Vertical distributions of non-methane hydrocarbons and halocarbons in the lower troposphere over northeast China

Vertical distributions of air pollutants are crucial for understanding the key processes of atmospheric transport and for evaluating chemical transport models. In this paper, we present measurements of non-methane hydrocarbons (NMHCs) and halocarbons obtained from an intensive aircraft study over northeast (NE) China in summer 2007. Most compounds exhibited a typical negative profile of decreasing mixing ratios with increasing altitude, although the gradients differed with different species. Three regional plumes with enhanced VOC mixing ratios were discerned and characterized. An aged plume transported from the northern part of the densely populated North China Plain (NCP; i.e. Beijing–Tianjin area) showed relatively higher levels of HCFC-22, 1,2-dichloroethane (1,2-DCE) and toluene. In comparison, the plume originating from Korea had higher abundances of CFC-12, tetrachloroethene (C₂Cl₄) and methyl chloride (CH₃Cl), while regional air masses from NE China contained more abundant light alkanes. By comparing these results with the earlier PEM-West B (1994) and TRACE-P (2001) aircraft measurements, continuing declining trends were derived for methyl chloroform (CH₃CCl₃), tetrachloromethane (CCl₄) and C₂Cl₄ over the greater China–northwestern Pacific region, indicating the accomplishment of China in reducing these compounds under the Montreal protocol. However, the study also provided evidence for the continuing emissions of several halocarbons in China in 2007, such as CFCs (mainly from materials in stock) and HCFCs.     

Application of CORINE land-cover mapping to estimate carbon stored in the vegetation of Ireland

The CORINE land cover database for Ireland (in ARC/INFO) is used to estimate the amount of carbon stored (tonnes) by each land-cover (vegetation) type. Carbon store is the area of each CORINE land-cover type multiplied by its carbon density (t C ha⁻¹). Derivations of these carbon densities are described and limitations of data and other empirical evidence discussed. The total vegetation-carbon stores are calculated for Northern Ireland (3·81 Mt), the Republic of Ireland (19·27 Mt) and Ireland (23·08 Mt). Carbon densities are grouped into classes and their distributions across Ireland are mapped. The vegetation-carbon store is taken to include stems, branches, foliage and roots. It does not include litter, microbial biomass and organic carbon in the soil. Forests store 49% of the vegetation carbon on less than 5% of the total CORINE land area, with a further 22% in other semi-natural vegetation. In contrast, pastures account for 56% of the land-cover area, but only 19% of the carbon store. High carbon densities are found in the west and in uplands, reflecting the distribution of forests and semi-natural vegetation, particularly peatland and moors. The inventory of vegetation-carbon stores is an important first step in attempts to monitor changes in carbon sequestration from, and emissions to, the atmosphere by terrestrial vegetation. Greenhouse gas fluxes, including CO₂, and climate warming are global issues which require responses by all countries. Inventories of carbon stores and fluxes therefore need to be comparable between countries so that agreed reductions can be targetted. CORINE land-cover data are available for 19 European Union and adjacent countries and could be used to provide an inventory of carbon stores, and through updating of CORINE, changes in those stores. Commonality in determining the carbon densities of CORINE classes would be required. This study exemplifies how that was achieved in two countries using their national data.     

Mesocosm-scale evaluation of faunal and microbial communities of aerated and unaerated leachfield soil

