Environmental Problems in Search of Analytical Solutions
A
tmospheric chemists are looking for a few good analytical chemists. During a two-day symposium at the recent ACS meeting in Boston these environmental researchers de tailed measurement problems for which analytical chemists might pro vide solutions. Unfortunately, the sym posium fell at the end of a very busy convention week. For those analytical chemists who headed home early, here is some of what you missed. One of the most difficult species at mospheric chemists measure is HNO3. Merely collecting samples of the acid poses a problem. The acid adheres to many surfaces, including most Teflons. "It is so sticky that we lose it in the plumbing," explained Barry Huebert from the University of Rhode Island's Graduate School of Oceanography. Furthermore, it is difficult to discrimi nate between the various forms of the acid. Depending on conditions, the acid could exist as H N 0 3 , Η Ν 0 3 · Η 2 0 , or NH4NO3. "Nitric acid is the final member of a chain of odd nitrogen compounds," said Huebert. The chain begins with roughly equal amounts of natural and anthropogenic NO, which is converted by O3 to NO2, and that in turn reacts over hours or days with OH· to generate HNO3. In urban areas HNO3 concen trations can reach 40 ppbv, but the con
centration of the free acid is often lim ited by reaction with ambient NH3 to form NH4NO3 (Kd at 20 °C = 20 ppbv 2 ). In rural areas values drop to 5 200 pptrv, although high concentra tions of NH3 from fertilizers can also generate the ammonium salt. IR spectroscopy is one method for determining the acid. Using the sun as a light source, IR detectors on orbiting satellites have measured HNO3 in the stratosphere. FT-IR measurements in the lower atmosphere have suffered from low sensitivity (~5 ppbv) and "sticky" losses on cell walls. On the
FOCUS other hand, detection by tunable dye laser absorption spectroscopy offers a sensitivity of 50-150 pptrv and, by low ering the pressure, lines sharp enough to differentiate hydrated HNO3 from free HNO3. However, by comparison, the tech nique of choice for many situations is simple and inexpensive. Air samples are funneled through two filters ar ranged in series. The first filter, either Teflon or acid-washed quartz, collects NO3 aerosols. That leaves the second filter—a Whatman 41 filter impregnat ed with NaCl/glycerol or tetrabutyl-
ammonium hydroxide, or a nylon fil ter—to quantitatively adsorb HNO3 vapor. Thus values from the filters sep arately determine N O j aerosol and va por HNO3 levels. Analytes are washed from the filters with either water or an ion chromatog raphy eluant. According to Huebert, these filter packs can boast a precision of < 1%, but the accuracy is affected by a host of artifacts. For instance, the ammonium salt may dissociate to NH3 and HNO3, affecting measurements of NH4NO3. Therefore, NH4NO3 sam pling times must be kept short and heating of the sample must be avoided, or values for the different acid phases can be skewed. In addition, alkaline sea spray or dust, caught on the first filter, may scavenge HNO3 vapor. To better discriminate between HNO3 in the aerosol and vapor phases, researchers add a coated tube called a diffusion dénuder ahead of the filter pack. As the air flows through the diffusion dénuder, an alkaline coating on the inside of the tube captures the HNO3 vapor, allowing only the aerosols to pass through to the filter pack. Analysis provides an aerosol-only value. The difference between the dénuder tube filter pack value and the total HNO3 concentration (i.e., the sum of both filters) determined with a simple filter pack yields the vapor concentration.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990 · 847 A
FOCUS The dénuder tube's trapping efficiency depends on a laminar airflow and is related to DL/Q, where D is the diffusion rate of analyte, L is the length, and Q is the flow rate. Diameter is not a factor, but short dénuder tubes are limited to low flow rates. A double dénuder wall design circumvents that problem, allowing flow rates up to 30 L/min. Because nitric acid is difficult to handle, calibrations of these different approaches with standards may not be accurate. Worse yet, when intercomparisons of these techniques are performed there is little agreement in values—especially at pptrv concentrations. Taking stock of all the problems, Huebert appealed to analytical chemists for new techniques with a precision of 5-10% at 5-pptrv levels; better specificity; and, for monitoring fluxes, a speed of 1-10 Hz. The nitrogen family is lumped together as NO x (NO + N0 2 ) and NO y (all oxidized nitrogen species except N2O). Total NO y currently is measured by running air samples over a catalyst, such as Au or Mo, at elevated temperatures. The catalyst reduces the NO y constituents to NO, which is then quantified