predicting evapotranspiration (ET) has largely relied on relatively simple (e.g., linear) relationships which may not be sufficiently accurate to predict small—yet ecologically significant—changes in plant phenology that are expected to occur in response to global climate change. A dearth of experimental data reflects the difficulties in quantifying these relationships against the background of large environmental variability that occurs in the field. Our main objective was to quantify how plant phenology (leaf area index [LAI] and root length density [RLD]) affect ET and its components during an entire vegetation cycle in large-scale model grassland (Bromus tectorum) ecosystems using the Ecologically Controlled Enclosed Lysimeter Laboratory (EcoCELL)—a unique open flow and mass balance laboratory. We also aimed to compare the three methods employed by the EcoCELL laboratory to measure ecosystem ET (whole-ecosystem gas exchange, weighing lysimetry, and weighing lysimetry combined with time domain reflectometry [TDR]) in order to independently confirm the performance of the unique gas exchange technology. Cumulative ET during the 190 days of the experiment measured with the three different methods compared very well with each other (mean errors <1%). We found that ET reached maximum levels at relatively low LAI (2–3), but as LAI increased beyond this value, small increase in transpiration were more than offset by decreases in soil evaporation, thereby causing declines in ET. A combined rectangular hyperbola (effects on transpiration) and linear (effects on soil evaporation) function between LAI and ET accounted for almost 90% of all variability in measured daily ET. RLD showed relationships to ET similar to those observed for LAI due to high covariance between RLD and LAI, but root length densities did not explain any additional variability in daily ET beyond that explained by LAI under the well-watered conditions of the experiment. Taken together, our results show that: (i) the EcoCELL mesocosm laboratory can precisely and accurately quantify hydrologic processes of large soil–plant monoliths under controlled environmental conditions; (ii) plant canopy phenological changes affect ecosystem ET, and the contribution of transpiration, in non-linear ways; (iii) these non-linear responses must be accounted for when assessing the consequences of changes in plant phenology—e.g., due to global environmental change—on ecosystem hydrology.

Assessment of Nontailpipe Hydrocarbon Emissions from Motor Vehicles

This report evaluates tailpipe and nontailpipe hydrocarbon (HC) emissions from light-duty spark-ignition (SI) vehicles. The sources of information were unpublished data sets, generated mainly from 1990 through 1994, on emissions from volunteer fleets of in-use vehicles in chassis dynamometer and sealed housing for evaporative determination tests, and published chemical mass balance (CMB) source apportionments of HC in roadway tunnels and in urban air. The nontailpipe emissions evaluated comprise running-loss, hot soak, diurnal emissions, and resting-loss emissions. Relations between pressure and purge test failures and actual nontailpipe emissions were also examined.

Measurements of gaseous peroxides near the Grand Canyon—Implication for summertime visibility impairment from aqueous-phase secondary sulfate formation

Abstract Warm cloud processes in which dissolved peroxides and ozone oxidize SO2 to sulfate are important contributors to acidic sulfate deposition, and may also contribute to visibility impairment if the clouds evaporate prior to rainfall, e.g. under summer monsoonal conditions in the Southwest U.S.A. However, data from summer season gaseous peroxide measurements are sparse for this region, specifically in the area of the Grand Canyon National Park (GCNP). Results are reported herein from a gaseous peroxide measurement campaign at Meadview, AZ (between GCNP and various sources of S02 to the west and south) during July–August 1992. Total peroxide and H2O2 concentrations were measured by the concurrent coil collection/enzyme catalyzed fluorescent technique (Lazrus et al., 1986, Anal. Chem. 58, 594–597 Total gaseous peroxide levels observed were generally in the range of 1.0–5 ppbv. Results from measurements of peroxides are used with other preliminary data for ozone, SO2, temperature and humidity, and from other observations to estimate the potential contribution of in-cloud oxidation processes to visibility degradation due to increased sulfate levels. The data show that peroxide concentrations and estimated H2O2 concentrations nearly always exceeded SO2 levels at Meadview, indicating the absence of “oxidant limited” conditions in air in which clouds might be formed. Only modest quantities of aerosol sulfate (

An Analysis of Regional Haze Using Tracers of Opportunity

A two-year record of hourly concentrations of halocarbon tracers (methylchloroform and perchloroethylene) and hourly averages of particle light scattering (Bsp) has been analyzed In an effort to understand the sources of haze In the U.S. southwestern deserts and mountains. Measurements were taken on top of Spirit Mountain in southern Nevada. In conjunction with photographs used to interpret visual quality, haze episodes at Spirit Mountain were usually coincident with elevated concentrations of tracers originating from urban sources. Haze obscured an 88-km-distant mountain 17 percent of the total observation time. Of those Incidents, 69 percent were associated with long-range transport of haze from the Los Angeles Basin.

Tracers of oppurtunity and pollutant transport in Southern California

Abstract The Greater Los Angeles Distant Impact Study is an ongoing program to investigate the fate of urban pollutants in the southwestern United States. As an initial phase of this program the source strengths of halocarbon emissions in the Los Angeles Basin were investigated. These inert compounds, emitted in large quantities by various industrial and commercial activities, offer an oppurtunity to routinely trace polluted air leaving the Basin. Results show widespread emissions of halocarbons with two areas of peak concentrations found in the western part of the Basin; however, the distribution of pollutants leaving the Basin indicates that considerable mixing has taken place. Diurnal halocarbon concentrations peak during the late evening at the eastern end of the Basin due to the prevailing westerly winds. Pollutants are transported out of the Basin in discrete episodes of about 14 h duration with the highest concentrations lasting for about 6 h. Surface and upper air meteorological measurements were used to determine the dynamic and thermodynamic structure of air leaving the Basin. Under the prevailing conditions of strong onshore flow, pollutants were transported out of the Basin in a shallow surface layer; above this, the flow reversed in a deep well-mixed layer containing elevated pollutant concentrations. This layer was capped by the planetary boundary layer inversion. Halocarbons appear to be very useful tracers of urban air in the southwestern United States.

Semicontinuous method for monitoring SO2 at low parts-per-trillion concentrations

A technique is described for low parts-per-trillion (ppt) detection of SO 2 with high temporal resolution compared to current filter sampling methods. A sulfur chemiluminescence detector, equipped with a quartz burner chamber and a sample probe with a critical orifice, was used as a single detector to analyze SO 2 in ambient air by alternately cycling the sample airstream through a SO 2 denuder. The method provides one SO 2 concentration reading every 10 min. The limits of detection and quantitation were found to be 20 and 70 pptv, respectively. Results from field measurements at a remote site on the south rim of the Grand Canyon during November 1991 are presented, and they indicate a highly variable distribution of SO 2 from the limit of detection to near 2 ppbv