Aeration improves the capacity of leachfields to decontaminate and reduce the nutrient load of wastewater. To gain a better understanding of the effects of aeration, we examined the faunal and microbial communities of septic system leachfield soil (0-4 and 4-13 cm) using replicated (n = 3) mesocosms that were actively aerated (AIR) or unaerated (LEACH). Protozoa were 40 to 140 times more abundant in AIR than in LEACH soil. No nematodes were found in LEACH soil, whereas AIR soil contained 5 to 14 x 10(3) nematodes (all bacteriovores) kg(-1). Active microbial biomass was four to five times higher in AIR than LEACH soil. Proteobacteria and actinomycetes/sulfate-reducing bacteria constituted a higher proportion of the community in AIR soil, whereas anaerobic Gram-negative bacteria/firmicutes were more prominent in LEACH soil. Ratios of prokaryotic to eukaryotic phospholipid fatty acids (PLFAs) were higher in LEACH soil, as were membrane stress index values, whereas the starvation index was higher in AIR soil. Community-level physiological profiles showed that 29 and 30 different substrates were used for growth by LEACH and AIR soil microorganisms, respectively. The AIR soil had more microorganisms capable of growing on 10 substrates, whereas growth on two substrates was higher in LEACH soil. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis of 16S rRNA gene fragments revealed greater diversity of dominant phylotypes in AIR than LEACH soil, with communities separated by treatment. Aerated leachfield soil had a larger and more diverse faunal and microbial community than unaerated soil, possibly due to differences in the type and availability of electron acceptors.     

Air distribution in the Borden aquifer during in situ air sparging

A field experiment was conducted at Canadian Forces Base Borden (CFB Borden) to assess the air distribution from a single in situ air sparging injection point. This aquifer consists of fine to medium sand deposited in horizontal layers. The permeability at the study location varied from 10(-10) to 10(-14) m2 and distinct low permeability horizons were present at approximately 1.2, 2.0, and 2.9 m below the water table. Prior to air injection, a 15x15-m portion of the vadose zone was excavated to the water table (approximately 1 m below ground surface) in order to visually observe air release distribution at the water table. The water table was actively maintained 5 cm above the excavated surface. The sparging system operated for a period of 7 days with an injection flow rate of 200 m3/days (5 scfm). The resulting subsurface air distribution was assessed using a variety of techniques including neutron logging, borehole and surface ground penetrating radar, piezometric head measurements, surface visualization, and hydraulic testing. Through this combination of tests, it was demonstrated that variations in permeability and, hence, capillary pressure at the site were sufficient to cause the injected air to spread laterally, forming stratigraphically trapped air pockets beneath the low permeability horizons. The formation of these air pockets eventually resulted in a buildup of capillary pressure that exceeded the air entry pressure and allowed some air to migrate up through the lower permeability layers. Each of the assessment techniques employed generated information at different spatial scales that prevented a direct comparison of the results from the various techniques; however, the results from all techniques proved to be critical in the interpretation of the experimental data. As a consequence, the different assessment techniques should not be viewed as alternatives, but rather as complimentary techniques.     

Decomposition of nitrous oxide on pillared clays

Alumina-pillared smectites have been found to abate nitrous oxide in the presence of methane. The results indicate that the yield of the reaction (N20 --> N2 + (1/2)O2) increases when pillared clays are exchanged with transition metals, single-pass conversion rates of >70% being attainable. In particular, when double exchanged (calcium and subsequently copper) alumina pillared montmorillonite/beidellite is used as a catalyst, de-N2O activity reaches a maximum, which is maintained even after 4 h of work at a space velocity of 5.5 h(-1). A mechanism for the reaction is suggested, which implies that N2O is first adsorbed by the catalyst and then decomposes through two different paths: catalyst oxidation and catalyst reduction. Such a redox process explains the kinetic data.     

Soil carbon stocks and changes in the Republic of Ireland

The soil carbon (C) stock of the Republic of Ireland is estimated to have been 2048 Mt in 1990 and 2021 Mt in 2000. Peat holds around 53% of the soil C stock, but on 17% of the land area. The C density of soils (tCha(-1)) is mapped at 2 km x 2 km resolution. The greatest soil C densities occur where deep raised bogs are the dominant soil; in these grid squares C density can reach 3000 tCha(-1). Most of the loss of soil C between 1990 and 2000-up to 23 MtC (1% of 1990 soil C stock)-was through industrial peat extraction. The average annual change in soil C stocks from 1990 to 2000 due to land use change was estimated at around 0.02% of the 1990 stock. Considering uncertainties in the data used to calculate soil C stocks and changes, the small average annual 'loss' could be regarded as 'no change'.