by its chemiluminescent reaction with O3. Alternatively, each of the constituents—including NO, N 0 2 , C3-C5 alkyl nitrates, HNO3, PAN [peroxyacetylnitrate, CH 3 C(0)0 2 N02], nitrate particulates, and so on—are determined independently and summed together to give NOy. A variety of techniques are employed, such as the filter packs for HNO3 and GC for PAN. Ideally, the results should be the same in either approach and provide an intercomparison. "At polluted sites we are independently measuring almost all the nitrogen species," explained David Parrish of the National Oceanic and Atmospheric Administration's Aeronomy Laboratory in Boulder, CO. However, in "clean" air the sum of individual nitrogen constituents is less than the NOy value. Parrish suspects that the missing species are longer chain alkyl nitrates and alkyl nitrates containing other organic functional groups—materials currently lacking reliable methods of field analysis. Sophisticated spectroscopic techniques have also been employed for measuring individual nitrogen species. For example, ammonia is detectable by a two-photon-induced fluorescence. A vacuum-UV laser initially photodissociates the molecule to metastable excited N H and then a second laser induces fluorescence. Similar multiphoton approaches have been applied to NO,
N 0 2 , and HNO2. However, each requires a "trailer-full" of instruments. Ideally, researchers would like to detect each nitrogen species, measuring the various constituents simultaneously. That, says Parrish, will require a suite of instruments. Another group of molecules that atmospheric chemists struggle to measure are free radicals that play key roles in the chemistry of the atmosphere. For instance, Robert O'Brien, of the Department of Chemistry at Portland State University, calls the simple hydroxyl radical "nature's policeman" for scrubbing trace gas pollutants from the lower atmosphere. For that reason, explains O'Brien, OH· lasts for seconds in clean air but for just milliseconds in polluted air. Several reactions generate OH·, such as the photolysis of peroxide, nitrous acid, or aldehydes; the reaction of singlet oxygen with water; and the transfer of an oxygen from HO2· to NO. Concentrations of OH· fall in the part-perquadrillion range (106 molecules/cc). Fortunately, the radical has a large extinction coefficient and cross section, allowing laser fluorescence detection. Excitation at 282 nm with a Nd-YAG laser gives rise to a 309-nm fluorescence. However, the laser technique suffers from numerous interferences: spurious OH· from laser photolysis of ambient ozone, light losses from scattering, broadband fluorescence, and rapid OH· quenching. The overall effect is to drop photon recovery efficiencies to around 10~16 relative to laser output. O'Brien's group has developed a new laser fluorescence system that eliminates many of the interferences. This system excites OH· at 308 nm using a Cu vapor laser with a high repetition rate (5.6 kHz) but less energy per pulse than the Nd-YAG laser. In addition, OH· is detected in a cell at a reduced pressure of ~ 4 Torr. Spurious OH- formation, saturation of the detector's photomultiplier tube, broadband fluorescence, and stray light illumination of cell walls are greatly reduced. As a result, signal-averaging times have dropped from 1 h to just 4 min. One product of OH· reactions in the atmosphere is peroxy radicals (ROO·). Like OH·, peroxy radicals form mainly from a complex assortment of photolytic reactions. Thus, explains Chris Cantrell from the National Center for Atmospheric Research (Boulder, CO), ROO· levels lie in the part-per-trillion range during the day, but at night concentrations drop dramatically. In the laboratory peroxy radicals are detected in the IR with a tunable diode laser or by laser magnetic resonance,
848 A · ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
which looks at vibrational levels in the mid- to the far-IR that are split by a magnetic field. Detectors on research balloons measure peroxy radicals in the upper atmosphere by their spectroscopic signature in the far-IR (100 cm - 1 ) against the emission of the daytime atmosphere. One of the better methods for measuring these radicals is electron paramagnetic resonance (EPR), which can differentiate some of the peroxy species. However, EPR's sensitivity limit of 2-4 pptr is barely sufficient for detection. Thus air samples are frozen and preconcentrated prior to analysis. Samples are also collected using spin traps, but these trap molecules generally are photochemically unstable. O'Brien and Thomas Hard, also at Portland State University, have developed a new method of analysis based on the multistep chemical conversion of ROO· to hydroxyl radical in the presence of NO (e.g., reaction 1). Laser-induced fluorescence then measures OHwith a detection limit of < 0.1 pptr. A similar analysis scheme, now being developed by Cantrell, measures NO2 as a marker for OH·. Prior to detection, the concentration of N 0 2 is magnified via a chain reaction (reactions 1-3). The trick of this chemical amplification reaction is to accurately determine the number of cycles before termination. H0 2 - + NO — OH· + N 0 2
