A sodium carbonate coated denuder for determination of nitrous acid in the atmosphere

A denuder technique for sampling and analysing nitrous acid at sub ppb levels in air is described. After sampling, the sodium carbonate coated denuder is leached in water, and the NO⁻₂ concentration is determined spectrophotometrically. Field tests show that PAN is partly sorbed and hydrolized to nitrite in the sodium carbonate layer. It seems as HNO₂ also can be formed by heterogeneous reactions between NO and NO₂ at the denuder wall. These sampling artifacts were overcome by sampling with two or three denuders in series. The presence of PAN deteriorates the detection limit, which during optimal conditions is about 0.5 nmole m⁻³ (0.01 ppb). The method is therefore not recommended for measurements in background air, where HNO₂ concentrations normally are low compared to PAN concentrations.     

Atmospheric gas and particle measurements at a rural northeastern U.S. site

Ammonia, nitric acid, sulfur dioxide and particles in two size ranges were collected at a rural site in northeastern U.S. in January–March 1984. Ammonia was collected with an oxalic acid coated denuder, all other components were collected on filters. The concentrations of ammonia ranged between 0.0 and 0.5 ppbv, nitric acid: 0.1 and 2.3 ppbv and sulfur dioxide: 1 and 52 ppbv. Ammonium and sulfate in the fine particles were highly correlated, the regression line indicated that the most abundant compound was ammonium sulphate. The content of free hydronium ions in the fine particles was well below the ammonium content. No correlation between NH₃ and NH⁺₄, HNO₃ and NO₃ SO₃ and SO⁻²₄ could be observed.     

Performance of the Annular Denuder System with Different Arrangements for HNO3 and HNO2 Measurements in Taiwan

ABSTRACT

Experiments on different annular denuder system (ADS)arrangements for sampling nitrous acid (HNO₂) and ni-tric acid (HNO₃) gases were conducted in this study toevaluate their sampling artifacts. The evaluation basis isthe one that employed one sodium chloride denuder forsampling HNO₃ gas and two sodium carbonate (Na₂CO₃)denuders for sampling HNO₂ gas, which is a commonlyemployed ADS arrangement in many field applicationsin the United States. A field study was conducted inHsinchu, Taiwan, and the results indicated that this ADSarrangement may yield over 80% relative errors for HNO₃gas. It also showed that the relative errors for HNO₂ gascan be less than 10% as sampled with only one Na₂CO₃denuder. This is attributed to the fact that the ambientHNO₃ concentration measured in this study was relativelylow while the HNO₂ concentration was high, as comparedto typical concentrations of these two gases measured inthe United States.

The sampling error of HNO₃ gas may be due to highconcentrations of N-containing interfering speciespresent in Taiwan’s atmosphere. Because the relative sam-pling errors of HNO₃ and HNO₂ gases depend mainly ontheir concentrations in the atmosphere as well as con-centrations caused by interfering species, the risk for higherror while measuring low HNO₂ concentrations by onlyone Na₂CO₃ denuder is also possible. As a result, it is sug-gested that pretests are necessary to evaluate possiblesources and degrees of sampling errors before fieldsampling of HNO₂ and HNO₃ gases. The sampling errorsof these two gases can, therefore, be minimized with abetter arrangement of the ADS.     

Use of Two Different Acidic Aerosol Samplers To Measure Acidic Aerosols in Hsinchu,Taiwan

ABSTRACT

Acidic aerosol concentrations measured by an annular denuder system (ADS) and a honeycomb denuder system (HDS) in Hsinchu, Taiwan, were compared. Aerosols were also sampled by a MOUDI (micro-orifice uniform deposit impactor) and analyzed by an ion chromatograph to determine the size distributions of different species. Using the measured aerosol size distribution, theoretical analysis showed that positive HNO₃ artifact due to volatiliza-tion of NH₄NO₃ is generally negligible for both samplers.4 3Comparing two different denuder samplers, the average concentration of HNO₃ measured by the ADS was found3to be lower than that measured by the HDS, while the difference between the two samplers for the average concentration of other species was found to be within ±15%. A possible cause of the difference in HNO₃ con-3centrations is due to a greater loss of HNO₃ in the cyclone3 used by the ADS than in the impactor used by the HDS. The study also showed incomplete absorption of the evaporated HCl and HNO₃ from the particles on the Teflon3filter by the first nylon filter in the filter pack of the ADS. Collection efficiency and capacity of HCl and HNO₃ by3the nylon filters need further investigation.     

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.     

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.     

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.     

