Using soil-borne polycyclic aromatic hydrocarbons (PAHs) and organic carbon in soil for projecting pollution states of PAHs in ambient air.

摘要

Polycyclic Aromatic Hydrocarbons (PAHs) are of environmental concern due to their ubiquitous nature, long-range transport, long half-lives, and toxicity. PAHs enter the environment as a consequence of incomplete combustion and cause adverse health effects. Because they are omnipresent pollutants, PAH distribution is not only a local issue, but also a global issue. Therefore, monitoring PAHs in ambient air and identifying their sources and distribution patterns have tremendous importance in assessing public health. However, sampling and analyzing airborne PAHs with conventional methods is time consuming and costly. In addition, an active sampling method remains unfeasible in many cases because it requires electrical power to operate. To promote inexpensive and simple PAH analysis method, this study investigated the feasibility of local soils as passive samplers to predict ambient air pollution states.;PAH studies are abundant in the literature; however, studies reporting PAHs in semi-arid or arid conditions are scarce. The fate and transport of PAHs in semi-arid or arid regions may differ from the current knowledge that has emerged from many studies conducted in temperate, cold, and tropical regions. This study fills the gap in our knowledge and contributes a better understanding of PAH behavior in semi-arid and arid regions.;A total of 30 soil samples and 33 air samples were simultaneously collected in six sampling sites in the arid region of El Paso, TX, in winter and summer of 2009. Soil samples were fractioned into coarse (125microm--500microm) and fine (<125microm), rendering four soil sample groups: summer coarse, summer fine, winter coarse, and winter fine. Soil organic matter (SOM) content was also analyzed for each group. Statistical analyses were performed using SPSS 15.0 and 17.0 (SPSS Inc.). The principal approaches and findings are listed below: (1) Pearson correlation analyses showed that soil-borne PAH concentrations and SOM levels were statistically correlated. For the winter coarse fraction set, all PAH ring number groups showed a significant correlation, and the three-ring group gave the highest r-squared value, 0.735, with a linear regression model. However, for the fine fraction set from the summer samples, no ring group exhibited a correlation. (2) Fitting models were sought on the relations of PAH levels in coarse-fine fractions. Sixteen PAHs were categorized based on the aromatic ring number in their chemical structure. Quadratic models exhibited higher r-squared values than a linear model in all groups, indicating different PAH sorption kinetics and indefinite relations occurring in soil with different particle size. (3) Soil-borne PAHs were studied on correlations with airborne PAHs and SOM. Statistical analyses underscored soil grain size as a seminal factor on the correlation. In addition, soil-borne PAHs and airborne PAH levels showed a larger correlation coefficient with propinquity to the soil-sampling event in summer coarse fractions while a relatively smaller correlation existed in winter samples. The results indicate that high temperature accelerates soil-borne PAH turnover in environmental media, that seasonal variation of PAH residence time occurs, and that soil-borne and airborne PAHs achieve the equilibrium state faster in summer than in winter. (4) Diagnostic ratios of isomeric PAHs and principal component analyses on 16 PAHs were also compared between soil-borne and airborne PAHs. Pyrogenic and petrogenic source inputs were observed in both media with a greater seasonal variation in airborne PAHs. Nonetheless, the comparison confirmed that soil-borne PAHs do not represent freshly emitted PAHs in the ambient air, implying possible time dependent factors affecting on air-soil flux. This result also demonstrated that meticulous data interpretations including soil grain size and sampling season are required to apportion sources with regard to applying the popular diagnostic ratio method on soil-borne PAHs. (5) Although statistically significant correlations were found between soil-borne PAH levels and airborne PAH levels, multiple linear regression analyses with other factors including soil organic matter, temperature, and KOA yielded poor r-squared values on regression models: no efficient regression model was obtained. The result suggests that soil may not be a suitable passive sampler for predicting airborne PAH levels in urban environment where complex sources are involved. (6) Lastly, this research adopted green chemistry practice. A simple, cost effective, and environmentally friendly sample preparation method for determination of PAH in solid samples was proposed. The optimized method, which uses ultrasonic extraction method and acetone/hexane mixture (2:3 and 1:1 v/v), demonstrated satisfactory recoveries ranging from 63.3% (indeno[1,2,3-cd]pyrene) to 122% (benzo[b]fluoranthene) on the U.S. EPA 16 PAHs in solid samples (NIST SRM 1649a).