A wide range of toxic VOCs measured by dual-sorbent passive sampling with validation by field online measurements.

This study modified a passive sampling technique similar to the US EPA Method 325 A/B method but extended to include more toxic volatile organic compounds (VOCs) under varied climate conditions to enhance field applicability. A mixing chamber was built to determine uptake rates (Us) for the target compounds. It was found that the Us of 27 air toxics previously reported in the literature agreed reasonably well with our findings within 18%, thus proving the chamber's integrity. To broaden the compound coverage, both Carbopack X and Carboxen 569 were studied for a suite of toxic VOCs to meet stringent quality control (QC) criteria of correlation coefficients (R-square), method detection limits (MDL), back diffusion (BD), storage stability, as well as a wide range of climate conditions in temperature and humidity. After excluding the species that failed to pass any of the QC criteria, Carbopack X was found to fit 50 air toxics, whereas Carboxen 569 held 37. After excluding the overlapped species, 61 toxic VOCs can be determined with robust Us for a broad range of climate conditions when the two sorbents are used in pairs. A one-week field measurement was conducted to compare with the online thermal desorption gas chromatography-mass spectrometry (TD-GC-MS) with hourly data resolution. The field passive sampling showed comparable results to the means of the online hourly measurements, despite the high variability of selected target compounds, such as toluene from 0.3 ppbv as the 5th percentile to the maximum of about 80 ppbv. Passive sampling clearly demonstrated the ability to smooth out concentration variability and thus the time-averaging strength of toxic VOCs, revealing its ideal role as an exposure monitor over time. The passive sampling method can be more desired than active sampling or online methods when the aim is simply the knowledge of prolonged time-averaged concentrations.

Taiwan ozone trend in response to reduced domestic precursors and perennial transboundary influence.

Two decades of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in ambient concentrations have been continuously reduced by nearly 60% since 1994. However, the annual mean ozone (O3) concentrations in Taiwan had leveled off for more than a decade. Furthermore, the significant cuts in precursors did lead to reduction in episodic days (O3≥ 100 ppbv) over time, implying the weakened photochemical production of O3. Simultaneously, the number of low O3 days (O3 < 20 ppbv) also decreased due to weakened NO titration. Nevertheless, the reduction of both high and low O3 days resulted in an increase in the number of medium O3 days (O3 from 60-80 ppbv). While the nighttime mean O3 continued to increase, the daytime mean O3 leveled off and coincided with the marine background concentration of approximately 40 ppbv, revealing a gradual decrease in the daytime-nighttime mean O3 difference. In addition to multiple chemical reasons, transboundary O3 from Asian continental outflow is thought to be another major contributor to the prolonged leveling of domestic O3. To quantitatively illustrate the role of the transboundary effect, model simulations were conducted in two ways: one assessed the time percentage of a year affected by the transboundary influence; and the other involved decoupling the transboundary O3 from the domestically produced O3 to quantify the amount of domestic O3 that can be mitigated by prescribing scenarios; namely, the so-called sensitivity test. The sensitivity test suggests that although controlling domestic emissions is still vital to contain O3 under weak transboundary conditions, the overall O3 averages and the long-term trend at the island scale are closely associated with the transboundary influence. The gradual increased medium O3 concentrations to approximate the marine background level could imply that future O3 concentration on the island will be more influenced more by the transboundary conditions and less relevant to the domestic containment measures.

Identifying an unknown compound in flue gas of semiconductor industry - Forensics of a perfluorocarbon.

This study investigated the source of a false positive signal in the measurement of total non-methane organic compounds (TNMOCs) by an on-line analyzer based on flame ionization detection (FID) in the flue gas released from a semiconductor fabrication plant. Since no release of volatile organic compounds (VOCs) into the waste gas stream in acid/base ventilation was assured by the plant authority, the positive detection of VOCs became a subject of dispute. In addition to the TNMOC analysis of 5 samples, the investigation used the method that coupled thermal desorption (TD) with gas chromatography mass spectrometry (GC/MS), dubbed TD-GC/MS, to identify the substance that produced the FID signals of TNMOCs. The waste gas was collected with sampling canisters and analyzed by in-laboratory TD-GC/MS. However, mass scanning from 45 to 250 m/z to remove interference from air matrix of O2, N2 and CO2 forbid detecting any ion fragments smaller than 45 m/z and, thus, led to poor match in mass (MS) library search. As a result, a highly retentive porous layer open tubular (PLOT) capillary column was employed to separate the unknown away from the air peak. The success of acquiring all key ion fragments of 31, 50, 69, and 131 m/z resulted in an excellent match with octafluorocyclobutane (C4F8) in the NIST database. A gas standard was then prepared and injected to confirm the identity of C4F8 by the identical mass spectrum and GC retention time. The concentrations of C4F8 found in the 5 flue gas samples varying from 3.32 to 6.21 ppmv were very close to the NMOC range of 3.48-6.62 ppmv as reported by the TNMOC analyzer, proving that the FID signals observed from TNMOC method were mostly produced from C4F8. Consequently, the method of TD-GC/MS would be an ideal method to pre-screen the presence of PFCs before a non-distinguishable TNMOC analyzer is applied to approximate the VOC level as part of the integrated effort to monitor VOC in flue gas.

Characterization of industrial plumes of volatile organic compounds by guided sampling.

Instead of manual sampling in a random way near a source area, this study used trigger sampling guided by an analyzer at a fixed site near a refinery plant to obtain the chemical composition of volatile organic compounds (VOCs) representative of the source. The analyzer was built in-house to measure total VOC (TVOC) levels by subtracting methane from total combustible organic compounds (TOC) with flame ionization detection. The analyzer with minute resolution provided instantaneous measurements of TVOCs to trigger canister sampling at the moments of VOC plumes in a source area. The chemical composition of the 13 trigger samples were compared with the other non-trigger samples randomly collected either within the refinery or on an urban street. All samples were analyzed by gas chromatography-mass spectrometry/flame ionization detection (GC-MS/FID) for detailed speciation. High agreement in total VOC abundance between the analyzer and GC-MS/FID indicates internal consistency of the two techniques and the robustness of the TVOC analyzer to guide sampling of VOC plumes. The trigger samples showing very high consistency in the overall composition imply that sampling at the right moments of plume arrivals can facilitate characterization of the source profiles, which can hardly be achieved by random sampling. The coupling of the fast-and-slow analytical techniques to guide sampling is proven to be an effective means to probe source characteristics.