دانلود رایگان مقاله LED دوگانه مبتنی بر ابزار دیفرانسیل طیف سنجی جذب نوری برای اندازه گیری هیدروکربن های آروماتیک

abstract Aromatic hydrocarbons are well known air toxics which are regulated by the US EPA and other air quality agencies. Accurate, long-term monitoring of these compounds at low part-per-billion levels, as well as identifying emission point sources is therefore crucial to protect human health in neighborhoods near large emission sources. Here we present a new long-path differential optical absorption spectroscopy (LP-DOAS) instrument specifically designed to monitor aromatic hydrocarbons. The system is based on a novel dual - light emitting diode (LED) light source, which eliminates the requirement to suppress spectrometer stray light. This light source, together with a high stability fiber-based sending/receiving telescope, allows the measurement of aromatic hydrocarbons on once-folded absorptions paths of 200 e1200 m length. The new instrument shows very good agreement with simultaneous in-situ measurements if inhomogeneities of the trace gas spatial distributions are considered. The new instrument performed well during a three-month field test as an automated fence-line monitor at a refinery, successfully distinguishing upwind background levels of ~1 ppb from emissions reflected in elevated mixing ratios of 3e4 ppb. A two-dimensional measurement network based on two identical LP-DOAS instruments operating on seven crossed light paths was operated successfully in Houston, TX. Qualitative and quantitative analysis of two events with toluene and xylene plumes demonstrate how this setup can be used to derive the spatial distribution of aromatic hydrocarbons, and identify point sources.

5. Conclusions

The goal of this study was to demonstrate a novel LP-DOAS system specifically developed for the measurement of aromatic hydrocarbons. Two applications were presented: one LP-DOAS as a long term fence-line monitor, and a dual LP-DOAS network to perform two-dimensional observations of trace gas distributions. The following conclusions can be drawn from our experience with the new LP-DOAS system:
A new light source, which combines two LEDs using a fiber mixer, is suitable for LP-DOAS observations of aromatic hyrdrocarbons. The use of LEDs solves the problem of large stray light levels due to light with wavelengths smaller than 290 nm encountered when using Xe-arc lamps. It is also longer lived and more stable than comparable Xe-arc lamps. While we demonstrated this setup in the UV wavelength range, it should also be applicable for DOAS observations in other wavelength ranges by using other LED combinations.
The new instrument, together with an improved analysis algorithm, allows the absolute determination of BTX concentrations without the use of an atmospheric reference spectrum. This is a significant improvement compared to many commercial systems, which predominately measure relative to this reference spectrum.
We demonstrated that dedicated LP-DOAS instruments are capable of measuring aromatic hydrocarbons on light paths from 270 to 1200 m length, with detection limits below 1 ppb. Comparison with in-situ data shows very good agreement in cases where a well-mixed plume is encountered. However, close to sources the high variability of the data makes a comparison difficult. To our knowledge this is the first time a BTX LP-DOAS system has successfully been operated on light paths longer than 700 m. This ability expands the possible applications of BTX LP-DOAS.