Laser-induced Fluorescence Instrument Research Articles

Development of an ambient pressure laser-induced fluorescence instrument for nitrogen dioxide

Concerns about the health effects of nitrogen dioxide (NO2) and its role in forming deleterious atmospheric species have made it desirable to have low-cost, sensitive ambient measurements of NO2. We have developed a continuous-wave laser-diode laser-induced fluorescence (LIF) system for NO2 that operates at ambient pressure, thereby eliminating the need for an expensive pumping system. The current prototype system has achieved sensitivity several orders of magnitude beyond previous efforts at ambient pressure (limit of detection of 2 ppb, 60 s averaging time). Ambient measurements of NO2 were made in Portland, Oregon using both the standard NO2 chemiluminescence method and the LIF instrument and showed good agreement (r2 = 0.92).

Measurements of OH and HO<sub>2</sub> concentrations during the MCMA-2006 field campaign – Part 1: Deployment of the Indiana University laser-induced fluorescence instrument

Abstract. Measurements of tropospheric hydroxyl (OH) and hydroperoxy (HO2) radicals were made during the MCMA (Mexico City Metropolitan Area) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These radicals were measured using a laser-induced fluorescence instrument developed at Indiana University. This new instrument takes advantage of the Fluorescence Assay by Gas Expansion technique (FAGE) together with direct excitation and detection of OH at 308 nm. HO2 is indirectly measured as OH by titration with NO inside the fluorescence cell. At this stage of development, IU-FAGE is capable of detecting 3.9×105 molecule/cm3 of both OH and HO2, with a signal to noise ratio of 1, an averaged laser power of 10-mW and an averaging time of 5-min. The calibration accuracies (1σ) are ±17% for OH and ±18% for HO2 using the water-vapor photolysis/O2 actinometry calibration technique. OH and HO2 concentrations were successfully measured at an urban site in Mexico City, with observed concentrations comparable to those measured in other polluted environments. Enhanced levels of OH and HO2 radicals were observed on several days between 09:30–11:00 a.m. and suggest an intense photochemistry during morning hours that may be due to elevated sources of HOx (OH+HO2) and a fast cycling between the radicals under the high NOx (NO+NO2) conditions of the MCMA.

Laser induced fluorescence instrument for NO 2 measurements: Observations at a central Italy background site

A laser induced fluorescence (LIF) instrument has been developed to measure tropospheric NO 2 with low detection limit. The instrument design, development and first measurements are reported. There are also details of the temporal gate system built for the fluorescence acquisition. The instrument is able to make fast measurements (up to 4 Hz) and shows a limit of detection of 10 pptv/60 s. Continuous observations (2 weeks in summer 2007) in a small town in central Italy were used to test the performance of the instrument and to study the photochemistry of ozone in a background site. LIF and a commercial chemiluminescence (CL) instrument simultaneous observations of NO 2 show a good linearity (LIF = 1.02 CL + 0.6 (ppb), R 2 = 0.98) but there is a bias of the commercial instrument of about 0.60 ppbv on average. The overestimation of the CL system is probably due to conversion of NO y species into NO by the molybdenum converter used in the CL instrument to detect NO 2. Analysis of 1 s data is used to test the instrument response and the coupling between nitrogen oxides and ozone.

Measurement of tropospheric RO2 and HO2 radicals by a laser-induced fluorescence instrument

A new method (ROxLIF) for the measurement of atmospheric peroxy radicals (HO(2) and RO(2)) was developed using a two-step chemical conversion scheme and laser-induced fluorescence (LIF) for radical detection. Ambient air is sampled into a differentially pumped flow reactor, in which atmospheric RO(x) radicals (=RO(2)+RO+HO(2)+OH) are chemically converted to HO(2) by a large excess of NO and CO at reduced pressures (ROx mode). When only CO is added as a reagent, the sum of atmospheric HO(2)+OH is converted to HO(2) (HOx mode). At the reactor outlet, part of the air flow is transferred into a low-pressure detection chamber, where the HO(2) is further converted by reaction with NO to OH, which is then detected with high sensitivity by LIF at 308 nm. The ROxLIF technique has been implemented in an existing LIF instrument that is also capable of measuring atmospheric OH. From the concurrent measurements of RO(x), HO(x) and OH, concentrations of HO(2) and RO(2) can be determined. The system is calibrated using the quantitative photolysis of water vapor at 185 nm as a radical source. Addition of CO or hydrocarbons to the calibration gas yields well-defined concentrations of HO(2) or RO(2), respectively, providing an estimated accuracy for the calibration of about 20%. The ROxLIF technique is extremely sensitive and has detection limits (signal-to-noise ratio=2) of about 0.1 pptv of HO(2) or RO(2) at a time resolution of 1 min. The paper describes the technique and its calibration, discusses the chemistry in the conversion reactor and possible interferences, and gives an example of ambient air measurements to demonstrate the performance of the new technique.

