Soil desiccation for Loess soils on natural and regrown areas

In the Loess Plateau, soil desiccation has become a serious problem for forest and grass vegetation. Soil desiccation leads to the formation of a dried soil layer (DSL). This paper presents the results of research carried out in the central part of the Loess Plateau. The objective of the research was to produce a statistically supported set of indicators for evaluating soil desiccation of forestlands, to present a heuristic idea for soil desiccation and to supply scientific support for replacing farmland with forest or grass in the Loess Plateau and other regions of China. Here, we suggest that more attention should be paid to soil desiccation and its effects on the ecosystem of the region in the future. The results showed that natural Quercus liaotungensis forestlands (NQF) retained more water content than regrown Robinnia pseudoscacia forestlands (RRF). Significant DSLs were formed in the RRF but not in the NQF. A possible reason for no formation of DSL in NQF could be due to the presence of an arbor-shrub-herb stand structure and large humus and litter accumulation, which increased the natural forest's (NF) adaptability to the environmental conditions. Soil water content in the north-facing slope was significantly larger than in the south-facing slope. DSLs formed in the 0-500 cm layer of the south-facing slope. When slope gradient was greater than 25 degrees, soil water content deceased sharply and showed significant difference compared with 9 degrees, 15 degrees and 20 degrees (P < 0.05). So, we conclude that plant species, aspect and slope angle could be the predicators for the formation of DSLs. The analysis on soil physical properties of 0-60 cm layer indicated that plant species, aspect and slope angle also have significant effects on bulk density, porosity, plant-available capacity, and hydraulic conductivity, especially in the 0-20 and 20-40 cm layers. In the NQF and RRF with north-facing slope, soil physical properties were improved. (C) 2008 Elsevier B.V. All rights reserved

Individual and pooled tree-ring stable-carbon isotope series in Chinese pine from the Nan Wutai region, China: Common signal and climate relationships

To investigate the differences in the climatic signals in stable-carbon isotopic composition captured by averaging series from individual trees versus raw wood of trees pooled prior to analysis, we analyzed two groups of Chinese pine (Pinus tabulaeformis Carr.) from the Nan Wutai region of the Qinling Mountains, China. One group included three trees that were analyzed separately, and the other group comprised four other trees that were pooled prior to preparation and analysis. All &delta;13C series were positively correlated (r = 0.50&ndash;0.58, p &lt; 0.0001) for the period AD 1901&ndash;2003. After removing the effects of changing &delta;13C of atmospheric CO2, correlations between the meteorological data and all individual and pooled discrimination (∆13C) series revealed significant negative responses to temperature for several specific months and for mean January to September (TJ&ndash;S) temperature. We used a &ldquo;numerical mix method&rdquo; (NMM, equivalent to unweighted mean), by averaging individual ∆13C series (NPS1+NPS2+NPS3), to generate a new series that more strongly correlated to climate series TJ&ndash;S (r = &minus; 0.67, p &lt; 0.0001). This time interval from January through September (J&ndash;S) includes the growing season and months prior to the growing season, but the temperature prior to the growing season may provide energy necessary for timely initiation of growth. Thus, the mean TJ&ndash;S is significant for plant growth and is consistent with the tree physiology in this region. Our results suggest that the numerical mix method with tree-ring stable isotope data from three trees provides a series quite satisfactory for climatic reconstruction. The relationship of the numerical mix model ∆13C with temperature was stronger than for the pooled series, suggesting numerical mixing of series can be more effective than raw wood sample pooling at least according to the trees in this study.</p

Tree-ring stable carbon isotope-based May-July temperature reconstruction over Nanwutai, China, for the past century and its record of 20th century warming

Growth anomaly of trees in some regions was detected under current episode of rapid warming. This raises a dilemma for temperature reconstructions by using tree-ring width which is believed to be the most important proxy on inter-annual temperature reconstruction during the past millenniums. Here we employed the tree-ring &delta;13C to reconstruct temperature variations for exploring their potential on capturing signals of rapid warming, and to test how its difference with the tree-ring width based reconstruction. In this study the mean May-July temperature (TM-J) was reconstructed over the past century by tree-ring &delta;13C of Chinese pine trees growing in the Nanwutai region. The explained variance of the reconstruction was 43.3% (42.1% after adjusting the degrees of freedom). Compared to a ring-width temperature reconstruction (May-July) from the same site, the tree-ring &delta;13C-based temperature reconstruction offered two distinct advantages: 1) it captured a wider range of temperature variability, i.e., at least May-July, even over a longer part of the year, January-September; and 2) the reconstruction preserved more low-frequency climate information than that of ring width did.The 20th century warming was well represented in the Nanwutai tree-ring &delta;13C temperature reconstruction, which implied that stable carbon isotope of tree rings potentially represents temperature variations during historical episodes of rapid warming. A spatial correlation analysis showed that our temperature reconstruction represented climate variations over the entire Loess Plateau in north-central China. Significant positive correlations (p&lt;0.1) were found between the temperature reconstruction and ENSO, as well as SSTs in the Pacific and Indian Oceans. The reconstruction showed the periodicities of 22.78-, 4.16-, 3.45-3.97- and 2.04-2.83-year quasi-cycles at a 95% confidence level. Our results suggested that temperature variability in the Nanwutai region may be linked to Pacific and Indian Ocean SST variations and solar activity.</p

