M. A. H. Khan

Analysis of low concentration reduced sulfur compounds (RSCs) in air: Storage issues and measurement by gas chromatography with sulfur chemiluminescence detection

Reduced sulfur compounds (RSCs) were measured at low concentrations in small volume air samples using a cryo-trapping inlet system and gas chromatograph outfitted with a sulfur chemiluminescence detector (GC-SCD). The relative sensitivity of the system to the RSCs follows the sequence H(2)S<CH(3)SH<OCS∼DMS<CS(2). The analytical system achieves a detection limit of 120ppt in a 100mL air sample, which is suitable for measuring reactive RSCs (e.g., H(2)S and CH(3)SH) at ambient or near ambient atmospheric concentrations. The inlet system allows for replicate sampling from a stored air sample (sub-sampling), thereby improving estimates of instrumental precision and demonstrating the reproducibility of the analytical method. Although the SCD theoretically provides linear responses equivalent to the sulfur mass injected, we found that the response properties for each RSC differed. At concentrations below 2ppb, the compounds H(2)S and CH(3)SH have diminished responses, leading to larger measurement uncertainties. Two generations of commercially available SilcoCan canisters were tested to evaluate the relative RSC loss due to storage in the canister and loss of inertness because of coating age. The older generation canister (>6 years from initial coating) saw significant loss of H(2)S and CH(3)SH within 2 days, while the more recent generation canister (<1 year from initial coating) yielded percent recoveries of RSCs in the range of 85% (H(2)S and CH(3)SH) to 95% (OCS, DMS and CS(2)) after 7 days of storage, suggesting that these canisters may be suitable for the short-term storage of low level RSCs. The development of this low concentration, low sample volume method is well suited for measuring RSC gas fluxes from natural soils in laboratory incubations and in field flux chamber studies.

Stable carbon isotope analysis of selected halocarbons at parts per trillion concentration in an urban location

Environmental Context. Halocarbons can have significant effects on the atmosphere and the environment, particularly with regard to ozone depletion and climate change impacts. The determination of isotopic concentrations for selected halocarbon species should provide useful information to identify and constrain halocarbon sources and sinks within the urban environment. In the present study, a new instrumental method is described to determine these isotope ratios for selected halocarbons and the resultant data are interpreted in terms of local sources and sinks. Abstract. δ13C values of a suite of halocarbons have been determined in an urban background site in Bristol, UK. A novel mobile preconcentration system, based on the use of multi-adsorbent sample tubes, has been developed for trapping relatively large-volume air samples in potentially remote areas. An Adsorption Desorption System–Gas Chromatography–Electron Capture Detector was used to measure the mixing ratios of the selected halocarbon species, while a Gas Chromatography–Combustion–Isotope Ratio Mass Spectrometer was used to determine δ13C values. For the species with strong local sources, the variation of isotope ratios has been observed over the experimental period. Some of the results reported in the present study differ from previously reported values and reasons for this are discussed. The reporting of different δ13C values for selected halocarbons from different areas in the present study suggests that δ13C values may be used to determine the relative magnitudes of anthropogenic and biogenic sources.

Leaf Cutter Ants: A Possible missing source of biogenic halocarbons

Environmental context. With large reductions in anthropogenic emissions of many ozone-depleting gases in response to the Montreal Protocol, gases with biogenic sources have become relatively more important in recent years. The global budgets of the biogenic halocarbons are unbalanced with known sinks outweighing sources, suggesting that additional natural sources are required to balance the budgets. In the present study, an investigation has been carried out to determine the importance of leaf cutter ants as a missing source of the biogenic halocarbons, which will reduce the discrepancy of the global budget of the halocarbons. Abstract. Leaf cutter ant colonies are shown to be a potentially significant new source of biogenic halocarbons. Fungus cultivated by these ant species may emit CH3Br, CH3I, CH3Cl, CH2Cl2 and CHCl3 in significant quantities, contributing to their respective global atmospheric budgets. The study suggests that the mixing ratios of CH3Br, CH3I, CH3Cl, CH2Cl2 and CHCl3 in the ant colony under test were significantly higher than background levels, by on average a factor of 1.5–5.0. Sampling was carried out during three stages of ant colony development (new, moderately active and highly active) and it was found that levels of these halocarbons were elevated during the active phases of the ant colony life cycle. A very rough estimate of the possible emission of CH3Br, CH3I, CH3Cl, CH2Cl2 and CHCl3 from ant colonies globally are 0.50, 0.02, 0.80, 0.15 and 0.22 Gg year–1.

