Tungstic acid surfaces chemisorb NH, and HNOa. Coilectlon of these gases is posslbie by sampling with a hollow tube, the interior of which is coated wlth tungstic acid. Under conditions of laminar flow, the gases diffuse to the wail and are chemisorbed while particles are carried through the tube. Particles are collected with an in-line tube packed with tungstic acid coated sand. Thermal desorption followed by a simple separation and detection by a chemiluminescent NO, analyzer permits analyses at the parts per bllllon and lower concentration range for gaseous and particulate forms of the analytes. Detection limits are 1-3 nglsampie. The precision of analyses of ambient air samples is in the 5-10% relative standard deviation range. Coated hollow tubes conform sufficiently to the mathematical model of tube gas dynamics so as to permit reasonable determination of gaseous diffusion coefficients at ambient concentrations.Ammonia, nitric acid, and sulfuric acid aerosol are important factors influencing the acidity of air and acidity of rain. Ammonium nitrate and ammonium sulfate are solid compounds resulting from atmospheric acid-base reactions of the three acid rain components. Thus, both gaseous and particulate forms of ammonia and nitric acid are expected to be present in air.The determination of gaseous ammonia and nitric acid at the trace concentrations (0.1-5.0 pg/m3) present in comparatively nonpolluted, ambient air as well as discrimination of gaseous from particulate forms is a difficult task. A number of workers have noted particle to gas or gas to particle conversion reactions on filters used to separate particles from the gas phase. A discussion of this aspect of previous work is given in our companion article (I).Analytical procedures for gaseous ammonia have involved absorption on an oxalic acid treated surface as in the ring oven technique of Shendrikar and Lodge (2) modified by Cattell and DuCross (3). Gillett and Ayers (4) report the detection limit of this method to be near 0.05 pg/sample in routine use. Ferm (5) has used an oxalic acid coated diffusion tube to collect NH3 later determined by ion-selective electrode. McClenny and Bennett (6) have reported a method for gaseous ammonia based upon absorption on Teflon beads after filtration to remove particulate ammonia. Collected gaseous ammonia was desorbed by heating and detected by using a photoacoustic detector. The detection limit was approximately 5 ng/sample.Gaseous nitric acid has been collected on sodium chloride impregnated filters after removal of particles by a nontreated prefilter (7,8). Total gaseous and particulate nitrates can be collected on filter combinations such as, for example, a quartz fiber filter and a nylon filter (9). Shaw et al. (IO) developed a method for nitric acid in air employing two parallel sampling lines, one of which includes a diffusion denuder tube to eliminate "OB.Both total and particulate HN03 are determined and the difference is gaseousThe work reported here started with a preliminary study of new approaches t...

First, we briefly review the atmospheric chemistry and previous intercomparison measurements for HCHO, with special reference to the diffusion scrubber Hantzsch reaction based fluorescence instrument used in the field studies reported herein. Then we discuss summertime HCHO levels in five major U.S. cities measured over 1999-2002, primarily from ground-based measurements. Land-sea breeze circulations play a major role in observed concentrations in coastal cities. Very high HCHO peak mixing ratios were observed in Houston (>47 ppb) where the overall median mixing ratio was 3.3 ppb; the corresponding values in Atlanta were approximately >18 and 7.9 ppb, respectively. The peak and median mixing ratios (9.3 and 2.3 ppb) were the lowest for Tampa, where the land-sea breeze also played an important role. In several cities, replicate HCHO measurements were made by direct spectroscopic instruments; the instruments were located kilometers from each other and addressed very different heights (e.g., 106 vs 10 m). Even under these conditions, there was remarkable qualitative and often quantitative agreement between the different instruments, when they were all sampling the same air mass within a short period of each other. Local chemistry dominates how HCHO is formed and dissipated. The high concentrations in Houston resulted from emissions near the ship channel; the same formaldehyde plume was measured at two sites and clearly ranged over tens of kilometers. Local micrometeorology is another factor. HCHO patterns measured at a high-rise site in downtown Nashville were very much in synchrony with other ground sites 12 km away until July 4 celebrations whence HCHO concentrations at the downtown site remained elevated for several days and nights. The formation and dissipation of HCHO in the different cities are discussed in terms of other concurrently measured species and meteorological vectors. The vertical profiles of HCHO in and around Tampa under several different atmospheric conditions are presented. The extensive data set represented in this paper underscores that urban HCHO measurements can now be made easily; the agreement between disparate instruments (that are independently calibrated or rely on the absolute absorption cross section) further indicates that such measurements can be done reliably and accurately for this very important atmospheric species. The data set presented here can be used as a benchmark for future measurements if the use of formaldehyde precursors such as methanol or methyl tert-butyl ether (MTBE) as oxygenated fuel additives increases in the future.

