A compact differential optical absorption spectrometer with a 2.5 m base path has been constructed for indoor air measurements. The mirror system can achieve a total pathlength of up to 220 m. A cooled photodiode array is used as a detector with a detection limit equivalent to an absorbance of less than 0.0001 (3 α level with 2 mm averaging). This paper describes the setup of the instrument and explains the data acquisition and analysis procedures used.

A newly constructed DOAS is described. The instrument was specially designed for making automated measurements over diurnal cycles in relatively clean environments, and has been employed since 1989 to measure NO3, NO2, 03 3fld CH2O. A comparative study of NO3 chemistry in two very different environments is presented. The first study took place in the marine boundary layer over Biscayne Bay in South Florida during 1989. Of particular interest was to determine the importance of dimethyl sulfide (DMS) as a sink for NxOr through its rapid reaction with NO3. Modelling the observations indicates that in the clean marine boundary layer, ([O2] 2 ppb), DM5 accounts for up to about 50% removal of NxOr whereas in more polluted conditions, ([NO2] 25 ppb), DMS only removes about 10%. The other significant sink is probably adsorption of N205 onto moist aerosol surfaces. The second study took place near Edison, California, as part of the San Joaquin Valley Air Quality Study. At this dry and polluted location, NO3 exhibits marked variability during the night, although rarely exceeding 80 ppt. The presence of very high levels of industrial and agricultural hydrocarbons appears to limit both the maximum concentration and the duration of significant levels of NO3.

This is an extended abstract of a report on our ongoing development of a field instrument for long-path diode array (LPDA) differential optical absorption spectroscopy (DOAS) measurements of trace tropospheric constituents. We discuss the pros and cons of using intensified detectors and fiber optics, new spectrograph designs and a coaxial telescope arrangement for projection and reception of the analyzing beam.

During July and August of 1990, a differential optical absorption spectrometer (DOAS) made by OPSIS Inc. was used to measure gaseous air pollutants over three separate open paths in Atlanta, GA. Over path 1 (1099 m) and path 2 (1824 m), ozone (03), sulfur dioxide (SO2) nitrogen dioxide (NO2), nitrous acid (HNO2) formaldehyde (HCHO), benzene, toluene, and o-xylene were measured. Nitric oxide (NO) and ammonia (NH3) were monitored over path 3 (143 m). The data quality and data capture depended on the compound being measured and the path over which it was measured. Data quality criteria for each compound were chosen such that the average relative standard deviation would be less than 25%. Data capture ranged from 43% for o-xylene for path 1 to 95% for ozone for path 2. Benzene, toluene, and o-xylene concentrations measured over path 2, which crossed over an interstate highway, were higher than concentrations measured over path 1, implicating emissions from vehicles on the highway as a significant source of these compounds. Federal Reference Method (FRN) instruments were located near the DOAS light receivers and measurements of 03, NO2, and NO were made concurrently with the DOAS. Correlation coefficients greater than 0.85 were obtained between the DOAS and FRM's; however, there was a difference between the mean values obtained by the two methods for 03 and NO. A gas chromatograph for measuring volatile organic compounds was operated next to the FRN's. Correlation coefficients of about 0.66 were obtained between the DOAS and GC measurements of benzene and o- xylene. However, the correlation coefficient between the DOAS and GC measurements of toluene averaged only 0.15 for the two DOAS measurement paths. The lack of correlation and other factors indicate the possibility of a localized source of toluene near the GC. In general, disagreements between the two measurement methods could be caused by atmospheric inhomogeneities or interferences in the DOAS and other methods.

