The performance of a gas and aerosol monitoring system (GAMS) for the determination of acidic water soluble organic and inorganic gases and ammonia as well as related particles from the atmosphere

A new application of the quasi-simultaneous gas/particle phase sampling and analysis principle first proposed by Simon and Dasgupta (Anal. Chem. 34 (1995) 71) is described. For the first time, a gradient chromatograph is used in connection with such a sampling system to allow the simultaneous determination of major organic (formic, acetic, propionic, oxalic, malonic and succinic) and inorganic (SO₂, HNO₂, HNO₃, HCl and H₂F₂) acidic gases and related particles. Another addition to the previous systems is the analysis of cations other than ammonium from the particulate phase. The time resolution of the instrument still remains high, 1 h, during which gaseous water-soluble acidic compounds, ammonia, as well as related anionic particles and inorganic major cations are analysed. Sampling is based on diffusion in a wetted parallel plate denuder for gases and on growth in supersaturated water vapour for particles. The determination limits range from 2 ppt (acetate) to 0.4 ppb (ammonia) in the gas phase and 0.01 μg m⁻³ (citric acid) to 0.79 μg m⁻³ (calcium) for particulate matter. Collection efficiencies for gas and aerosol sampling were quantified and the supersaturation in the aerosol sampling apparatus investigated. The system has been used for field measurements at a background station; selected results of these measurements are presented.     

HNO3 losses within the cyclone inlet of a diffusion-denuder system under simulated marine environments

We evaluated the loss of HNO₃ within a Teflon-coated aluminum cyclone of an annular diffusion denuder atmospheric sampling system (ADS) under simulated marine conditions. To simulate marine environment, the cyclones were pre-coated with NaCl aerosol droplets. Loss of vapor-phase HNO₃ within the NaCl-coated cyclone was generally greater than 30% at relative humidities (RH) of 60 and 80% and as large as 67% when the cumulative HNO₃ dosages were lower than 3 μg. In contrast, there was little loss of HNO₃ (<8%) in cyclones with no NaCl coating at RHs ranging from 0 to 80%, at HNO₃ air concentrations of 4.3±1.6 μg m⁻³, and at cumulative HNO₃ dosages of greater than 5 μg. However, at lower HNO₃ cumulative dosages (<3 μg), losses in the non-coated cyclones were strongly influenced by RH, ranging from 9% in dry air to 58% at 80% RH. The enhanced loss of HNO₃ in the NaCl-coated cyclone was most likely caused by the reaction between HNO₃ and NaCl on the cyclone wall.     

A continuous monitor-sulfur chemiluminescence detector (CM-SCD) system for the measurement of total gaseous sulfur species in air

A continuous monitor-sulfur chemiluminescence detector (CM-SCD) system with a flameless, temperature-controlled furnace combustion source was developed for real-time measurement of total sulfur gases in air. This measurement system demonstrated a linear dynamic range exceeding five orders of magnitude and equimolar sensitivity to the most prevalent atmospheric sulfur gases. A detection limit of 10 pptv was obtained using 10 min signal averages. On a real-time basis, detection in the 20–50 pptv range was demonstrated. After modification of its sample inlet system, the CM-SCD showed no appreciable interference effects from the addition of H₂O vapor, NO₂ or O₃ to the sample matrix. In the recent Gas-Phase Sulfur Intercomparison Experiment (GASIE), the CM-SCD compared favorably with SO₂ measurement methods.     