Diffusive sampling of a mixture of 42 volatile organic compounds (VOCs) in humidified, purified air onto the solid adsorbent Carbopack X was evaluated under controlled laboratory conditions. The evaluation included variations in sample air temperature, relative humidity and ozone concentration. Linearity of samples with loading was examined both for a constant concentration with time varied up to 24 h and for different concentrations over 24 h. Reverse diffusion and its increase with accumulation of sample were determined for all compounds. Tubes were examined for blank levels, change of blanks with storage time, and variability of blanks. Method detection limits were determined based on seven replicate samples. Based on this evaluation, 27 VOCs were selected for quantitative monitoring in the concentration range from approximately 0.1 to 4 ppbv. Comparison results of active and diffusive samples taken over 24 h and under the same simulated ambient conditions at a constant 2 ppbv were interpreted to estimate the effective diffusive sampling rates (ml min(-1)) and their uncertainties and to calculate the corresponding diffusive uptake rates (ng ppmv(-1) min(-1)).

The capping of stationary source emissions of NO x in 22 states and the District of Columbia is federally mandated by the NO x SIP Call legislation with the intended purpose of reducing downwind O 3 concentrations. Monitors for NO, NO 2 , and the reactive oxides of nitrogen into which these two compounds are converted will record data to evaluate air quality model (AQM) predictions. Guidelines for testing these models indicate the need for semicontinuous measurements as close to real time as possible but no less frequently than once per hour. The measurement uncertainty required for AQM testing must be less than ±20% (±10% for NO 2 ) at mixing ratios of 1 ppbv and higher for NO, individual NO z component compounds, and NO y . This article is a review and discussion of different monitoring methods, some currently used in research and others used for routine monitoring. The performance of these methods is compared with the monitoring guidelines. Recommendations for advancing speciated and total NO y monitoring technology and a listing of demonstrated monitoring approaches are also presented.

This paper evaluates the accuracy of ozone measurements made by monitors that determine ozone concentrations in ambient air by UV absorption. These monitors are typically used to measure ozone for the purpose of establishing local compliance to air-quality standards. The study was predicated by the concern that commercially available UV absorbance O 3 monitors may be subject to interference from volatile organic carbon (VOC) species that absorb light at 254 nm. To test for these and other effects, we compared simultaneous O 3 measurements made by a commercial UV O 3 monitor with an O 3 -NO chemiluminescence instrument, which is not subject to interference by VOC compounds. The comparisons were carried out in the summers of 1999 and 2000 at urban/industrial sites in Nashville and Houston, and in 2004 aboard a ship in the Gulf of Maine. In the two urban areas, we also compared the O 3 measurements from these two methods with O 3 measurements made by a long-path differential optical absorption spectrometer (DOAS). Our tests indicate that, with wellmaintained monitors, there are no significant interferences even in areas with significant ambient concentrations of potentially interfering VOCs.

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An automated gas chromatographic (autoGC) system was used to collect and analyze both nonpolar and polar volatile organic compounds (VOCs) in ambient air. This system combines the use of dual multiadsorbent traps to provide continuous air sampling for 57 min of each hour; a dry helium purge to remove extraneous gases, including some residual water vapor retained in the sorbent packing; thermal desorption of analytes onto a VOC-focusing trap cooled by a small Stirling-cycle refrigerator; and GC/mass spectrometric detection using ion trap technology. Cleanliness, linearity, method detection limits (MDLs), precision, and accuracy of the autoGC were determined for 41 VOCs. For most of the compounds tested, MDLs were less than 0.10 ppbv, response was linear over the 1-40 ppbv range, accuracy was (20%, and trap-to-trap precision was (20%. Linear response for a set of polar VOCs was also determined over the 5-50 ppbv range. The autoGC was successfully operated in a mobile laboratory at a field site in Axis, AL, for 10 days, during which time the system was in operation 24 h/day with minimal interruptions. This autoGC is designed for monitoring subsets of the 97 VOCs among the 189 hazardous organic compounds that are listed in Title III of the Clean Air Act Amendments of 1990.

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