Intercomparison of Clean Air Status and Trends Network Nitrate and Nitric Acid Measurements with Data from Other Monitoring Programs

Abstract

The U.S. Environmental Protection Agency Clean Air Status and Trends Network (CASTNET) utilizes an open-face filter pack system to measure concentrations of atmospheric sulfur and nitrogen species. Concentration data for nitrogen species measured with filter pack systems sometimes deviate from data collected by other measurement systems used to measure the same species. The nature of these differences suggests that more than one sampling mechanism or atmospheric process is involved. The study presented here examines these differences by intercomparing CASTNET data with data from other studies, examining the results from earlier intercomparison studies, and conducting a field test to investigate the effect of particle size on filter pack measurement systems. Measurements of nitrogen species from the Maryland Aerosol Research and Characterization (MARCH) monitoring site were compared with nitrogen concentrations at three nearby CASTNET sites. Results indicate that CASTNET measured higher particulate nitrate (NO₃ ^-) and lower gaseous nitric acid (HNO₃) concentrations. Comparisons of NO₃ ^- from 34 collocated CASTNET and Inter-agency Monitoring of Protected Visual Environments (IMPROVE) sites show that CASTNET NO₃ ^- measurements were typically higher than the corresponding IM PROVE values. Also, results from the Lake Michigan Air Director’s Consortium Midwest Ammonia Monitoring Project demonstrated NO₃ ^- dissociation on Teflon filters. To investigate the effect of particle size, filter pack measurement systems were operated at three CASTNET sites with and without cyclones during six 7-day measurement periods from March to August 2006. Results indicate the size-selection cyclones had a significant effect on both NO₃ ^- and HNO₃ concentrations, but little effect on sulfate (SO₄ ^2-) and ammonium (NH₄ ⁺) levels. NO₃ ^- concentrations sampled with the open-face filters were significantly higher than concentrations measured with a 2.5-μm cut point, as were HNO₃ concentrations. Although limited in spatial and temporal coverage, the field study showed that the use of an open-face filter pack may allow for the collection of coarse NO₃ ^- particles and for the reaction of HNO₃ with metals/salts on the Teflon filter.     

Source Identification and Trends in Concentrations of Gaseous and Fine Particulate Principal Species in Seoul,South Korea

Abstract

Ambient measurements were made using two sets of annular denuder system during the four seasons (April 2001 to February 2002) and were then compared with the results during the period of 1996–1997 to estimate the trends and seasonal variations in concentrations of gaseous and fine particulate matter (PM_2.5) principal species. Annual averages of gaseous HNO₃ and NH₃ increased by 11% and 6%, respectively, compared with those of the previous study, whereas HONO and SO₂ decreased by 11% and 136%, respectively. The PM_2.5 concentration decreased by ～17%, 35% for SO₄ ^2?, and 29% for NH₄ ⁺, whereas NO₃ ^? increased by 21%. Organic carbon (OC) and elemental carbon (EC) were 12.8 and 5.98 μg/m^-3, accounting for ～26 and 12% of PM_2.5 concentration, respectively. The species studied accounted for 84% of PM_2.5 concentration, ranging from 76% in winter to 97% in summer.

Potential source contribution function (PSCF) analysis was used to identify possible source areas affecting air pollution levels at a receptor site in Seoul. High possible source areas in concentrations of PM_2.5, NO₃ ^?, SO₄ ^2?, NH₄ ⁺, and K⁺ were coastal cities of Liaoning province (possibly emissions from oil-fired boilers on ocean liners and fishing vessels and industrial emissions), inland areas of Heibei/Shandong provinces (the highest density areas of agricultural production and population) in China, and typical port cities (Mokpo, Yeosu, and Busan) of South Korea. In the PSCF map for OC, high possible source areas were also coastal cities of Liaoning province and inland areas of Heibei/Shandong provinces in China. In contrast, high possible source areas of EC were highlighted in the south of the Yellow Sea, indicating possible emissions from oil-fired boilers on large ships between South Korea and Southeast Asia. In summary, the PSCF results may suggest that air pollution levels in Seoul are affected considerably by long-range transport from external areas, such as the coastal zone in China and other cities in South Korea, as well as Seoul itself.     

An experimental study of the dry deposition of gaseous nitric acid to snow

A chamber placed in a constant temperature freezing room was used to study the surface resistance during deposition of HNO₃ to a snow surface. The resistance decreased with increasing temperature from larger than 5 s mm⁻¹ at − 18°C to about l s mm⁻¹ at −3°C. Measurements of gaseous and particulate nitrate concentrations during winter at a rural site in south central Sweden gave concentrations in the range of 0.4–5 μg HNO₃ m⁻¹ and 0.3–3 μg NO⁻₃ m⁻³ with a mean value of 1.3 μg HNO₃ m⁻³ and 0.7 μg NO⁻₃ m⁻³, respectively. The results indicate that for periods with temperatures below − 2°C estimated dry deposition of HNO₃ to snow is at most 4 % of measured wet deposition of nitrate in the area.     