(1)
OH· + CO — H· + C 0 2
(2)
H· + 0 2 + M — Η 0 2 · + Μ
(3)
All these techniques fail to characterize the individual peroxy radicals, infor mation that could benefit atmospheric research. Not all the problems were related to field measurements. Richard Flagan, of California Institute of Technology's Environmental Engineering Science Department, is investigating aerosol nucleation in a laboratory smog cham ber. In particular he has been looking at the reaction of dimethyl sulfide with NO, N 0 2 , and O3, which generates sul furic acid among other products. Flagan's group follows the nucle ation process by monitoring the grow ing size of the aerosols. The charged aerosol particles are electrically sepa rated by mobility, resolving sizes in the range 5-200 nm. In 15 min the aerosols in these experiments fully nucleate. "However," said Flagan, "we don't know the chemistry." He would like to couple his size detector to analytical techniques such as ICP, AES, FT-IR, and MS; measure equilibrium partial pressures of the reactants; determine
particle surface tension; and extend his studies to particles > 200 nm, where aerosols may affect human health and atmospheric visibility. Finally, because nucleation occurs quickly, he requires sampling rates of < 1 min. Time response becomes especially important for researchers collecting atmospheric data from airplanes. "The slowest plane moves at 50 m/s," said Peter Daum of Brookhaven National Laboratory's Environmental Chemistry Division, "and speeds can be over 200 m/s." Thus spatial averages for a single measurement can run on the order of kilometers. A nitric acid filter pack, for instance, requires about 1 h of flying time to collect a sufficient sample, whereas many trace gas instruments have time responses on the order of 1060 s. However, a typical smokestack plume measures 0.5 km, and clouds can have diameters of < 1 km. Obtaining the required spatial resolution requires rapid and sensitive analysis. "The first law of airplane sampling," said Daum, "is that if the sensitivity is high enough then the response time is too slow, and vice versa. The second law is that if there is the desired response and sensitivity, then
it is too big or requires too much power." Most ofthe airborne analytical devices are commercial instruments that have been modified for flight. Air samples are composed of several phases—aerosols, gases, and droplets—that need to be distinguished. One method of separating phases is to pass the sample air through a conventional cyclone wherein the heavier particles are thrown to the sides and drop to the bottom. Alternatively, the axial flow separator spins the air sample via a static vane assembly, throwing particles to the sides of the sampling tube where they may be drawn off. For both these devices adiabatic heating and subsequent evaporation of water droplets is of concern. A newer, more sophisticated collector—the counterflow impactor—has been developed to obtain size-distributed cloud droplet composition. An inlet nozzle lined with a sintered metal tube sticks out of the aircraft. A counterflow of air is fed through this tube at a controlled rate so as to determine the size particles that will pass through the inlet. As the counterflow velocity increases, the smaller particles are excluded from the inlet and only the larger particles are sampled.
The trickiest sample to collect by plane is snow. Usually, says Daum, a cyclone collector is employed. However, placement of the cyclone is critical because snowflakes typically have a low density and tend to follow the flow stream around the plane, bypassing the collector. These sample inlets require careful attention, and a history of their activity must be maintained. Once again, sticky molecules such as HNO3, H2O2, and NH3 cause problems. Given the recent political changes on the international scene that have brought a new awareness of the environment, Daum sees the need for air sampling at more and increasingly remote sites. "We are going to need the techniques to measure lower concentrations from airplanes," said Daum. The symposium was organized by Leonard Newman of Brookhaven National Laboratory's Environmental Chemistry Division and will be published in the ACS Advances in Chemistry series under the title Measurement Challenges in Atmospheric Chemistry. Newman is looking for additional contributions to the volume and invites interested researchers in this field to contact him. Alan R. Newman
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