Ambient Mixing Ratios Research Articles

Diurnal fluctuations of secondary photooxidants in air and of detoxification systems in the foliage of Mediterranean forest trees

Gas-phase peroxide mixing ratios were measured in the canopy area of a Mediterranean oak/pine forest. Concurrently the activities of cell wall localized peroxidases, which protect leaves from external oxidants, were investigated. The diurnal cycles of peroxidase activity showed a strong decrease when the gas-phase concentration of highly phytotoxic hydroxy hydroperoxides increased. These results suggest that ambient gas-phase mixing ratios of organic peroxides may injure the plants' oxidative defence system.

A high-frequency modulated tunable diode laser absorption spectrometer for measurements of CO2, CH4, N2O, and CO in air samples of a few cm3

Investigations of the relationship between changes in climate and the variation of composition of the atmosphere can be performed most directly and with the highest resolution possible by analyzing the air sampled and preserved in the polar ice sheets for hundreds of thousands of years by nature itself. For the determination of CO2, CH4, N2O, and CO in air samples of 1–3 cm3 extracted from ice cores, a high-frequency modulated tunable diode laser absorption spectrometer has been developed. The instrument can measure CO2, CH4, N2O, and CO at ambient mixing ratios of 300 ppmv (1 ppmv=10−6 volume mixing ratio), 1700, 300, and 100 ppbv (1 ppbv=10−9 volume mixing ratio), respectively, with a precision of 1%–2%. The measurement of high CO2 mixing ratios is not constrained by the detection limit. For other gases mixing ratios of 20 ppbv of CH4, 0.9 ppbv of N2O, and 1.6 ppbv of CO are detectable with the instrument in 2 cm3 standard temperature and pressure. These detection limits and the measurement precision are sufficient for the determination of past changes in atmospheric composition. The technique is also suitable for other applications in which several infrared active trace gases have to be determined in the low ppbv and possibly even in the upper pptv range in air samples of a few cm3 or small samples of other matrices. The sensitivity or the reproducibility of the instrument can be improved by another order of magnitude by using more powerful lasers and by averaging over longer periods. Such improvement could result in a capability to detect sub-ppbv mixing ratios in samples of the present size or ppbv measurements in even smaller air samples. The higher reproducibility would also make the technique interesting for measurements of isotope ratios.

Determination of Atmospheric Methyl Bromide by Cryotrapping-Gas Chromatography and Application to Soil Kinetic Studies Using a Dynamic Dilution System

Methyl bromide (CH(3)Br) is considered to be a major source of stratospheric Br, which contributes to the destruction of ozone. It is therefore necessary to understand the natural sinks of this compound and to accurately measure ambient mixing ratios. Methodology is described for the measurement of atmospheric CH(3)Br by cryotrapping-gas chromatography and its application to soil kinetics. A 2-propanol/dry ice cryotrap was used to preconcentrate CH(3)Br in standard and air samples, with subsequent detection using a gas chromatograph equipped with an O(2)-doped electron capture detector (GC-ECD). The GC-ECD cryotrapping method had a detection limit of 0.23 pmol of CH(3)Br. This is equivalent to the amount of CH(3)Br in a 500 mL sample of ambient air at the estimated northern hemisphere atmospheric mixing ratio of 11 parts per trillion by volume (pptv). A dynamic dilution system was developed to produce mixing ratios of CH(3)Br ranging between 4 and 1000 pptv. Calibrated mixing ratios of CH(3)Br produced with the dilution system were used to determine soil uptake kinetics employing a dynamic soil incubation method.

Estimates of regional natural volatile organic compound fluxes from enclosure and ambient measurements

Natural volatile organic compound (VOC) emissions were investigated at two forested sites in the southeastern United States. A variety of VOC compounds including methanol, 2‐methyl‐3‐buten‐2‐ol, 6‐methyl‐5‐hepten‐2‐one, isoprene and 15 monoterpenes were emitted from vegetation at these sites. Diurnal variations in VOC emissions were observed and related to light and temperature. Variations in isoprene emission from individual branches are well correlated with light intensity and leaf temperature while variations in monoterpene emissions can be explained by variations in leaf temperature alone. Isoprene emission rates for individual leaves tend to be about 75% higher than branch average emission rates due to shading on the lower leaves of a branch. Average daytime mixing ratios of 13.8 and 6.6 ppbv C isoprene and 5.0 and 4.5 ppbv C monoterpenes were observed at heights between 40 m and 1 km above ground level the two sites. Isoprene and monoterpenes account for 30% to 40% of the total carbon in the ambient non‐methane VOC quantified in the mixed layer at these sites and over 90% of the VOC reactivity with OH. Ambient mixing ratios were used to estimate isoprene and monoterpene fluxes by applying box model and mixed‐layer gradient techniques. Although the two techniques estimate fluxes averaged over different spatial scales, the average fluxes calculated by the two techniques agree within a factor of two. The ambient mixing ratios were used to evaluate a biogenic VOC emission model that uses field measurements of plant species composition, remotely sensed vegetation distributions, leaf level emission potentials determined from vegetation enclosures, and light and temperature dependent emission activity factors. Emissions estimated for a temperature of 30°C and above canopy photosynthetically active radiation flux of 1000 μmol m−2 s−1 are around 4 mg C m−2 h−1 of isoprene and 0.7 mg C m−2 h−1 of monoterpenes at the ROSE site in western Alabama and 3 mg C m−2 h−1 of isoprene and 0.5 mg C m−2 h−1 of monoterpenes at the SOS‐M site in eastern Georgia. Isoprene and monoterpene emissions based on land characteristics data and emission enclosure measurements are within a factor of two of estimates based on ambient measurements in most cases. This represents reasonable agreement due to the large uncertainties associated with these models and because the observed differences are at least partially due to differences in the size and location of the source region (“flux footprint”) associated with each flux estimate.

