portance of the roles played by raindrop impact and overland flow (Gilley and Finkner, 1985; Bradford et Quantification of soil detachment rates is necessary to establish a al., 1987), the effect of flow depth and sediment load basic understanding of soil erosion processes and to develop fundamental-based erosion models. Many studies have been conducted on on splash (Hirschi and Barfield, 1988; Kemper et al., the detachment rates of disturbed soils, but very little has been done 1985), and transport capacity (Guy et al., 1987; Kinnell, to quantify the rates of detachment for natural soil conditions. This 1993) have been simulated and analyzed. The relationstudy was conducted to evaluate the influence of flow discharge, slope ship between soil detachment by raindrop impact, raingradient, flow velocity, shear stress, stream power, and unit stream drop size and mass, drop velocity, kinetic energy, soil power on detachment rates of natural, undisturbed, mixed mesic typistrength, water drop impact angle, and surface sealing cal Udorthent soil. Flow rates ranged from 0.25 to 2.0 L s Ϫ1 and slope have also been investigated (Nearing and Bradford, gradient ranged from 8.8 to 46.6%. This study was compared with a 1985; Bradford et al., 1987; Sharma et al., 1991; Sharma previous study that used disturbed soil prepared by static compression.

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Knowledge of the relative abundance of primary organic aerosol (POA) and secondary organic aerosol (SOA) forms an important scientific basis for formulating particulate matter (PM) control policies. Taking advantage of a comprehensive chemical composition data set of PM2.5 including both POA and SOA tracers (most notably, SOA tracers of a few biogenic voltaic organic compound precursors), we investigate the impact of inclusion of SOA tracers on the source apportionment of organic carbon (OC) and PM2.5 in the Pearl River Delta region of China using positive matrix factorization (PMF). In PMF runs incorporating SOA tracers (PMFw), ten PMF factors were resolved including four secondary factors: (1) SOA I (α-pinene, β-caryophyllene, and naphthalene-derived SOA), (2) SOA II (isoprene-derived SOA), (3) a secondary sulfate factor, and (4) a secondary nitrate factor. In PMF tests without SOA tracers (PMFwo), the SOA I and SOA II factors could not be extracted, but the remaining eight source factors were resolved. Among the eight common source factors, the industrial emission factor, identified by high loadings of Zn and Pb, showed the largest variations between PMFw and PMFwo solutions. The source contributions of SOA I and SOA II resolved in PMFw were largely shifted to the industry emission source in PMFwo. Secondary organic carbon (SOC) summed from the four secondary factors in PMFw contributed ∼40% (4.47 μgC/m3), and the SOC estimate by PMFwo (3.51 μgC/m3) was 21% lower due to the inability to extract SOA I and SOA II. Secondary PM2.5 by PMFwo was 6% lower than that by PMFw (23.7 vs 25.2 μg/m3). The PMFw results indicated that SOC from specific precursors may have different formation pathways than secondary sulfate and nitrate formation processes, and their source contributions could not be properly resolved without the indicative tracers included in PMF. This study demonstrates the utility of biogenic SOA tracers in resolving isoprene-derived SOA and highlights the need for more SOA tracers, especially those specific to anthropogenic precursors, in improving the source apportionment for those broad OA sources such as industrial emissions.

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The organic composition of airborne fine particulate matter (PM 2.5 , aerodynamic diameter less than 2.5 μm) at a molecular level has yet to be achieved, hindering a full understanding of the climatic impacts and health effects of PM 2.5 . Compounds containing aromatic rings are closely associated with optically active brown carbon and toxicologically important quinones. In this work, a group of ten aromatic organic acids including three phthalic acids, four phenolic acids, and three benzene-tricarboxylic acids (BTCAs) in PM 2.5 were studied for their abundance and potential sources through quantifying their ambient concentrations at four sites in the Pearl River Delta (PRD) region in Southern China, where biomass burning and anthropogenic emissions are both significant PM sources. Average concentrations of individual aromatic acids in a total of 240 PM 2.5 samples collected throughout 2012 were in the order of 0.1−20 ng/m 3 with p-and o-phthalic acid being the most abundant. Interspecies correlation analysis with known PM source tracers reveals different source origins for the ten aromatic acids. The four phenolic acids, all possessing partial lignin structures, are highly correlated with levoglucosan, indicating their association with biomass burning emissions. Specific lignin tracer ratios characteristic of different types of biomass fuels (i.e., cinnamyl-to vanillyl-phenol ratio) revealed the significant influence of crop burning emissions in the PRD region. The three BTCAs have moderate correlation with sulfate but no correlation with levoglucosan, suggesting a strong association with secondary formation origins while negating a strong link with biomass burning. The three phthalic acids are moderately correlated with sulfate, levoglucosan, and a number of polycyclic aromatic hydrocarbons (PAHs), indicating multiple significant sources. This study provides a valuable data set toward establishing quantitative links between molecular composition of organic matter and the optical and toxicological properties of PM 2.5 as well as assisting identification of tracers for PM 2.5 sources.

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The electron transfer capacities (ETCs) of soil humic substances (HSs) are linked to the type and abundance of redox-active functional moieties in their structure. Natural temperature can affect the chemical structure of natural organic matter by regulating their oxidative transformation and degradation in soil. However, it is unclear if there is a direct correlation between ETC of soil HS and mean annual temperature. In this study, we assess the response of the electron-accepting and -donating capacities (EAC and EDC) of soil HSs to temperature by analyzing HSs extracted from soil set along glacial-interglacial cycles through loess-palaeosol sequences and along natural temperature gradients through latitude and altitude transects. We show that the EAC and EDC of soil HSs increase and decrease, respectively, with increasing temperature. Increased temperature facilitates the prevalence of oxidative degradation and transformation of HS in soils, thus potentially promoting the preferentially oxidative degradation of phenol moieties of HS or the oxidative transformation of electron-donating phenol moieties to electron-accepting quinone moieties in the HS structure. Consequently, the EAC and EDC of HSs in soil increase and decrease, respectively. The results of this study could help to understand biogeochemical processes, wherein the redox functionality of soil organic matter is involved in the context of increasing temperature.