Christophe Guimbaud

Effects of pH on nitrogen transformations in media-based aquaponics.

To investigate the effects of pH on performance and nitrogen transformations in aquaponics, media-based aquaponics operated at pH 6.0, 7.5 and 9.0 were systematically examined and compared in this study. Results showed that nitrogen utilization efficiency (NUE) reached its maximum of 50.9% at pH 6.0, followed by 47.3% at pH 7.5 and 44.7% at pH 9.0. Concentrations of nitrogen compounds (i.e., TAN, NO2(-)-N and NO3(-)-N) in three pH systems were all under tolerable levels. pH had significant effect on N2O emission and N2O conversion ratio decreased from 2.0% to 0.6% when pH increased from 6.0 to 9.0, mainly because acid environment would inhibit denitrifiers and lead to higher N2O emission. 75.2-78.5% of N2O emission from aquaponics was attributed to denitrification. In general, aquaponics was suggested to maintain pH at 6.0 for high NUE, and further investigations on N2O mitigation strategy are needed.

A portable infrared laser spectrometer for flux measurements of trace gases at the geosphere–atmosphere interface

A portable infrared laser absorption spectrometer named SPIRIT (SPectrometre Infra-Rouge In situ Tropospherique) has been set up for the simultaneous flux measurements of trace gases at the geosphere–atmosphere interface. It uses a continuous wave distributed feedback room temperature quantum cascade laser and a patented new optical multi-pass cell. The aim of SPIRIT field studies is to get a better understanding of land and water bodies to atmosphere exchange mechanisms of greenhouse gases (GHG). The analytical procedures to derive concentrations and fluxes are described, as well as the performances of the instrument under field conditions. The ability of SPIRIT to assess space and time dependence emissions of two GHG—nitrous oxide (N2O) and methane (CH4)—for different types of ecosystems is demonstrated through in situ measurements on peatland, on fertilized soil, and on water body systems. The objectives of these investigations and preliminary significant results are reported.

Stratospheric chemical ionization mass spectrometry: nitric acid detection by different ion molecule reaction schemes

Abstract Detailed height profiles of stratospheric nitric acid mixing ratios have been derived with a baloon borne chemical ionization mass spectrometer by applying several ion molecule reaction schemes, each associated to a specific and selective ion source. These ions (CO 3 − , Cl n − , CF 3 O − , and CF 3 O − H 2 O) give rise to specific product ions (mainly CO 3 − HNO 3 , NO 3 − HCl, NO 3 − HF, and CF 3 O − HNO 3 ) upon reaction with ambient nitric acid molecules. This paper reports on the instrumental details as well as on the results obtained during two balloon flights with the instrument. Within the accuracy of the measurements, the nitric acid height profiles obtained with the three different ion sources are in good agreement with one another as well as with literature data.

High-pressure flowing-afterglow setup validated by the study of the CO3− + HNO3 reaction

Abstract A high-pressure flowing-afterglow apparatus has been built in order to study in the laboratory gas-phase reactions of ions with neutral molecules playing an important role in the stratospheric ozone chemistry. The instrument consists of a flow tube, a first intermediate pressure chamber located between two conical electrodes, a quadrupole guide in a second intermediate pressure chamber, and a quadrupole mass analyzer in a third chamber. A method of measurement of the residence time for the ions in the flow tube has been used to derive absolute reaction rate coefficients. The validation of this setup was performed at room temperature over a pressure range of 1–3 hPa by the study of the well-known reaction CO3− + HNO3. The rate coefficient was measured as (1.2 ± 0.3) × 10−9 cm3 s−1, and CO3−(HNO3), NO3−, and NO3−(OH) were detected as primary product ions, in agreement with results previously reported. For the first time, the study was extended up to 24 hPa and at lower temperature. The rate coefficient was found to be independent of pressure above 1.1 hPa, but increased to a value of (2.4 ± 0.7) × 10−9 cm3 s−1 at 212 K. The relative yield of CO3−(HNO3) increased with pressure to a value ≥ 60% above 12 hPa at 298 K, and ≥ 85% above 6 hPa at 212 K. These results are fundamental to the derivation of a HNO3 mixing ratio in the stratosphere with balloon-borne instruments using active chemical ionization mass spectrometry.