The Differential Optical Absorption Spectroscopy technique (DOAS) is used to determine the concentrations of several atmospheric gaseous components with one single system. The technique, which was developed mainly in West Germany and Sweden, makes use of a broadband light source, e.g. a high pressure xenon lamp or a halogene lamp.19 This light is collimated by a parabolic mirror to a narrow beam, which passes through the atmosphere over a path of several hundred metres to several kilometres. At the end of this absorption path the light is captured again and focused onto the end of an optical fibre. The light is then led through the fibre (the length of which can be up to several tens of metres) to the main part of this measuring systern - the opto-analysis unit. This unit consists of a spectrometer together with electronics for acquiring and processing measurement data. It is also equipped with a PC with the required hard- and software. The necessary reference spectra for both the measured and the interfering components are pre-calibrated and stored in the computer. A number of atmospheric trace gases with absorption lines in the visible and UV are possible to monitor with this system, among them are nitrogen dioxide, sulphur dioxide, ozone, toluene, benzene and formaldehyde. In this paper the principle of the DOAS technique is described. We also discuss the design of the optical system and the evaluation technique. Finally a few results from different applications are presented.

Two rapid scanning (~3000 scan min^-1) differential optical absorption spectrometers were interfaced to 25 m basepath open, multiple reflection systems operated routinely at a total optical path of 800 m at Long Beach and Claremont, CA during the 1987 Southern California Air Quality Study. These instruments permitted measurements of atmospheric concentrations of nitrous acid, nitrogen dioxide and formaldehyde at the part per billion levels, and detection of the nitrate radicals with sensitivity of several tens of parts per trillion.

The hydroxyl radical plays a key role in the photochemistry of the troposphere, and its measurement can form a crucial test of models of that chemistry. However, the OH concentration is often sub-part-per- trillion, so its measurement demands highly sensitive and interference- free techniques. The method of laser-induced fluorescence can meet these requirements, if properly designed to avoid interference due to OH generated by laser photolytic processes. Quantitative laser detection of OH requires knowledge of collisional processes affecting the measurement and these interference effects. Collisions induce quenching of the excited A-state, and vibrational and rotational energy transfer in the excited and ground states. The state-specific nature of these processed is important, particularly the rotational level dependence of quenching and vibrational transfer in excited OH. The temperature dependence is also important: quenching cross sections increase sharply with decreasing temperature. Laboratory experiments investigating these collisional processes are described and their implications for laser detection of tropospheric OH are discussed.

Vacuum-ultraviolet/photofragmentation-laser-induced fluorescence (VUV/PF-LIF) has been demonstrated to be a highly specific and sensitive method for the quantitative measurement of atmospheric ammonia (NH3). The fluorescence detected in this approach results from the two photon (193 nm) photofragmentation of NH3 followed by the LIF excitation of the NH(b1(summation)+) yields NH(c1II) (at 452 nm) and the monitoring of fluorescence from the NH(c1II) yields NH(a1(Delta) ) transition at 325 nm. Limits of detection for the instrument presented here are < 10 pptv and < 4 pptv for one and five minute integration periods, respectively, under ambient sampling conditions. The technique is free from interferences and system performance does not significantly degrade under adverse sampling conditions (i.e. rain, fog, clouds, haze, etc.). Spectroscopic selectivity in the NH(b1(summation)+) yields NH(c1II) transition is sufficient to resolve 15NH3 and 14NH3 contributions for use in atmospheric tracer studies. Average ammonia measurements at Stone Mtn., GA, range from approximately equals 110 pptv for air temperatures < 5 degree(s)C to approximately equals 240 pptv for air temperatures >= 5 degree(s)C over the period from December 1987 to the end of April 1988. Ammonia levels measured at Green Mountain Mesa, Boulder CO, ranged from 10 pptv to 10 ppbv for measurements made during March 1989. Ammonia levels were seen to vary from about 300 pptv to greater than 5 ppbv over time scales of < 10 minutes in this latter data set. These results and future instrument improvements are discussed.

A tandem Raman laser has been constructed using a 1.06 (mu) Nd:YAG laser as the fundamental pump source to generate a backward propagating stimulated Raman (D2 gas) source near 1.5 (mu) (185 mJ/pulse), which in turn is used to generate a second backward propagating stimulated Raman (CH4 gas) source near 2.9 (mu) . This 2.9 (mu) Raman Laser can now produce 18 mJ of near diffraction limited output energy. The anticipated sensitivity of a TP-LIF OH sensor using this mid-IR source would give signal limited (i.e. background free) limits of detection of 1.4 X 105 OH/cm3 under boundary layer conditions, and 5.5 X 104 OH/cm3 under free troposphere conditions for a five minute signal integration period. This level of performance coupled with the techniques nonperturbing nature (i.e. direct measurement) and freedom from both interferences and background would allow reliable tropospheric OH measurements to be obtained under virtually any ambient condition of current interest, including interstitial cloud sampling.