Measurements of atmospheric nitrous acid and nitric acid

A highly sensitive technique for the measurement of atmospheric HONO and HNO₃ is reported. The technique is based on aqueous scrubbing using two coil samplers, followed by conversion of HNO₃ to nitrite, derivatization of nitrite to a highly light-absorbing azo dye with sulfanilamide (SA) and N-(1-naphthyl) ethylenediamine (NED), and high performance liquid chromatography (HPLC) analysis. HNO₃ concentration was obtained by the difference of the two channels. Two scrubbing solutions were used for sampling the two species: a 1-mM phosphate buffer solution (pH 7) for the measurement of HONO and a 180 mM NH₄Cl/NH₃ buffer solution (pH 8.5) for the measurement of HONO+HNO₃. The scrubbing solution flow rate was 0.24 ml min⁻¹ and the gas sampling flow rate was 2 l min⁻¹. HNO₃ in the NH₄Cl/NH₃ buffer solution was quantitatively reduced to nitrite along an on-line 0.8-cm Cd reductor column. Nitrite in both channels was derivatized with 2 mM SA and 0.2 mM NED in 25 mM HCl. Quantitative derivatization was achieved within 5 min at 55°C. The azo dye derivative was then separated from the SA/NED reagent by reversed-phase HPLC and detected with a UV-vis detector at 540 nm. With an on-line SEP-PAK C-18 cartridge for the reagent purification, the method detection limit is estimated to be better than 1 pptv for HONO and about 20 pptv for HNO₃. The sample integration time was about 2 min and the sampling frequency is every 10 min. Data collected in downtown Albany and Whiteface Mountain, NY, are shown as examples of applications of this technique in both urban and remote clean environments.     

A theoretical evaluation on the HNO3 artifact of the annular denuder system due to evaporation and diffusional deposition of NH4NO3-containing aerosols

A mathematical model was developed to evaluate HNO₃ artifact of the annular denuder system due to evaporation and diffusional deposition of nitrate-containing aerosols. The model performance was validated by comparing its numerical solutions with laboratory and numerical data available in the literature for evaporation and diffusional deposition of monodisperse and polydisperse NH₄NO₃ aerosols. Measurement artifacts were evaluated by varying typical sampling ranges of ambient temperature, HNO₃ gas concentration, aerosol number concentration, aerosol mass median diameter, and nitrate mass fraction of <2.5 μm aerosols to see their respective effects. Potential application of the present model on estimating HNO₃ artifacts was demonstrated using literature data sampled in USA, Taiwan, Netherlands, Korea and Japan. Significant measurement artifact could be found in Taiwan and Netherlands due either to low HNO₃ gas concentration and high nitrate concentration in <2.5 μm aerosols or to high ambient temperature.     

Comparing field performances of denuder techniques in the high Arctic

A field evaluation between two annular denuder configurations was conducted during the spring of 2003 in the marine Arctic at Ny-Ålesund, Svalbard. The IIA annular denuder system (ADS) employed a series of five single-channel annular denuders, a cyclone and a filter pack to discriminate between gas and aerosol species, while the EPA-Versatile Air Pollution Sampler (VAPS) configuration used a single multi-channel annular denuder to protect the integrity of PM_2.5 sample filters by collecting acidic gases. We compared the concentrations of gaseous nitric acid (HNO₃), nitrous acid (HONO), sulfur dioxide (SO₂) and hydrochloric acid (HCl) measured by the two systems. Results for HNO₃ and SO₂ suggested losses of gas phase species within the EPA-VAPS inlet surfaces due to low temperatures, high relative humidities, and coarse particle sea-salt deposition to the VAPS inlet during sampling. The difference in HNO₃ concentrations (55%) between the two data sets might also be due to the reaction between HNO₃ and NaCl on inlet surfaces within the EPA-VAPS system. Furthermore, we detected the release of HCl from marine aerosol particles in the EPA-VAPS inlet during sampling contributing to higher observed concentrations. Based on this work we present recommendations on the application of denuder sampling techniques for low-concentration gaseous species in Arctic and remote marine locations to minimize sampling biases. We suggest an annular denuder technique without a large surface area inlet device in order to minimize retention and/or production of gaseous atmospheric pollutants during sampling.     