A quantum cascade laser infrared spectrometer for CO2 stable isotope analysis: Field implementation at a hydrocarbon contaminated site under bio-remediation

Real-time methods to monitor stable isotope ratios of CO2 are needed to identify biogeochemical origins of CO2 emissions from the soil-air interface. An isotope ratio infra-red spectrometer (IRIS) has been developed to measure CO2 mixing ratio with δ(13)C isotopic signature, in addition to mixing ratios of other greenhouse gases (CH4, N2O). The original aspects of the instrument as well as its precision and accuracy for the determination of the isotopic signature δ(13)C of CO2 are discussed. A first application to biodegradation of hydrocarbons is presented, tested on a hydrocarbon contaminated site under aerobic bio-treatment. CO2 flux measurements using closed chamber method is combined with the determination of the isotopic signature δ(13)C of the CO2 emission to propose a non-intrusive method to monitor in situ biodegradation of hydrocarbons. In the contaminated area, high CO2 emissions have been measured with an isotopic signature δ(13)C suggesting that CO2 comes from petroleum hydrocarbon biodegradation. This first field implementation shows that rapid and accurate measurement of isotopic signature of CO2 emissions is particularly useful in assessing the contribution of contaminant degradation to the measured CO2 efflux and is promising as a monitoring tool for aerobic bio-treatment.

Combining Geoelectrical Measurements and CO2 Analyses to Monitor the Enhanced Bioremediation of Hydrocarbon-Contaminated Soils: A Field Implementation

Hydrocarbon-contaminated aquifers can be successfully remediated through enhanced biodegradation. However, in situ monitoring of the treatment by piezometers is expensive and invasive and might be insufficient as the information provided is restricted to vertical profiles at discrete locations. An alternative method was tested in order to improve the robustness of the monitoring. Geophysical methods, electrical resistivity (ER) and induced polarization (IP), were combined with gas analyses, CO2 concentration, and its carbon isotopic ratio, to develop a less invasive methodology for monitoring enhanced biodegradation of hydrocarbons. The field implementation of this monitoring methodology, which lasted from February 2014 until June 2015, was carried out at a BTEX-polluted site under aerobic biotreatment. Geophysical monitoring shows a more conductive and chargeable area which corresponds to the contaminated zone. In this area, high CO2 emissions have been measured with an isotopic signature demonstrating that the main source of CO2 on this site is the biodegradation of hydrocarbon fuels. Besides, the evolution of geochemical and geophysical data over a year seems to show the seasonal variation of bacterial activity. Combining geophysics with gas analyses is thus promising to provide a new methodology for in situ monitoring.

Suitable real-time monitoring of the aerobic biodegradation of toluene in contaminated sand by spectral induced polarization measurements and CO2 analyses

Hydrocarbons commonly contaminate aquifers and, in certain cases, can be successfully treated through biodegradation. Biodegradation is an effective technique for cleaning up pollution by enhancing pollutant-degrading bacteria in situ. However, in situ sampling for monitoring processes occurring into the ground during the treatment is expensive and invasive. In this article, an alternative method was tested. Spectral Induced Polarization (SIP) was combined with gas analyses, CO2 concentration and its carbon isotopic ratio, to monitor toluene aerobic biodegradation in laboratory columns. Microbial activity was characterized by an evolution of the SIP response in correlation with a CO2 production with the same carbon isotope signature as toluene. The spectral induced polarization response followed the variations of bacterial activity and displayed a phase shift up to 15 mrad. These results support the feasibility of using geophysical measurements, supported by CO2 analyses, to monitor in situ hydrocarbon biodegradation, and they are proving to be highly promising for real field scale monitoring.

CO2 and CH4 budgets and global warming potential modifications in Sphagnum-dominated peat mesocosms invaded by Molinia caerulea

Plant communities play a key role in regulating greenhouse gas (GHG) emissions in peatland ecosystems and therefore in their ability to act as carbon (C) sinks. However, in response to global change, a shift from Sphagnumdominated to vascular-plant-dominated peatlands may occur, with a potential alteration in their C-sink function. To investigate how the main GHG fluxes (CO2 and CH4) are affected by a plant community change (shift from dominance of Sphagnum mosses to vascular plants, i.e., Molinia caerulea), a mesocosm experiment was set up. Gross primary production (GPP), ecosystem respiration (ER) and CH4 emission models were used to estimate the annual C balance and global warming potential under both vegetation covers. While the ER and CH4 emission models estimated an output of, respectively, 376± 108 and 7± 4 g C m−2 yr−1 in Sphagnum mesocosms, this reached 1018± 362 and 33± 8 g C m−2 yr−1 in mesocosms with Sphagnum rubellum and Molinia caerulea. Annual modeled GPP was estimated at −414±122 and−1273±482 g C m−2 yr−1 in Sphagnum and Sphagnum + Molinia plots, respectively, leading to an annual CO2 and CH4 budget of−30 g C m−2 yr−1 in Sphagnum plots and of−223 g C m−2 yr−1 in Sphagnum + Molinia ones (i.e., a C sink). Even if CH4 emissions accounted for a small part of the gaseous C efflux (ca. 3 %), their global warming potential value makes both plant communities have a climate warming effect. The shift of vegetation from Sphagnum mosses to Molinia caerulea seems beneficial for C sequestration at a gaseous level. However, roots and litter of Molinia caerulea could provide substrates for C emissions that were not taken into account in the short measurement period studied here.