The paper describes recent developments in TDL (tunable lead-salt diode laser) manufacturing technology at Laser Photonics Analytics Division, resulting in higher-temperature operation (greater than 200 K CW) and improved spectroscopic performance and reliability. These factors have led to lower costs and general system simplifications that are necessary for routine monitoring and diagnostic applications. System design considerations are discussed in the light of these developments, and new TDL developments currently under way are presented.

Tunable diode laser absorption spectrometry can provide unequivocal identification of many of the trace gases of atmospheric interest with rapid response and high sensitivity. Instrumentation is described which can measure trace gases with the required sensitivity, specificity and response time dictated by widely differing environments including ambient air, from both mobile laboratories and aircraft, smog chambers and in exhaust gas emissions. Representative examples are discussed of applications to measurements of ambient air under clean and highly polluted conditions, CH4 fluxes from airborne and ground based platforms and combustion gases in diluted and undiluted exhaust from Diesel and automobile engines.

Frequency modulations spectroscopy (FMS) with infrared lasers is an attractive technique for a number of environmental chemical sensing problems. The technique combines high detection sensitivity with high detection speed and, when implemented with tunable infrared laser sources, is capable of detecting numerous chemical species in the atmosphere. To date, the technique has been demonstrated with semiconductor diode lasers and carbon dioxide lasers, and absorptions at the 10-7 level have been detected. We will review the principles and status of FMS for chemical sensing and discuss applications in atmospheric and environmental monitoring.

Absorption spectroscopy with tunable diode lasers (TDLAS) is increasingly being used to monitor atmospheric trace gases down to high pptv-levels. These sensitivities have been achieved with low frequency wavelength modulation (WM). A sensitivity improvement by about two orders of magnitude was demonstrated with a high frequency modulation (FM) technique in experiments with single pass absorption cells. In practical TDLAS instruments however, the achievable sensitivity improvement is limited by several factors. In this paper, limitations due to the use of multipass absorption cells and due to the stability of the instrument are discussed.

A FM-TDLAS-instrument for simultaneous measurement of two atmospheric trace gases has been constructed. The instrument is designed to enable time or frequency multiplex operation after minor setup changes. Noise properties for the frequency modulated setup were investigated. It was found that the noise from both lasers combines as expected by square root addition. Additional noise from strong absorption lines has been observed. Consequences of these findings on the design of multi-species. FM-TDLAS-instruments are discussed.

The ambient fluctuations of long-lived atmospheric gases such as carbon dioxide, methane, and carbonyl sulfide, to name a few, contain important information about sources, sinks and potential secular trends. Since such fluctuations can be quite small, typically less than a few percent, high precision instruments are required. In the present study, we describe a versatile tunable diode laser system for this purpose. This system employs a number of novel features for increased system control and versatility. In addition to high precision, such versatility enables us to acquire the ratio of 2 spectral features with high precision, even when the signal amplitudes are a factor of 26 different and when the spectral features are separated by as much as 0.424 cm-1. This capability can be used to address many important applications in atmospheric studies.

An airborne tunable diode laser instrument is described that is capable of operating in two measurement modes. One mode provides high precision (0.1 percent CH4; 1 percent CO) measurements of CH4 and CO with a 5 second response time, and a second mode achieves the very fast response time that is necessary to make airborne eddy correlation flux measurements. Examples of data from atmospheric expeditions of the Global Tropospheric Experiment are presented.

Biogenic emissions from and dry deposition to terrestrial surfaces are important processes determining the trace gas composition of the atmosphere. We have developed an instrument for flux measurements of gases such as CH4, N2O, and O3 based on the eddy correlation technique which combines trace gas fluctuation measurements with simultaneous windfield measurements. The instrument combines a tunable diode laser infrared light source with an open-path multipass absorption cell in order to provide the fast time response (approximately equals 0.1 s) and short base pathlength (0.6 m) required for the eddy correlation method. Initial field tests using the instrument to measure methane emissions from a local wetland demonstrate the capability for high precision eddy correlation flux measurements.