HNO3 analyzer by scrubber difference and the NO–ozone chemiluminescence method

A fast response analyzer for HNO₃ in highly polluted air is described. The time resolution attainable was 12 s. The method is based on the difference in a technique for HNO₃-scrubbed and non-scrubbed air and the reduction of HNO₃ to NO with the use of a line of catalytic converters and a method for the subsequent NO-ozone chemiluminescence. A sample air stream, in which particulates are removed with a Teflon filter, is divided into two channels. CH-1 is directly connected to the converter line, and CH-2 contains a HNO₃ scrubber packed with a nylon fiber that goes to another converter line. Each converter line is composed of a hot quartz-bead converter (QBC) and a molybdenum converter (MC) in a series. A QBC reduces HNO₃ to (NO+NO₂), which is called NO_x. The MC reduces the NO_x to NO.For CH-1, the analyzer detects most compounds that typically comprise NO_y (J. Geophys. Res. 91 (1986) 9781). These CH-1 compounds are called NO_y′ hereafter (NO_y-particulate nitrate) because the particulates are removed by the filter. A difference in the detector signal for the two channels indicates HNO₃. For a blank test, atmospheric air in which HNO₃ was pre-scrubbed by an extra nylon fiber was introduced to the analyzer. Variations in the blank value were 0.38±0.42 and 0.34±0.55 ppb during the high readings (NO_y′-HNO₃ ) (called NO_y^* hereafter) (111±12 ppb, N=180), and low NO_y^* readings (62±8 ppb, N=180), respectively, indicating that the lowest detection limit of the analyzer is 1.1 ppb (2σ). When the data obtained with the analyzer is compared to the data using the denuder method, a linear correlation with the regression of Y=0.973X+0.077 (r²=0.916 (N=20)) in the range of 0–6.5 ppb HNO₃ is obtained, which is an excellent agreement. Atmospheric monitoring was carried out at Kobe. Although the average concentration of HNO₃ was 2.6±1.3 ppb, ca.10 ppb for a HNO₃ concentration was occasionally observed when the NO_y^* concentration was high, i.e., more than 100 ppb.     

Measurements of nitrogen dioxide on Funen using diffusion tubes

Monitoring of nitrogen dioxide (NO₂) by passive sampling on the Danish island Funen (Fyn) show that the concentration of nitrogen dioxide is low (2–20 ppb). The level of NO₂ in rural and suburban areas is governed by imported airpollution, and elevated NO₂ concentrations due to local traffic are of limited importance. These results are supported by diffusion denuder measurements of nitric acid (HNO₃) and particulate nitrate. Measurements of NO₂ with chemiluminescence and diffusive passive sampling showed good agreement between the methods. The special mounting device for the diffusive samplers used in this work seem to have reduced the turbulence at the open end of the tube. The product from the reaction between nitrogen dioxide and triethanolamine was investigated and tentatively identified as triethanolamine N-oxide, which is in accordance with the observed 1 : 1 stoechiometry in the conversion of NO₂ to nitrite ions.     

Atmospheric corrosion effects of HNO3—Influence of temperature and relative humidity on laboratory-exposed copper

The effect of HNO₃ on the atmospheric corrosion of copper has been investigated at varied temperature (15–35 °C) and relative humidity (0–85% RH). Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) confirmed the existence of cuprite and gerhardtite as the two main corrosion products on the exposed copper surface. For determination of the corrosion rate and for estimation of the deposition velocity (V_d) of HNO₃ on copper, gravimetry and ion chromatography has been employed. Temperature had a low effect on the corrosion of copper. A minor decrease in the mass gain was observed as the temperature was increased to 35 °C, possibly as an effect of lower amount of cuprite due to a thinner adlayer on the metal surface at 35 °C. The V_d of HNO₃ on copper, however, was unaffected by temperature. The corrosion rate and V_d of HNO₃ on copper was the lowest at 0% RH, i. e. dry condition, and increased considerably when changing to 40% RH. A maximum was reached at 65% RH and the mass gain remained constant when the RH was increased to 85% RH. The V_d of HNO₃ on copper at ⩾65% RH, 25 °C and 0.03 cm s⁻¹ air velocity was as high as 0.15±0.03 cm s⁻¹ to be compared with the value obtained for an ideal absorbent, 0.19±0.02 cm s⁻¹. At sub-ppm levels of HNO₃, the corrosion rate of copper decreased after 14 d and the growth of the oxide levelled off after 7 d of exposure.     