Long-Term Trends in Visibility and at Chengdu, China

Long-term (1973 to 2010) trends in visibility at Chengdu, China were investigated using meteorological data from the U.S. National Climatic Data Center. The visual range exhibited a declining trend before 1982, a slight increase between 1983 and 1995, a sharp decrease between 1996 and 2005, and some improvements after 2006. The trends in visibility were generally consistent with the economic development and implementation of pollution controls in China. Intensive PM2.5 measurements were conducted from 2009 to 2010 to determine the causes of visibility degradation. An analysis based on a modification of the IMPROVE approach indicated that PM2.5 ammonium bisulfate contributed 27.7% to the light extinction coefficient (bext); this was followed by organic mass (21.7%), moisture (20.6%), and ammonium nitrate (16.3%). Contributions from elemental carbon (9.4%) and soil dust (4.3%) were relatively minor. Anthropogenic aerosol components (sulfate, nitrate, and elemental carbon) and moisture at the surface also were important determinants of the aerosol optical depth (AOD) at 550 nm, and the spatial distributions of both bext and AOD were strongly affected by regional topography. A Positive Matrix Factorization receptor model suggested that coal combustion was the largest contributor to PM2.5 mass (42.3%) and the dry-air light-scattering coefficient (47.7%); this was followed by vehicular emissions (23.4% and 20.5%, respectively), industrial emissions (14.9% and 18.8%), biomass burning (12.8% and 11.9%), and fugitive dust (6.6% and 1.1%). Our observations provide a scientific basis for improving visibility in this area.</p

Observation of atmospheric aerosols at Mt. Hua and Mt. Tai in central and east China during spring 2009-Part 1: EC, OC and inorganic ions

PM(10) and size-segregated samples were simultaneously collected at Mt. Hua (2060 ma.s.l.) and Mt. Tai (1545 ma.s.l.) in central and east coastal China during spring, 2009 including an intensive dust storm event occurring on 24 April, and determined for EC, OC and inorganic ions. During the non-dust storm period particles, EC, OC and ions except for SO(4)(2-) were 2-10 times more abundant at Mt. Tai than at Mt. Hua. SO(4)(2-) (13 +/- 7.1 mu g m(-3)) at Mt. Hua was the dominant ion, followed by NO(3)(-) (5.0 +/- 3.9 mu g m(-3)), NH(4)(+) (2.5 +/- 1.3 mu g m(-3)) and Ca(2+) (1.6 +/- 0.8 mu g m(-3)). In contrast, at Mt. Tai NO(3)(-) was most abundant (20 +/- 14 mu g m(-3)), followed by SO(4)(2-) (16 +/- 13 mu g m(-3)), NH(4)(+) (12 +/- 8.9 mu g m(-3)) and Ca(2+) (3.9 +/- 2.1 mu g m(-3)). The fact of NO(3)(-) exceeding over SO(4)(2-) at Mt. Tai may suggest the changes in chemical composition of the atmosphere over east China due to sharply increasing vehicle emission. pH values of the water-extracts of PM(10) samples indicate that at the two mountain sites aerosols transported from the south regions are more acidic than those from the north and more acidic at Mt. Tai than at Mt. Hua during the non-dust storm period. During the dust storm event particle mass, OC, Na(+), K(+), Mg(2+) and Ca(2+) at both sites increased by a factor of 1-9, while EC, NO(3)(-) and NH(4)(+) decreased by 20-80 %. However, SO(4)(2-) concentrations (13 +/- 7.7 mu g m(-3) at Mt. Hua and 15 +/- 5.6 mu g m(-3) at Mt. Tai, respectively) at the two sites during the episode were comparable and did not change significantly compared to those in the non-dust storm period, probably due to a similar level of free tropospheric SO(2) in central and east China.</p