A new photolysis laser-induced fluorescence instrument for the detection of H2O and HDO in the lower stratosphere

We present a new instrument, Hoxotope, for the in situ measurement of H(2)O and its heavy deuterium isotopologue (HDO) in the upper troposphere and lower stratosphere aboard the NASA WB-57. Sensitive measurements of deltaD are accomplished through the vacuum UV photolysis of water followed by laser-induced fluorescence detection of the resultant OH and OD photofragments. The photolysis laser-induced fluorescence technique can obtain S/N>20 for 1 ppbv HDO and S/N>30 for 5 ppmv H(2)O for 10 s data, providing the sensitivity required for deltaD measurements in the tropopause region. The technique responds rapidly to changing water concentrations due to its inherently small sampling volume, augmented by steps taken to minimize water uptake on instrument plumbing. Data from the summer 2005 Aura Validation Experiment Water Isotope Intercomparison Flights (AVE-WIIF) out of Houston, TX show agreement for H(2)O between Hoxotope and the Harvard water vapor instrument and for HDO between Hoxotope and the Harvard ICOS water isotope instrument, to within stated instrument uncertainties. The successful intercomparison validates Hoxotope as a credible source of deltaD data in the upper troposphere and lower stratosphere.

Measurement of IO radical concentrations in the marine boundary layer using a cavity ring-down spectrometer

An open-path cavity ring-down spectroscopy (CRDS) instrument for measurement of atmospheric iodine monoxide (IO) radicals has been tested in the laboratory and subsequently deployed in Roscoff on the north-west coast of France as part of the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) project in September 2006. In situ measurements are reported of local IO mixing ratios in the marine boundary layer. To obtain these mixing ratios, accurate absorption cross sections of IO are required at the selected wavelengths used for spectroscopic measurements. Absorption cross sections at the bandheads of the IO A2Π3/2–X2Π3/2 (3,0) and (2,0) vibronic bands were thus verified by a combination of spectral simulation methods, inter-comparison of prior determinations of cross-sections at high and low spectral resolution, and by measurement of rates of loss of IO by its self-reaction. The performance of the open-path CRDS instrument was tested by measuring concentrations of NO2 in ambient air, both within and outside the laboratory, with results that were in excellent agreement with a previously validated continuous wave CRDS apparatus for NO2 detection. During the RHaMBLe campaign, the open-path CRDS instrument was located within a few metres of the shoreline and operated at wavelengths close to 435 nm to detect the absorption of light by trace levels of IO. The IO mixing ratios were obtained on two days, peaked close to low tide, and were approximately 5–10 times higher than values calculated from column densities previously reported by long-path, differential optical absorption spectroscopy (DOAS) in coastal regions. The typical detection limit of the instrument was estimated to be 10 pptv of IO, with some fluctuation around this value depending on the conditions of wind and atmospheric aerosol particles, and the total accumulation time was 30 s for each data point. The observations of relatively high concentration of IO, compared to the values previously reported by DOAS, are consistent with the concurrent observations using a LIF (Laser induced Fluorescence) instrument (Whalley et al. in press). The first such measurements of localized IO by CRDS and LIF should contribute to an improved understanding of the chemistry of halogen compounds and the formation of iodine oxide aerosol particles in the marine boundary layer.

Gating a channel photomultiplier with a fast high-voltage switch: reduction of afterpulse rates in a laser-induced fluorescence instrument for measurement of atmospheric OH radical concentrations

By employing a commercially available high-voltage switch in a time-gating circuit to drive a channel photomultiplier (CPM), the afterpulse rates are significantly reduced in the time window to collect fluorescence >200 ns after the pulsed laser excitation. The CPM, kept deactivated under normal conditions (normally off), is turned on immediately after the passage of the laser pulse by shifting the voltage applied to the photocathode by 150 V to collect the fluorescence. When the detection system is used as part of a laser-induced fluorescence instrument to measure atmospheric OH radicals with the photon-counting method, the background signal is reduced by more than a factor of 10 as compared with our previous case where a conventional dynode-gated photomultiplier tube (PMT) is used, while the sensitivity toward the fluorescence is almost unchanged. A detection limit as low as 2 x 10(5) radicals cm-3 or 0.008 parts per trillion by volume is achieved for OH, with an integration time of 1 min and a signal-to-noise ratio of 2, enabling sensitive detection of the important radical in the atmosphere. This system is a superior choice with higher sensitivity and cost effectiveness as compared with the gated PMITs utilizing a microchannel plate as an electron multiplier, and could also be used effectively in light detection and ranging (lidar) instruments, where a delayed scattering signal would be efficiently discriminated from afterpulses.