Atmospheric corrosion effects of HNO3—Comparison of laboratory-exposed copper,zinc and carbon steel

The influence of nitric acid (HNO₃) on the atmospheric corrosion of copper, zinc and carbon steel was investigated in laboratory exposures at 65% relative humidity (RH), 25 °C and 0.03 cm s⁻¹ air velocity. The deposition velocity (V_d) of HNO₃ on the specimens, the corrosion rates and corrosion products were determined by gravimetry, ion chromatography, X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) microspectroscopy. Comparisons were also made with literature data on the corrosion effects of sulfur dioxide (SO₂), nitrogen dioxide (NO₂) and ozone (O₃).At 65% RH, the V_d of HNO₃ on all metals was at least 70% of that of an ideal absorbent, i.e., an impregnated filter with perfect absorption for HNO₃. The V_d of HNO₃ was much higher than that of SO₂, NO₂ or O₃, which is mainly attributed to the relatively high sticking coefficient, high solubility and high reactivity of HNO₃ compared to the other gases. During identical exposures to HNO₃, the corrosion rate of carbon steel was nearly three times higher than that of copper or zinc. However, when comparing the corrosion effects induced by HNO₃ with those induced by SO₂ alone or in combination with either NO₂ or O₃, HNO₃ turned out to be far more aggressive than SO₂. Relative to SO₂, zinc is the metal most sensitive to HNO₃, followed by copper and with carbon steel least sensitive to HNO₃.     

Deposition and adsorption of the air pollutant HNO3 vapor to soil surfaces

Deposition of nitric acid (HNO₃) vapor to soils has been evaluated in three experimental settings: (1) continuously stirred tank reactors with the pollutant added to clean air, (2) open-top chambers at high ambient levels of pollution with and without filtration reducing particulate nitrate levels, (3) two field sites with high or low pollution loads in the coastal sage plant community of southern California. The results from experiment (1) indicated that the amount of extractable NO₃⁻ from isolated sand, silt and clay fractions increased with atmospheric concentration and duration of exposure. After 32 days, the highest absorption of HNO₃ was determined for clay, followed by silt and sand. While the sand and silt fractions showed a tendency to saturate, the clay samples did not after 32 days of exposure under highly polluted conditions. Absorption of HNO₃ occurred mainly in the top 1 mm layer of the soil samples and the presence of water increased HNO₃ absorption by about 2-fold. Experiment (2) indicated that the presence of coarse particulate NO₃⁻ could effectively block absorption sites of soils for HNO₃ vapor. Experiment (3) showed that soil samples collected from open sites had about 2.5 more extractable NO₃⁻ as compared to samples collected from beneath shrub canopies. The difference in NO₃⁻ occurred only in the upper 1–2 cm as no significant differences in NO₃⁻ concentrations were found in the 2–5 cm soil layers. Extractable NO₃⁻ from surface soils collected from a low-pollution site ranged between 1 and 8 μg NO₃–N g⁻¹, compared to a maximum of 42 μg NO₃–N g⁻¹ for soils collected from a highly polluted site. Highly significant relationship between HNO₃ vapor doses and its accumulation in the upper layers of soils indicates that carefully prepared soil samples (especially clay fraction) may be useful as passive samplers for evaluation of ambient concentrations of HNO₃ vapor.     

The use of inferential models for estimating nitric acid vapor deposition to semi-arid coniferous forests