The Optical Properties of Urban Aerosol in Northern China: A Case Study at Xi’an

Simultaneous measurements of particle scattering coefficient (Bscat) and absorption coefficient (Babs) were conducted at Xi\u27an from mid-August to mid-October 2012 to estimate the particle single scattering albedo (SSA) and the Ångström coefficients in highly polluted urban air. The hourly averaged Bscat was 272 Mm− 1 at 532 nm and 82 Mm− 1 at 870 nm, while hourly averaged Babs was 31 Mm− 1 at 532 nm and 19 Mm− 1 at 870 nm. Similar diurnal variations for Bscat and Babs were observed between the two wavelengths. The averaged SSA was 0.88 at 532 nm and 0.78 at 870 nm. Based on the Ångström coefficients, anthropogenic fine particles show dominant contribution during the sampling period, accompanied by occasional dust events. Moreover, the major contributors to aerosol optical properties are attributed to the mixture of black carbon (BC) and brown carbon (BrC) with non-absorbing components over urban area in northern China. The findings provide useful insights into the factors affecting the visibility in northern Chinese cities and therefore essential knowledge for improving the air quality

Water-soluble ions in atmospheric aerosols measured in Xi'an, China: Seasonal variations and sources

Daily PM(2.5) and water-soluble inorganic ions (Na(+), NH(4)(+), K(+), Mg(2+), Ca(2+), Cl(-), NO(3)(-) and SO(4)(2-)) were collected in Xi&#39;an (34.23 degrees N, 108.88 degrees E), China from March 2006 to March 2007. PM(2.5) was collected using battery-powered mini-volume samplers. And the ions were determined by ion chromatography from the measured aerosol mass. The annual average mass concentration of PM(2.5) was found to be 194.1 +/- 78.6 mu g m(-3), which exceeded substantially the international guidelines for health concerns. The seasonal average mass concentration of PM(2.5) was highest in winter (266.8 mu g m(-3)) and lowest in summer (138.6 mu g m(-3)). The three highest abundant ions were SO(4)(2-), NO(3)(-), and NH(4)(-), with average concentrations of 35.6 +/- 19.5 mu g m(-3), 16.4 +/- 10.1 mu g m(-3), and 11.4 6.8 mu g m(-3), which were accounted for 18.7%, 8.0%, and 5.7% of the PM(2.5) mass, respectively. The major ions were in the species of (NH(4))(2)SO(4), NH(4)HSO(4) and NH(4)NO(3), and their concentrations were highest in winter, due to high coal combustion. The concentrations of Ca(2+) were higher in spring than other seasons, due to the higher mineral dust concentrations. Ca(2+) was strongly correlated with CO(3)(2-), which was calculated as the difference in the measured cations minus anions. Ion balance calculations indicate that the PM(2.5) was acidic, and this result is consistent with the measurement of pH values. Sulfur oxidation ratio was higher in summer and autumn, which implies that the formation of secondary sulfate-rich particles is favored by warm and relatively moist weather. Nitrogen oxidation ratio was highest in autumn.</p

Lead concentrations in fine particulate matter after the phasing out of leaded gasoline in Xi’an, China

Daily concentrations of lead (Pb) were determined for PM2.5 samples collected from an urban location in Xi&rsquo;an, China from 2007 to 2009 to assess the effects of the phasing out of leaded gasoline in 2000. The Pb concentrations (annual average: 0.306 &mu;g m&minus;3, range: below detection limit to 2.631 &mu;g m&minus;3) have declined after the phasing out of leaded gasoline, but the concentrations were still higher than those reported in many other cities. Seasonal variations of Pb were significant, with high concentrations in winter, presumably due to the burning of coal, and low concentrations in summer, due to a deep mixed layer and scavenging of aerosols by precipitation. Correlation analyses and enrichment factor calculations both indicated that anthropogenic sources had a large influence on atmospheric Pb. The lead isotope ratios were low in winter (the average 207Pb/206Pb ratio was 0.843 &plusmn; 0.032; 208Pb/206Pb was 1.908 &plusmn; 0.058) and high in summer (207Pb/206Pb was 0.860 &plusmn; 0.032; 208Pb/206Pb was 2.039 &plusmn; 0.057), suggesting that coal combustion was the major Pb source in winter and vehicular emission was the major Pb source in summer. Positive Matrix Factorization receptor model indicated that there were five major sources for Pb in PM2.5. Coal combustion was the major contributor, accounting for 39.0% PM2.5 mass, followed by vehicular emissions (30.4%). Other contributors included 17.8% from industrial emissions, 11.6% from biomass burning, and 1.2% from fugitive dust.</p