Rotationally Resolved Absorption Cross Sections of Formaldehyde in the 28100−28500 cm-1 (351−356 nm) Spectral Region: Implications for in Situ LIF Measurements

The rotationally resolved ultraviolet absorption cross sections for the 2(0)(0)4(1)(0) vibrational band of the A(1)A(2)-X(1)A(1) electronic transition of formaldehyde (HCHO) at an apodized resolution of 0.027 cm(-1) (approximately 0.0003 nm at 352 nm) over the spectral range 28100-28500 cm(-1) (351-356 nm) at 298 and 220 K, using Fourier transform spectroscopy, are first reported here. Accurate rotationally resolved cross sections are important for the development of in situ HCHO laser-induced fluorescence (LIF) instruments and for atmospheric monitoring. Pressure dependence of the cross sections between 75 and 400 Torr at 298 K was explored, and an average pressure broadening coefficient in dry air of 1.8 x 10(-4) cm(-1) Torr(-1) for several isolated lines is reported. Gaseous HCHO was quantitatively introduced into a flow cell by evaporating micron-sized droplets of HCHO solution, using a novel microinjector technique. The condensed-phase concentrations of HCHO were determined by iodometric titrations to an accuracy of <1%. Accuracy of the measured absorption cross sections is estimated to be better than +/-5%. Integrated and differential cross sections over the entire band at low resolution (approximately 1 cm(-1)) obtained with our calibration technique are in excellent agreement with previous measurements. A maximum differential cross section of 5.7 x 10(-19) cm(2) molecule(-1) was observed at high resolution-almost an order of magnitude greater than any previously reported data at low resolution.

In situ measurement of N 2O 5 in the urban atmosphere by thermal decomposition/laser-induced fluorescence technique

The laser-induced fluorescence (LIF) detector with a thermal converter has been developed for measuring atmospheric N 2O 5. The detection limit for N 2O 5 was 11 pptv for 10-min averaging ( S / N = 2 , [NO 2]=0). The field measurements of N 2O 5 were conducted in the urban atmosphere in winter. N 2O 5 was successfully monitored during four nights. Typically, observed N 2O 5 level was in the range of 0–200 pptv. Note that N 2O 5 reached 800 pptv at one night, when NO x level was extremely high and the temperature was low. After the data were selected by the stability of NO x , N 2O 5 chemistry was discussed for a representative case of the urban night. Observed trend of N 2O 5 was compared with the theoretically predicted one. The heterogeneous loss rate of N 2O 5 on the aerosol surfaces was estimated as 5.2×10 −4 s −1. Consequently, it was confirmed that N 2O 5 loss was critical for NO x budget in the urban atmosphere in winter, in comparison with NO x loss via NO 3. The LIF instrument proved to be useful for studying nocturnal chemistry of N 2O 5 in the source region.

Laser-induced fluorescence instrument for measuring atmospheric SO2

Abstract We report on the development of a high-sensitive detection system for measuring atmospheric SO2 using a laser-induced fluorescence (LIF) technique at around 220 nm. Second harmonics of a tunable broad-band optical parametric oscillator (OPO) pumped by the third harmonic of a Nd:YAG laser is used as a fluorescence excitation source. The laser wavelength is alternatively tuned to the peak and the bottom wavelengths in the photoabsorption spectrum of SO2 at 220.6 and 220.2 nm, respectively, and the difference signal at the two wavelengths is used to extract the SO2 concentration. This procedure can give a good selectivity for SO2 and avoid interferences of fluorescent or particulate species other than SO2 in the sample air. The SO2 instrument developed has a sensitivity of 5 pptv in 60 s and S / N = 2 . The practical performance of the detection system is tested in the suburban area. The inter-comparisons between the LIF instrument and a commercial instrument using Xe flash lamp excitation for the fluorescence detection have been performed. The correlation between two instruments is measured up to 70 ppbv. A good linear relationship between the LIF measurements and commercial instrument measurements is obtained.