Urban areas emit significant amounts of pollutants that impact forest ecosystems. One of the most important of these is nitric acid vapor (HNO₃), a nitrogen-containing gas that deposits efficiently to forest canopies. Since measuring HNO₃ fluxes directly is often impractical and costly in remote forest locales, inferential techniques are most often used to estimate HNO₃ flux. Given the highly efficient deposition of HNO₃, many of these inferential models assume that leaf surfaces are a ‘perfect sink’ for HNO₃ (i.e., that resistance to HNO₃ deposition is negligibly small or zero). This study tests the ‘perfect sink’ assumption in an open gas exchange system by exposing Abies magnifica, Abies concolor, and Pinus jeffreyi seedlings to concentrations of 1–13 ppb at 4–20% relative humidity. We find that, at these humidities and concentrations, cuticles are not perfect sinks for HNO₃, with cuticular resistance values ranging from 20 to 184 s m⁻¹. In addition, our results indicate that accumulating HNO₃ on leaf cuticles at these concentrations leads to higher cuticular resistance over 8–12 h exposure periods. Based on this laboratory data, we then parameterized cuticular resistance using a single-layer inferential model for semi-arid forests in the Lake Tahoe Basin. Modeled fluxes using this modification were 33% lower during well-mixed daytime conditions than the fluxes from an identical model run using the perfect sink assumption. Since HNO₃ can often account for more than half of atmospheric deposition, we conclude that inferential models that assume foliage to be perfect HNO₃ sinks are inaccurate, especially in semi-arid forests where significant amounts of HNO₃ can accumulate on leaf surfaces during dry periods.     

Seasonal variations of acidic air pollutants in Seoul,South Korea

The annular denuder system (ADS) was used to characterize seasonal variations of acidic air pollutants in Seoul, South Korea. Fifty- four 24 h samples were collected over four seasons from October 1996 to September 1997. The annual mean concentrations of HNO₃, HNO₂, SO₂ and NH₃ in the gas phase were 1.09, 4.51, 17.3 and 4.34 μg m^-3, respectively. The annual mean concentrations of PM_2.5(d_p≤2.5 μm in aerodynamic diameter, 50% cutoff), SO^2-₄, NO^-₃ and NH⁺₄ in the particulate phase were 56.9, 8.70, 5.97 and 4.19 μg m^-3, respectively. All chemical species monitored from this study showed statistical seasonal variations. Nitric acid (HNO₃) and ammonia (NH₃) exhibited substantially higher concentrations during the summer, while nitrous acid (HNO₂) and sulfur dioxide(SO₂) were higher during the winter. Concentrations of PM_2.5, SO^2-₄, NO^-₃ and NH⁺₄ in the particulate phase were higher during the winter months. SO^2-₄, NO^-₃ and NH⁺₄ accounted for 26–38% of PM_2.5. High correlations were found among PM_2.5, SO^2-₄, NO^-₃ and NH⁺₄. The mean H⁺ concentration measured only in the fall was 5.19 nmole m^-3.     

Heterogeneous reactions of soot aerosols with nitrogen dioxide and nitric acid: atmospheric chamber and Knudsen cell studies

Heterogeneous chemical processes involving trace atmospheric gases with solid particulates, such as carbonaceous aerosol, are not well understood. In an effort to quantify some relevant carbon aerosol systems, the heterogeneous chemistry of NO₂ with both commercial and freshly prepared hexane soot was investigated in an atmospheric reaction chamber. At approximately an atmosphere of total pressure (760 Torr) and under dry conditions (relative humidities⩽1%), kinetic measurements gave an uptake coefficient of (2.4±0.6)×10⁻⁸ for n-hexane soot when referenced to the BET surface area of the sample. Commercial carbon black samples were found to yield a similar uptake coefficient. The reaction of HNO₃ with commercial carbon black was also investigated and gas phase NO₂ was detected as a reaction product. Low-pressure Knudsen cell experiments were carried out to facilitate a quantitative comparison between the two different techniques. The agreement between our current results and previously reported values of the uptake coefficient, with different soot samples and under varied pressure and surface coverage conditions, are discussed along with the possible implications for atmospheric chemistry.     

Trace gas variations at Cape Point,South Africa,during May 1997 following a regional biomass burning episode