This paper reviews the influence of the optical design and of various optical components and their properties on the performance and the handling of tunable diode laser spectrometers for high-sensitivity trace gas measurement. The scheme of a novel anastigmatic White cell is introduced.

The NO/O3 chemiluminescence technique provides selectivity and high sensitivity for the measurement of NO in a flowing system. The technique requires the addition of reagent O3, produced by electric discharge in O2, and the detection of photons near 1 micrometers wavelength with a photomultiplier tube. The technique has been used extensively in measurements of reactive nitrogen species in the atmosphere with detection limits reaching as low as a few parts per trillion by volume (pptv) for NO. With a gold catalyst, other oxides of nitrogen can be converted to NO and detected, thereby providing a measurement of the sum of reactive nitrogen species, designated NOy. An instrument that measures NO or NOy on board the NASA high altitude ER-2 aircraft will be described. Missions of this aircraft have addressed stratospheric ozone depletion in both polar regions in recent years. The polar data provides evidence for the condensation of reactive nitrogen species on aerosol particles and for the removal of reactive nitrogen through the sedimentation of aerosol particles. These processes facilitate the destruction of ozone through catalytic cycles involving reactive chlorine species. In the future, similar instruments will address the chemical impact of proposed supersonic aircraft for civilian transport.

Small battery-powered instruments for the sensitive detection of nitrogen dioxide (NO2), and ozone (O3) are described. Because of their portability, they are ideal for studies of indoor air quality, measurements from aircraft, and other mobile platforms where power may be limited. For example, a miniature balloon-borne sonde, based on the chemiluminescence between luminol and NO2 has recently demonstrated the ability to measure ambient mixing ratios of NO2 between ground level and an altitude of 33 km. Recent 'add-on's to the model LMA-3 NO2 analyzer are discussed and the model LOZ-4 ozone monitor is described.

A high performance measurement system based upon NO/O3 chemiluminescence has been developed for simultaneous measurements of the title species using smaller aircraft such as the NCAR Sabreliner. NO is determined directly, NO2 is converted to NO by broad-band photolysis, NO(y) is converted to NO using gold-catalyzed reduction in the presence of CO, and O3 is determined using excess NO as the reagent. The system is capable of measurements of the odd nitrogen species at levels below 10 pptv. A description of the instrument and its application to some flights within a thunderstorm are presented.

A commercial instrument for measuring PAN (peroxyacetyl nitrate), NO2, and optionally, NOx, is described. It is based on a gas chromatograph (GC), and uses a LuminoxR luminol detector following conversion of the PAN to NO2. A detection limit (based on a peak height of 3 X noise) of 30 pptV is achieved for injections made every five minutes. The instrument is computer driven and contains its own peak integrator and graphics output. No interferences have been identified, including chlorinated hydrocarbons, NO2 and ozone. However, the PAN analog peroxypropanyl nitrate (PPN) is not fully resolved from PAN. The maintenance interval is monthly. The calibration of the instruments is relatively easy: known mixing ratios of NO2 in air are used to calibrate the sample injection system and LuminoxR detector. The passage of PAN through the column and the subsequent conversion of PAN to NO2 has been found to be stable over times of one year, and is reproducible between instruments. Atmospheric measurements for PAN have been carried out for both very polluted environments and very clean environments. The results from a month long formal intercomparison between the LuminoxR PAN and two GCs equipped with electron capture detectors is presented. Those results demonstrate that the luminol based PAN detector can yield accurate measurements of ambient PAN concentrations. A method for calibrating PAN instruments in the field is described.

Aqueous fluorescence and chemiluminescence methods have been used to measure hydrogen peroxide in natural waters and in the atmosphere. Ambient hydrogen peroxide and soluble organic peroxide data is presented from the EMEX, MLOPEX and SAGA-3 experimental programs, experiments conducted in the remote marine environment. Methods to measure organic peroxide using conventional collection strategies and direct analysis by chemiluminescence or fluorescence method is approximately two orders of magnitude more sensitive than the fluorescence method. Species specific measurements of organic peroxides are also in development using high pressure liquid chromatography (HPLC) and fluorescence or chemiluminescence detection.