レーザー誘起蛍光法による大気中ＮＯ３／Ｎ２Ｏ５測定装置

NO3 and N2O5 are important as intermediates of NOx loss in the nocturnal atmosphere. In this study, laser-in-duced fluorescence (LIF) technique is applied for measuring NO3 and N2O5 in the atmosphere. A tunable dye laser pumped by a pulsed Nd: YVO4 laser was utilized as an excitation light source. NO3 was excited around its 623-nm absorption peak and the induced fluorescence was detected. For measuring N2O5, thermal decomposition of N2O5 was carried out and then generated NO3 was detected. As a result of improvement and calibration of the instrument, the limits of detection for NO3 and N2O5 reached 4 and 6 pptv, respectively (S/N=1, 10-min averaging). A measurement test of N2O5 was conducted in the urban atmosphere to check the applicability of the instru-ment. The LIF instrument is promising to measure atmospheric NO3 and N2O5. This is a successful case of ap-plying a spectroscopic method to the environmental chemistry.

Validation of the calibration of a laser-induced fluorescence instrument for the measurement of OH radicals in the atmosphere

Abstract. An assessment of the accuracy of OH concentrations measured in a smog chamber by a calibrated laser-induced fluorescence (LIF) instrument has been made, in the course of 9 experiments performed to study the photo-oxidation of benzene, toluene, 1,3,5-trimethylbenzene, para-xylene, ortho-cresol and ethene at the European Photoreactor facility (EUPHORE). The LIF system was calibrated via the water photolysis / ozone actinometry approach. OH concentrations were inferred from the instantaneous rate of removal of each hydrocarbon species (measured by FTIR or HPLC) via the appropriate rate coefficient for their reaction with OH, and compared with those obtained from the LIF system. Good agreement between the two approaches was found for all species with the exception of 1,3,5-trimethylbenzene, for which OH concentrations inferred from hydrocarbon removal were a factor of 3 lower than those measured by the LIF system. From the remaining 8 experiments, an overall value of 1.15±0.13 (±1σ) was obtained for [OH]LIF / [OH]Hydrocarbon Decay, compared with the estimated uncertainty in the accuracy of the water photolysis / ozone actinometry OH calibration technique of 26% (1σ).

A Laser-induced Fluorescence Instrument for Detecting Tropospheric OH and HO2: Characteristics and Calibration

Measurement capability for the detection of atmospheric OH and HO2 has been developed at the Pennsylvania State Universityover the last decade. The instrument is used in two forms: an aircraft configuration, Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS), and the configuration used on towers, Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). The instrument uses ultraviolet laser induced fluorescence (LIF) to detect OH in air that is pulled by a vacuum pump through a small inlet into a low-pressure detection chamber; HO2 is detected by reacting it with NO to form OH, which is detected by LIF in a second detection chamber. In the calibration, equal amounts of OH and HO2 ranging from 0.15 pptv to 100 pptv are produced via photolysis of water vapor by the 185 nm emission from a low-pressure Hg lamp. Estimated absolute uncertainty at the 2σ confidence level is ± 32% for bothOH and HO2. The dependence of the instrument detection sensitivity hasbeen quantified for changes in ambient water vapor, pressure, laser power, and the flow velocity of ambient air past the inlet. During the last 7 years, the instrument has been deployed in multi-investigator intensive field studies 5 times on the NASA DC-8 aircraft and 8 times on ground-based towers. The descriptions in this manuscript detail our cumulative wisdom of the instrumental response and calibration techniques developed over this time.

Improved analyzer for nitrogen dioxide by laser-induced fluorescence technique

Abstract A compact, fast-response instrument for measuring atmospheric NO 2 by single-wavelength excitation laser-induced fluorescence (LIF) technique was improved. A single-pass alignment of a more powerful, pulsed laser was utilized to realize the pptv (=10 −12 by volume) of the limit of detection (LOD). As a result, the LOD reached 1.8 pptv for 1-min averaging ( S / N =1). To determine the instrumental zero accurately, interference by particles was cancelled out utilizing an NO 2 scrubber tube coated with a mixture of titanium dioxide (TiO 2 ) and hydroxyapatite. Consequently, it was confirmed that the combination of the improved single-wavelength LIF instrument with the scrubber was reasonable for measuring atmospheric NO 2 .