During the continuous monitoring of atmospheric parameters at the station Cape Point (34°S, 18°E), a smoke plume originating from a controlled fire of 30-yr-old fynbos was observed on 6 May 1997. For this episode, which was associated with a nocturnal inversion and offshore airflow, atmospheric parameters (solar radiation and meteorological data) were considered and the levels of various trace gases compared with those measured at Cape Point in maritime air. Concentration maxima in the morning of 6 May for CO₂, CO, CH₄ and O₃ amounted to 370.3 ppm, 491 ppb, 1730 ppb and 47 ppb, respectively, whilst the mixing ratios of several halocarbons (F-11, F-12, F-113, CCl₄ and CH₃CCl₃) remained at background levels. In the case of CO, the maritime background level for this period was exceeded by a factor of 9.8. Differences in ozone levels of up to 5 ppb between air intakes at 4 and 30 m above the station (located at 230 m above sea level) indicated stratification of the air advected to Cape Point during the plume event. Aerosols within the smoke plume caused the signal of global solar radiation and UV–A to be attenuated from 52.4 to 13.0 mW cm⁻² and from 2.3 to 1.3 mW cm⁻², respectively, 5 h after the trace gases had reached their maxima. Emission ratios (ERs) calculated for CO and CH₄ relative to CO₂ mixing ratios amounted to 0.042 and 0.0040, respectively, representing one of the first results for fires involving fynbos. The CO ER is somewhat lower than those given in the literature for African savanna fires (average ER=0.048), whilst for CH₄ the ER falls within the range of ERs reported for the flaming (0.0030) and smouldering phases (0.0055) of savanna fires. Non-methane hydrocarbon (NMHC) data obtained from a grab sample collected during the plume event were compared to background levels. The highest ERs (ΔNMHC/ΔCH₄) have been obtained for the C₂–C₃ hydrocarbons (e.g. ethene at 229.3 ppt ppb⁻¹), whilst the C₄–C₇ hydrocarbons were characterised by the lowest ERs (e.g. n-hexane at 1.0 and n-pentane at 0.8 ppt ppb⁻¹).     

Physical characterization of aerosol particles during the Chinese New Year’s firework events

Measurements for particles 10 nm to 10 μm were taken using a Wide-range Particle Spectrometer during the Chinese New Year (CNY) celebrations in 2009 in Shanghai, China. These celebrations provided an opportunity to study the number concentration and size distribution of particles in an especial atmospheric pollution situation due to firework displays. The firework activities had a clear contribution to the number concentration of small accumulation mode particles (100–500 nm) and PM₁ mass concentration, with a maximum total number concentration of 3.8 × 10⁴ cm^?3. A clear shift of particles from nucleation and Aitken mode to small accumulation mode was observed at the peak of the CNY firework event, which can be explained by reduced atmospheric lifetimes of smaller particles via the concept of the coagulation sink. High particle density (2.7 g cm^?3) was identified as being particularly characteristic of the firework aerosols. Recalculated fine particles PM₁ exhibited on average above 150 μg m^?3 for more than 12 hours, which was a health risk to susceptible individuals. Integral physical parameters of firework aerosols were calculated for understanding their physical properties and further model simulation.     

Dry deposition of ammonia,nitric acid,ammonium, and nitrate to alpine tundra at Niwot Ridge,Colorado

Micrometeorological measurements and ambient air samples, analyzed for concentrations of NH₃, HNO₃, NH₄⁺, and NO₃⁻, were collected at an alpine tundra site on Niwot Ridge, Colorado. The measured concentrations were extremely low and ranged between 5 and 70 ng N m⁻³. Dry deposition fluxes of these atmospheric species were calculated using the micrometeorological gradient method. The calculated mean flux for NH₃ indicates a net deposition to the surface and indicates that NH₃ contributed significantly to the total N deposition to the tundra during the August–September measurement period. Our pre-measurement estimate of the compensation point for NH₃ in air above the tundra was 100–200 ng N m⁻³; thus, a net emission of NH₃ was expected given the low ambient concentrations of NH₃ observed. Based on our results, however, the NH₃ compensation point at this alpine tundra site appears to have been at or below about 20 ng N m⁻³. Large deposition velocities (>2 cm s⁻¹) were determined for nitrate and ammonium and may result from reactions with surface-derived aerosols.     