A new Fourier transform infrared (FTIR) spectrometer has been set up at the California Department of Flood and Agriculture for monitoring gas phase pesticides and inert ingredients used in the formulation of pesticides. To allow detection of trace quantities down to the ppm range, the FTIR instrument has been interfaced to a custom, external open multiple reflection system with a base pathlength of 2.5 m. The mirror system is of the double corner cube White type design capable of achieving a total absorption pathlength of 140 m. Our initial application was to monitor the concentration of methyl bromide after controlled releases of this fumigant into an indoor environment.

The use of FTIR spectroscopy, coupled with computer programs for species quantification, has now become reasonably well established as a technique for quantitative analysis of gases and vapors. Classical Least Squares (CLS) fitting procedures allow rapid calculation of species concentrations even when there is sever overlap of spectral features. A major driving force in the development of FTIR-CLS procedures has been in the field of auto exhaust emissions analysis. Other areas where the technique has found application are indoor and outdoor air pollution monitoring, general combustion products analysis, process control, and the analysis of trace species present in nearly pure gases. In almost all applications the analyses can be carried out by technicians not trained in spectroscopy once the species of interest have been identified and analysis methods established. Auto emissions are analyzed either diluted by air (by a factor of 10 to 20) or as raw exhaust. The dilute exhaust gases are typically analyzed at either ambient temperature or 100 C in a 20-meter multiple-pass cell. The raw exhaust is typically analyzed at 185 C in a one-meter cell. Important considerations in the use of FTIR-CLS analysis for auto emissions are cell volume, gas flow rate, measurement time resolution, and spectral resolution. The Minimum Detectable Concentration (MDC) values for species in the exhaust depend on these factors. This paper will discuss these factors using examples from laboratory simulations and actual exhaust measurements. MDC values for a number of species will be given.

Exhaust from vehicles powered by reformulated gasoline and methanol/gasoline blends has been analyzed by FTIR spectroscopy in parallel with conventional techniques. Linear regression analysis of the data showed the following relationships between the FTIR and conventional measurements: methane (0.89X + 0.31, R2 equals 0.96), carbon monoxide (0.82X + 34, R2 equals 0.96), formaldehyde (0.84X + 0.003, R2 equals 0.97), methanol (0.72X + 4.0, R2 equals 0.86), nitric oxide (0.93X + 0.83, R2 equals 0.89), and total non- methane hydrocarbons (1.02X + 2.8, R2 equals 0.96), where the slope, intercept (ppm) and correlation coefficient are shown in parenthesis. With the exception of methanol, good linear correlations are indicated. The apparent non-linearity for methanol is most likely due to the coaddition of interferograms during large concentration transients. Although the validity of FTIR measurements must be assessed on a compound-by-compound basis, the results of this study indicate that valid measurements of motor vehicle exhaust components can be made with non specialized (non-real-time) FTIR instruments.

An improved real-time methane monitor based on infrared absorption of the 3.39 micron line of a HeNe laser is described. Real time in situ measurement of methane has important applications in stratospheric and tropospheric chemistry, especially when high accuracy measurements can be made rapidly, providing fine spatial-scale information. The methane instrument provides 5 ppb resolution in a 1 sec averaging time. A key feature in this instrument is the use of magnetic (Zeeman) broadening to achieve continuous tunability with constant output power over a range of 0.017/cm. The instruments optical absorption path length is 47 m through sampled air held at 50 torr in a multipass cell of the Herriott (off-axis resonator) type. A microprocessor controls laser frequency and amplitude and collects data with minimal operator attention. The instrument recently has been used to measure methane emissions from a variety of natural and artificial terrestrial sources.