Comparisons of in situ and long path measurements of NO2 in urban plumes

Measurements of NO2 were taken at Cornelia Fort Airpark in Nashville, TN, during the 1999 Southern Oxidant Study using three different techniques: photolysis to NO followed by chemiluminescence (PCL), laser‐induced fluorescence (LIF), and differential optical absorption spectroscopy (DOAS). This was an informal comparison of these techniques conducted during the 1999 Southern Oxidant Study. The PCL and LIF instruments were connected to a common manifold that sampled at the top of a 10‐m‐walkup tower. The DOAS instrument sampled over a 1.37‐km‐long light path with end points at 2 and 35 m above ground. The range of NO2 mixing ratios measured was 0.75 ppbv to over 60 ppbv and the median value was nearly 3 ppbv. While preliminary data analysis showed overall agreement between the LIF and PCL instruments to within 1% (least squares slope = 0.99; r2 = 0.98), subsequent analysis revealed a discontinuous shift of about 12% in the PCL data, which was confirmed by comparison to the DOAS data. A leak in the PCL inlet system was the likely cause. After adjustment of the affected PCL data, a comparison of all the coincident measurements showed high correlation (r2 &gt; 0.99) and overall agreement to within 5%. Analysis of the ratios of PCL NO2 to LIF NO2 showed that greater than 90% of individual data points agree to within the total combined instrumental uncertainties. However, the comparison over short time periods is more precise than the average over the campaign. We attribute this to the need for improved PCL instrument data reduction procedures. The two in situ instruments were also operated side by side a year later in Houston, TX, with similar results.

HOx concentrations and OH reactivity observations in New York City during PMTACS-NY2001

Abstract Hydroxyl (OH) and hydroperoxy (HO 2 ) radicals (collectively called HO x ) were measured by a laser-induced fluorescence instrument during the PMTACS-NY (PM2.5 Technology Assessment and Characterization Study–New York) intensive campaign in New York City in summer 2001. Measurement results for OH and HO 2 are presented for the month-long study. The detection limits were about 3.0×10 5 cm −3 for OH and 2.5×10 6 cm −3 (∼0.1 ppt) for HO 2 with a 1-min integration time and a 2 σ confidence level. The daytime maximum concentrations were 5–20×10 6 cm −3 for OH and 0.4–6×10 8 cm −3 (2–24 pptv) for HO 2 , usually appearing later than the peak of ozone photolysis frequency, J (O 1 D). Relative high OH and HO 2 persisted into early evening and were frequently observed during nighttime. The ratios of HO 2 to OH were typically between 5 and 40, which are smaller than those obtained in relatively clean environments. The OH reactivity, measured by an instrument named total OH loss rate measurement was on average 19±3 s −1 in this urban environment. It was the highest in the morning and the lowest in the afternoon. The comparison of measured OH and HO 2 with model calculations is given in a companion paper (OH and HO 2 chemistry in the urban atmosphere of New York City, Atmospheric Environment (2003a) this issue).

Tunable ultraviolet laser-induced fluorescence detection of trace plastics and dissolved organic compounds in water.

We developed a tunable (220-285-nm) UV and fixed 266-nm laser-induced fluorescence (LIF) system using a spectrometer and a cooled CCD imaging detector to measure the excitation-emission matrix spectra of various compounds in water, including quinine sulfate and plastic compound bisphenol-A. The LIF instrument was used for the fast, nonspecific determination of trace amounts of dissolved organic compounds present in natural water supplies and various brand name bottled distilled water and bottled drinking water. Plastic-related compounds that leached out of plastic utensils and containers were also detected with this instrument. The sensitivity of the system was approximately 1-2 orders of magnitude better than that for a commercial system.

Comparison of two hyperspectral imaging and two laser-induced fluorescence instruments for the detection of zinc stress and chlorophyll concentration in bahia grass ( Paspalum notatum Flugge.)