Heterogeneous conversion of nitric acid to nitrous acid on the surface of primary organic aerosol in an urban atmosphere

Nitrous acid (HONO), nitric acid (HNO₃), and organic aerosol were measured simultaneously atop an 18-story tower in Houston, TX during August and September of 2006. HONO and HNO₃ were measured using a mist chamber/ion chromatographic technique, and aerosol size and chemical composition were determined using an Aerodyne quadrupole aerosol mass spectrometer. Observations indicate the potential for a new HONO formation pathway: heterogeneous conversion of HNO₃ on the surface of primary organic aerosol (POA). Significant HONO production was observed, with an average of 0.97 ppbv event^?1 and a maximum increase of 2.2 ppb in 4 h. Nine identified events showed clear HNO₃ depletion and well-correlated increases in both HONO concentration and POA-dominated aerosol surface area (SA). Linear regression analysis results in correlation coefficients (r²) of 0.82 for HONO/SA and 0.92 for HONO/HNO₃. After correction for established HONO formation pathways, molar increases in excess HONO (HONO_excess) and decreases in HNO₃ were nearly balanced, with an average HONO_excess/HNO₃ value of 0.97. Deviations from this mole balance indicate that the residual HNO₃ formed aerosol-phase nitrate. Aerosol mass spectral analysis suggests that the composition of POA could influence HONO production. Several previously identified aerosol-phase PAH compounds were enriched during events, suggesting their potential importance for heterogeneous HONO formation.     

The impact of an 8 h ozone air quality standard on ROG and NOx controls in Southern California

The new National Ambient Air Quality Standard for ozone in the US uses 8 h averaging for the concentration. Based on the 1993 ambient data for Southern California, 8 h averaging has a moderate tendency to move the location of the peak ozone concentration east of the location of the peak 1 h ozone concentration. Reducing the area-wide peak 8 h ozone concentration to 80 ppb would require an effective reduction of the area-wide peak 1 h ozone concentration to around 90 ppb. The Urban Airshed Model with improved numerical solvers, meteorological input based on a mesoscale model and an adjusted emissions inventory was used to study the effect of reactive organic gases (ROG) and NO_x controls on daily-maximum and peak 8 h ozone concentrations under the 26–28 August 1987 ozone episodic conditions in Southern California. The NO_x disbenefit remains prominent for the case of 8 h ozone concentration but is somewhat less prominent, especially when areal ozone exposure is considered, than the case for 1 h ozone concentration. The role of two indicators – O₃/NO_y and H₂O₂/HNO₃ – for NO_x- and ROG-sensitivity for 1 and 8 h ozone concentrations were also studied. In general, the indicator trends are consistent with model predictions, but the discriminating power of the indicators is rather limited.     

Evaluation of the filter pack for long-duration sampling of ambient air

A 14-week filter pack (FP) sampler evaluation field study was conducted at a site near Bondville, IL to investigate the impact of weekly sampling duration. Simultaneous samples were collected using collocated filter packs (FP) from two independent air quality monitoring networks (CASTNet and Acid-MODES) and using duplicate annular denuder systems (ADS). Precision estimates for most of the measured species are similar for weekly ADS and composited FPs. There is generally good agreement between the weekly CASTNet FP results aggregated from weekly daytime and weekly nighttime samples and those aggregated from daily 24 h Acid-MODES samples; although SO₂ is the exception, and CASTNet concentrations are higher than Acid-MODES. Comparison of weekly ADS results with composited weekly FP results from CASTNet shows good agreement for SO^2-₄. With the exception of the two weeks where the FP exceeded the ADS, both HNO₃ and the sum of particulate and gaseous NO^-₃ show good agreement. The FP often provides good estimates of HNO₃, but when used to sample atmospheres that have experienced substantial photochemical reactivity, FP HNO₃ determinations using nylon filters may be biased high. It is suggested that HNO₂ or some other oxidized nitrogen compound can accumulate on a regional scale and may interfere with the FP determination of HNO₃. FP particulate NO^-₃ results are in fair agreement with the ADS. Since FP SO₂ results are biased low by 12–20%, SO₂ concentration in the CASTNet data archive should be adjusted upward. Nylon presents problems as a sampling medium in terms of SO₂ recovery and specificity for HNO₃. Additional comparative sampler evaluation studies are recommended at several sites over each season to permit comprehensive assessment of the concentrations of atmospheric trace constituents archived by CASTNet.