Estimation of Daytime NO3 Radical Levels in the UK Urban Atmosphere Using the Steady State Approximation Method

The steady state approximation has been applied to the UK National Environment Technology Centre (NETCEN) data at three urban sites in the UK (Marylebone Road London, London Eltham, and Harwell) over the period of 1997 to 2012 to estimate the concentrations of daytime NO3. Despite the common assertion that NO3 levels are negligible in the day as a consequence of photolysis, there are occasions where NO3 reaches a few pptv. A seasonal pattern in NO3 concentration was observed with higher levels in the spring with consistent peaks in April and May. A combination of temperature effects (the formation of NO3 from the reaction of NO2 with O3 has a high activation energy barrier), a distinct pattern in O3 concentration (peaking in spring), and loss via reaction with NO peaking in winter is responsible for this trend. Although reaction with OH is still the dominant loss process for VOCs during the day, there are VOCs (unsaturated) that will have an appreciable loss due to reaction with NO3 in the daytime. Since the addition reaction of NO3 with alkenes can lead directly to organic nitrate formation, there are implications for O3 formation and secondary organic aerosol formation during daytime and these are discussed.

A mitigation strategy for commercial aviation impact on NOx-related O3 change

An operational mitigation strategy for commercial aircraft impact on atmospheric composition, referred to as the turboprop replacement strategy (TRS), is described in this paper. The global air traffic between 2005 and 2011 was modeled with the TRS in which turbofan powered aircraft were replaced with nine chosen turboprop powered aircraft on all routes up to 1700 nautical miles (NM) in range. The results of this TRS double the global number of departures, as well as global mission distance, while global mission time grows by nearly a factor of 3. However, the global mission fuel and the emissions of aviation CO2, H2O, and SOx remain approximately unchanged, and the total global aviation CO, hydrocarbons (HC), and NOx emissions are reduced by 79%, 21%, and 11% on average between 2005 and 2011. The TRS lowers the global mean cruise altitude of flights up to 1700 NM by ~2.7 km which leads to a significant decrease in global mission fuel burn, mission time, distance flown, and the aircraft emissions of CO2, CO, H2O, NOx, SOx, and HC above 9.2 km. The replacement of turbofans with turboprops in regional fleets on a global scale leads to an overall reduction in levels of tropospheric O3 at the current estimated mean cruise altitude near the tropopause where the radiative forcing of O3 is strongest. Further, the replacement strategy results in a reduction of ground-level aviation CO and NOx emissions by 33 and 29%, respectively, between 2005 and 2011.

An estimate of the global budget and distribution of ethanol using a global 3-D atmospheric chemistry transport model STOCHEM-CRI

The atmospheric global budget and distribution of ethanol have been investigated using a global 3-dimensional chemistry transport model, STOCHEM-CRI. Ethanol, a precursor to acetaldehyde and peroxyacetyl nitrate (PAN), is found throughout the troposphere with a global burden of 0.024–0.25 Tg. The atmospheric lifetime of ethanol is found to be 1.1–2.8 days, which is in excellent agreement with estimates established by previous studies. The main global source of ethanol is from direct emission (99%) and the remainder (1%) being produced via peroxy radical reactions. In terms of removal rates of ethanol in the atmosphere, oxidation by hydroxyl radicals (OH) accounted for 51%, dry deposition 8% and wet deposition accounted for 41%. Globally there are significant concentrations of ethanol over equatorial Africa, North America and parts of Asia with considerably higher concentrations modelled over Saudi Arabia and Eastern Canada. Through comparison of measured and modelled ethanol data, it is apparent that the underestimation of the source strength of ethanol and the coarse resolution of the STOCHEM-CRI model produce the discrepancies between the model and the measured data mostly in urban areas. The increased vegetation and anthropogenic emissions of ethanol lead to an increase in the production of acetaldehyde (by up to 90%) and peroxyacetyl nitrate (by up to 10%) which disrupts the NOx-ozone balance, promoting ozone production (by up to 1.4%) in the equatorial regions.