Tunable diode laser absorption spectrometer measurements of ambient nitrogen dioxide, nitric acid, formaldehyde, and hydrogen peroxide in Parlier, California
Introduction
Nitrogen dioxide (NO2), nitric acid (HNO3), formaldehyde (HCHO), and hydrogen peroxide (H2O2) are important atmospheric pollutants. Although there has been significant effort in developing techniques for the measurement of these trace gases, few methods have shown the capability of making the required measurements with the degree of specificity and sensitivity necessary for atmospheric assessment. Accurate measurements of these species are important for assessing the environmental impacts and devising emission control strategies to reduce these impacts. Finlayson-Pitts and Pitts (2000) summarize the atmospheric importance, reactions, and measurement methods for these compounds.
Nitrogen dioxide is the precursor for tropospheric ozone, which is known to be harmful to organic tissues, including crops as well as humans (Haagen-Smit et al., 1952). It has been shown to be measurable in ambient air by various techniques: ozone chemiluminescence after reduction to NO (Fontijn et al., 1970), direct luminol chemiluminescence (Gaffney et al (1998), Gaffney et al (1999)), tunable diode laser absorption spectroscopy (TDLAS) (Schiff et al., 1994; Fried et al., 1998), and laser induced fluorescence (LIF) (Thornton et al., 2000; Matsumoto et al., 2001). A prototype luminol-based chemiluminescence instrument, which measures NO2 directly, was operated at the site. Because luminol is known to react with ozone, PAN and SO2 (Pisano et al., 1996) this instrument employed a capillary column to minimize these interferences. The TDLAS technique is advantageous for two major reasons. First, it is spectroscopically specific as the line width of a typical diode laser is better than 10 MHz, far narrower than the 200 MHz, typical of absorption line widths (Reid et al., 1978). Second, it is a direct measurement technique since it does not require conversion of NO2 into another species (e.g., NO) before detection.
Nitric acid is an important gaseous species to measure because it serves as the major sink for ambient airborne nitrogen oxides as well as a significant sink for hydroxyl radicals. The latter radical initiates many of the gas phase reactions that degrade hydrocarbons in ambient air. Due to its sorptive properties and equilibrium with ammonium nitrate, nitric acid has proved to be a difficult species to quantify in ambient air and it has been the subject of a number of comparison studies (Spicer et al., 1982; Anlauf et al., 1985; Fox et al., 1988; Hering et al., 1988; Tanner et al., 1989; Fitz et al., 2003). The present methods for measuring nitric acid include denuder-based collection and infrared spectroscopic techniques such as FTIR and TDLAS. Although there has been moderate success during their use in short-term monitoring programs, none of the methods has become widely accepted as meeting the combined criteria of high sensitivity, accuracy, portability, and consistency of performance. The TDLAS was again chosen as the measurement method of choice due to overall performance and commercial availability.
Formaldehyde is emitted directly into the troposphere and is also a product of the photochemical degradation of hydrocarbons. Furthermore, HCHO in itself acts as a photolytic radical source contributing to ozone formation. Commonly HCHO is measured via its reaction with 2,4-dinitrophenyl hydrazine (DNPH). However, ozone and other pollutants have been shown to interfere with this technique (Kleindienst et al., 1998). For typical ambient levels, DNPH cartridges must be collected for 1 h or more. Spectroscopic methods such as TDLAS, long-path FTIR, and differential optical absorption spectroscopy (DOAS) can provide measurements with both high specificity and fast time resolution. A number of inter-comparison studies (Lawson et al., 1990; Kleindienst et al., 1988; Sirju and Shepson, 1995; Benning and Wahner, 1998) have shown reasonable agreement between these methods, especially if ozone is removed prior to reaction with DNPH. TDLAS was used in the present study because of availability and compatibility with the simultaneous measurement of the other trace pollutants.
Hydrogen peroxide is the product of ambient hydrocarbon degradation in the absence of excess nitric oxide. As a result it forms a sink for HOx radicals. At the same time it also photolyzes to OH radicals, and, therefore, contributes to ozone formation. The most commonly used methods to measure atmospheric hydrogen peroxide use water absorption followed by reaction with a reagent to form a colored adduct (Li and Dasgupta, 2000; Komazaki et al., 2001). Ozone and sulfur dioxide are known to be interferents, and it is likely that other ambient oxidants may interfere as well. TDLAS and long-path FTIR have both shown to provide measurements with high time resolution and specificity. The TDLAS was chosen for its direct measurement capability.
Section snippets
Measurement site
The TDLAS measurement equipment was installed at the University of California Kearney Agricultural Station near Parlier, California, as a part of the air-monitoring component of the Central California Ozone Study (CCOS). The primary objectives of CCOS were to gather an aerometric database for modeling and to apply air models for the attainment demonstration portion of the State Implementation Plan (SIP) for the Federal 8-h and State 1-h ozone standards. To meet this objective additional
Results and discussion
Fig. 2 depicts the data collected during this study. All four species measured via TDLAS show pronounced diurnal profiles. The days shaded in gray were the intensive measurement days when the meteorological conditions were expected to produce episodes of high ozone concentrations. The CCOS management declared four such multi-day periods during the course of the study, comprising a total of 14 days. The non-intensive measurement days were primarily between the second and third intensive periods.
Conclusions
Two tunable diode laser systems were successfully employed for the measurement of four trace gases as part of the CCOS study. Results for non-intensive and three of the four intensive measurement periods are in accordance to what was previously recorded in the literature for similar areas. The data from the last intensive period in September 2000 are contrary to what has been published concerning HNO3, H2O2 and HCHO in similar studies. The buildup of nitric acid and formaldehyde from night to
Acknowledgments
This study was funded by the San Joaquin Valleywide Air Pollution Study Agency through contract 00-6CCOS. The authors would like to thank Chuck Bufalino and Carl Camp for their help during the setup and dismantling of the instruments. Jill Locke and Carl Camp from the San Joaquin Valley Air Pollution Control District in Fresno provided the ozone and weather data; John Bowen from the Desert Research Institute in Reno made the dual converter nitric acid measurements available, and Kurt Bumiller
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