Super-stiff polymers

Air samples in 46 cities all over China were collected in October and November 2010, and the levels of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) were measured and are discussed in this work. In most cities, the ambient mixing ratios of CFCs were within 20% enhancements of the background values, while the levels of HCFCs and HFCs were significantly higher than the background values (P < 0.01). Compared with the results of previous research conducted in 2001 and other single-city studies, the levels of CFC-11, CFC-12 and CFC-113 have decreased since then, while the levels of HCFCs and HFCs have increased rapidly. The results are consistent with the fact that CFCs have been replaced by the interim replacements (HCFCs and HFCs) in compliance with the Montreal Protocol.

Kinetics of CH4 oxidation in oxic soils exposed to ambient air or high CH4 mixing ratios

Kinetics of CH4 oxidation in oxic soils exposed to ambient air or high CH4 mixing ratios

Dehydration mechanism in the Antarctic stratosphere during winter

The growth of ice nuclei through deposition of water vapor at temperatures below frost point is investigated in the context of the Antarctic winter stratosphere. The altitude and the ambient water vapor mixing ratio, as well as the size of the nuclei determine the ice particle growth rate, with higher altitudes requiring colder temperatures for ice deposition. The magnitude of the temperature decrease below the frost point and its evolution over the winter determine the residence time of the growing ice particles and the loss of water vapor at any altitude. A winter‐long simulation, using the observed South Pole daily temperatures, suggests that, in the limit of weak latitudinal mixing over the austral winter, considerable dehydration can occur within the polar vortex, with the higher altitudes (above 22 km.) experiencing the least losses.

Nonmethane hydrocarbons in remote tropical, continental, and marine atmospheres

Atmospheric hydrocarbon measurements from remote areas are presented and analyzed to determine sources of ambient mixing ratios. Biomass burning appears to make major contributions to free tropospheric hydrocarbon burden over tropical forests and grasslands. Biogenic emissions constitute a large percentage of atmospheric hydrocarbon volume in continental areas of significant productivity. In marine areas studied, ambient hydrocarbons may include contributions from evaporation of dissolved hydrocarbons transported by ocean currents from areas of coastal upwellings or marine pollution. In all cases, hydrocarbons from local sources are major components of the atmospheric hydrocarbon composition.

Vertical Mass Transport in Cumulonimbus Clouds on Day 261 of GATE

Abstract The model of Sikdar, for inferring upward transport of mass in a cumulonimbus cloud from expansion of the anvil in a satellite picture sequence, is refined and tested. In the present mass transport model, the anvil is configured as a stack of slabs, and the thickness of each slab is inferred from cloud radiance, by means of multiple scattering results. In addition, mass transport from the base of the anvil to the base of the cumulonimbus cloud is required to decrease according to the formula of Austin and Houze, in which the entrainment is inversely proportional to total cloud depth. With a sounding of ambient temperature and water vapor mixing ratio, the model yields profiles of upward mass transport and production of condensate. The model is applied to seven cumulonimbus clouds in the eastern Atlantic Occean on 18 member 1974, one of the last days of the GARP Atlantic Tropical Experiment. Transports of the present model at the base of the anvil average 27 &times 1010 g s-1. This is 2.5 times la...

Nitrous Oxide Emissions from Drained, Cultivated Organic Soils of South Florida

Due to the intense microbial oxidation of organic soils in the Florida Everglades, approximately 1400 kg N/ha are mineralized annually. Most of this nitrogen is lost to the atmosphere through denitrification in the soil. Nitrous oxide is one of the gaseous products of denitrification, therefore the objectives of this study were to determine the quantities of N2O emitted from these soils and to measure the effect of this N2O on ambient mixing ratios in the Everglades. Nitrous oxide fluxes from these soils ranged from 4 g N/ha/day, during dry periods, to 4500 g N/ha/day following rainfall events. Nitrous oxide emissions increased with increasing soil moisture. From April through the end of December 1979, a total of 165, 97, and 48 kg N2O-N/ha were emitted from fallow, St. Augustine grass, and sugarcane fields, respectively. There was a diurnal variation in the N2O mixing ratios of air 8 m above the soil in the Everglades. This diurnal fluctuation was affected by wind speed. There was a significant linear co...