Bahia grass ( Paspalum notatum Flugge.) plants were grown in silica sand and irrigated daily with one of five levels of Zn (0, 0.5, 25, 50, or 100 mg l −1) to determine the effects of the heavy metal on the growth and development of plant canopies. Healthy and stressed plants were measured with two hyperspectral imagers, laser-induced fluorescence spectroscopy (LIFS), and laser-induced fluorescence imaging (LIFI) systems in order to determine if the four handheld remote sensing instruments were equally capable of detecting plant stress and measuring canopy chlorophyll levels in bahia grass. Symptoms of bahia grass plants grown at deficient (0 mg l −1) or toxic (25, 50, or 100 mg l −1) concentrations of Zn were dominated by leaf chlorosis and plant stunting. Leaf fresh weight, leaf dry weight, CO 2 assimilation, total chlorophyll, and leaf thickness followed (+) quadratic models in which control plants (0.5 mg l −1 Zn) exhibited higher responses than plants grown at either deficient or toxic levels of Zn. Normalized difference vegetation index [NDVI=(NIR−Red)/(NIR+Red)] and ratio vegetation index [RVI= R 750/ R 700, in which R denotes reflectance] values were calculated for calibrated digital images from both hyperspectral imagers. The NDVI and RVI values from both hyperspectral imagers were fit best by (+) quadratic models when treatments were constrained between 0 and 100 mg l −1 Zn, but were fit best by linear regression models with (−) slopes when treatments were constrained between 0.5 and 100 mg l −1 Zn. Furthermore, both NDVI and RVI algorithms were effective in predicting the concentrations of chlorophyll in canopies of bahia grass grown at the various levels of Zn. In contrast, red/far-red (R/FR) fluorescence ratios estimated from leaf fluorescence values measured with the LIFS and LIFI instruments were fit best by (−) quadratic models when treatments were constrained between 0 and 100 mg l −1 Zn, but were fit best by linear regression models with (+) slopes when treatments were constrained between 0.5 and 100 mg l −1 Zn. A series of regression analyses were conducted among plant biometric, biochemical, and leaf anatomical parameters (treated as independent variables) and the remote sensing algorithms, NDVI, RVI, blue/green (BL/GR), and R/FR (treated as dependant variables). In general, residuals were significantly higher for NDVI and RVI models compared to the BL/GR and R/FR models indicating that the NDVI and RVI algorithms were able to measure total chlorophyll and plant biomass more accurately than the BL/GR and R/FR algorithms. However, unique capabilities of LIFS and LIFI instruments continue to argue for the development of laser-induced fluorescence remote sensing technologies.

Nighttime variations in HO 2 radical mixing ratios at Rishiri Island observed with elevated monoterpene mixing ratios

HO 2 radical concentrations were measured by a laser-induced fluorescence instrument for three nighttime periods during the intensive field campaign at Rishiri Island, Japan, in June 2000. The HO 2 mixing ratio had temporal variations around its average of 4.2±1.2 (1 σ) pptv and showed a positive correlation with the summed mixing ratio of four monoterpene species, α-pinene, β-pinene, camphene, and limonene, that sometimes reached 1 ppbv. Our model calculations suggested that ozonolysis reactions of monoterpenes were the main source of nighttime radicals and they explained 58% of measured HO 2 concentration levels. The model roughly reproduced the dependence of the HO 2 mixing ratio on the square root of the radical production rate due to the ozonolysis reactions of the monoterpenes. However, the absolute HO 2 mixing ratio was significantly underpredicted by the model. We discuss possible reasons in terms of misunderstood RO 2 chemistry, RO 2 interference with HO 2 observations, unknown radical production process associated by high NO 2 mixing ratio, and the contribution of unmeasured olefinic species to radical production via their reactions with ozone.

Direct measurements of HOx radicals in the marine boundary layer: testing the current tropospheric chemistry mechanism.

OH and HO(2) radicals, atmospheric detergents, and the reservoir thereof, play central roles in tropospheric chemistry. In spite of their importance, we had no choice but to trust their concentrations predicted by modeling studies based on known chemical processes. However, recent direct measurements of these radicals have enabled us to test and revise our knowledge of the processes by comparing the predicted and observed values of the radical concentrations. We developed a laser-induced fluorescence (LIF) instrument and successfully observed OH and HO(2) at three remote islands of Japan (Oki Island, Okinawa Island, and Rishiri Island). At Okinawa Island, the observed daytime level of HO(2) agreed closely with the model estimates, suggesting that the photochemistry at Okinawa is well described by the current chemistry mechanism. At Rishiri Island, in contrast, the observed daytime level of HO(2) was consistently much lower than the calculated values. We proposed that iodine chemistry, usually not incorporated into the mechanism, is at least partly responsible for the discrepancy in the results. At night, HO(2) was detected at levels greater than 1 pptv at all three islands, suggesting the presence of processes in the dark that produce radicals. We showed that ozone reactions with unsaturated hydrocarbons, including monoterpenes, could significantly contribute to radical production.