Methods to increase the sensitivity of intracavity laser spectroscopy (ILS) have been investigated with application to the measurement of trace gases in the atmosphere. Recent theory has predicted that a longer laser cavity can give an increase in the limiting sensitivity of ILS. Measurements have been performed on a dye laser with two different cavity configurations, with lengths of 54 and 160 cm. No significant difference in the limiting generation time t(g) for these two laser lengths is found. The limiting t(g) of approximately 1000 microsec is achieved at pump powers of 150 to 300 mW, corresponding to 1.5 times the threshold power. When a tuning pellicle is introduced in the cavity, the laser linewidth is halved, the threshold power increases to 220-400 mW and the limiting t(g) decreases to approximately 400 microsec. Other effects at long generation times that will influence the use of ILS for atmospheric measures have been investigated. The spectral noise increases strongly near the limiting t(g), and the spectrum may show structure due to weak interferences in the laser cavity.

A method for inhomogeneous, molecular gas integrated absorption calculating is developed by using partial absorption, that depend on mean absorption coefficients K_n obtained in analytical form. These coefficients are evaluated numerically or analytically for the actual frequency dependence of the spectral absorption coefficients. A procedure for determination of mean coefficient for gas mixture is discussed.

Tunable Diode Laser Spectroscopy (TDLAS) of oil derived SO2 in automotive exhaust demonstrated acceptable repeatability in determination of oil consumption at steady state engine operating conditions. The response time of the instrument was approximately 30 sec, the time related to the flow rate of the sampling system. Instrument sensitivity is sufficient to measure SO2 levels of 0.1 to 1 ppm required to the oil consumption determination. Typical exhaust gas species were investigated for their interference effects and were observed to have less than a 10% interference on the SO2 signal for mixing ratios with SO2 typical of automotive exhaust. Water, on the other hand, did show a significant, but compensatible interference. Carbon deposition under rich engine conditions was observed and is expected to be a problem for any analytical device and is best solved by using a heated sampling line.

A Fourier transform infrared remote sensor (FTIR-RS) has been used at a Superfund site to make measurements of the concentrations of various gases. An attempt was made to measure benzaldehyde and benzonitrile along with methane and carbon monoxide. The results of these measurements are discussed. We also attempted to compare the FTIR-RS results with those from canister samples, and this comparison is also discussed.

FTIR spectroscopy has been shown to be a valuable tool in the analysis of complex gas phase mixtures, such a dilute vehicle exhaust. Regulated and non-regulated vehicle emissions have been routinely sampled and analyzed using prototype instrumentation developed in this laboratory, and in several other laboratories over the last decade. More recently, commercial versions of these FTIR analyzers have become available through several manufacturers. This paper reviews the data acquisition and processing techniques utilized by the FTIR analyzer developed in this laboratory. The statistical detection limits for 22 of the components analyzed by the system are presented. In addition, the linearity of the carbon monoxide (CO) analysis is demonstrated over several orders of magnitude. Experiments designed to study the effects of environmental parameters on the accuracy and the sensitivity of the system are also described.

Air emissions from Superfund sites can potentially have a significant impact on air quality affecting the health and safety of surrounding populations. Although the air pathway is well understood, these air emissions may be difficult to characterize. Remote sensors or open path monitors (OPMs) can have useful applications in measuring site emissions of air toxic compounds during the Superfund process. The air monitoring needs for each step of the Superfund process are identified in this paper and OPMs are compared and contrasted with traditional point monitoring techniques. Example applications of OPMs to Superfund activities are presented. Desirable improvements in OPM technology are also discussed.

We report measurements of atmospheric peroxy radicals which were made at two rural sites in the eastern United States using the technique of chemical amplification followed by luminol chemiluminescence detection. Several improvements which have been made to the instrumental systems since earlier reports are discussed. Also reported is a new field calibration procedure which allows a critical evaluation of the instrumental stability and provides directions for design changes to improve its performance.

Paper deals with diode lasers application in high resolution molecular spectroscopy and atmosphere pollutants monitoring. Results of spectral line parameters measuring (frequency, intensity, pressure broadening and shift) are presented. Diagnostic systems based on Diode Lasers and IR fibers have been developed. Mentioned above systems' use in atmosphere monitoring and some additional applications are considered.