A Fabric Denuder for Sampling Semi-Volatile Species

ABSTRACT

A new style of diffusion denuder has been evaluated specifically for sampling HNO₃. A coated fabric is used as the denuder substrate, which can be loaded directly into a standard filter holder. This approach allows direct denuder sampling with no additional capital costs over filter sampling and simplifies the coating and extraction process.

Potential denuder materials and coatings were evaluated in the laboratory to test the removal efficiency. NaCl coatings were used to assess more than 20 materials for HNO₃ collection efficiency. Particle retention, which would cause a denuder to have a positive bias for gas concentration measurements, was evaluated by ambient air sampling using particulate sulfate as the reference aerosol. Particle retention varied from 0 to 15%, depending on the denuder material tested. The best performing material showed an average particle retention of less than 3%.

Denuder efficiency of four fabric materials was tested under ambient conditions to determine removal efficiency. The fabric denuder method was compared with a long path-length Fourier transform infrared (FTIR) spectrometer, a tunable diode laser absorption spectrometer (TDLAS), and a denuder difference sampler to independently measure HNO₃. HNO₃ collection efficiency was typically 90% for the denuders, whether coated with NaCl or not. For 10-L/min sampling rates with the fabric denuder, the square of the correlation coefficient with the FTIR spectrometer was 0.73, compared to 0.24 with the TDLAS.     

Development of a MnO2-coated,cylindrical denuder for removing NO2 from atmospheric samples

In order to develop a diffusion denuder for the removal of NO₂ from ambient atmospheric samples, a number of materials were screened for their ability to adsorb NO₂:

• 1.(1) MgO;
• 2.(2) MnO₂ on alpha-Al₂O₃;
• 3.(3) water-treated MnO₂ on alpha-Al₂O₃; and
• 4.(4) MnO₂ (activated).

A simple cylindrical denuder coated with MnO₂ (activated) was very effective in the removal of NO₂ from a feedgas of NO₂ in air at ambient temperature and pressure. The other materials were unsatisfactory. The strong oxidizing properties, along with the hydrated surface of the MnO₂ (activated), appear to be important for the sorption of NO₂, as suggested in applications elsewhere. Quantification of denuder sorption efficiencies indicates that MnO2 (activated) is nearly a perfect sorbent for NO₂. The diffusion coefficient of NO₂ in air was found to be 10.8 ± 0.3 cm² min⁻¹ at 22–23°C, which compares favourably with a theoretical estimate. Although MnO₂ (activated)-coated denuders were also found to adsorb SO₂, interference with NO₂ sorption was not sufficient to impair ambient applications.     

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.     

Seasonal variation of atmospheric ammonia and particulate ammonium concentrations in the urban atmosphere of yokohama over a 5-year period

Measurements of ammonia and particulate ammonium were made in the daytime (1200–1500) at a urban site in Yokohama during the 5-year period, 1982–1986. Diurnal NH₃ concentrations showed a distinct seasonal trend with a maximum in summer. The diurnal monthly average concentrations were above 10 ppb during the late spring and summer months, while the concentrations during the winter months were between 1 and 5 ppb. The seasonal variation was found to be very similar to that of the average air temperature and showed a periodic pattern over 1 year. A good correlation was observed between diurnal NH₃ concentrations and average air temperatures during the 5-year period. The annual mean concentrations were in the range of 6.6–7.6 ppb with only a minor deviation. The diurnal monthly average concentrations of particulate NH₄⁺ were between 1 and 4 μg m⁻³ and no significant seasonal variations were seen. As a short-term study, simultaneous measurements of NH₃, HNO₃ and particulate NO₃⁻ were made. The diurnal mean concentrations of NH₃ and HNO₃ were 7.6 and 0.8 ppb, respectively. The concentration of particulate NO₃⁻ ranged from 0.3 to 6μg⁻³. Both HNO₃ and particulate NO₃⁻ concentrations were relatively low and constant. Thus, NH₃ and HNO₃ levels did not agree with the concentrations predicted from the NH₄NO₃ equilibrium constant.     

In situ measurements of HNO3, NOy,NO, and O3 in the lower stratosphere and upper troposphere

In situ measurements of nitric acid (HNO₃), reactive nitrogen (NO_y), nitric oxide (NO), and ozone (O₃) made in the upper troposphere (UT) and lower stratosphere (LS) between 29° and 33°N latitudes during September 1999 are used to examine NO_y partitioning and correlations between the measured species in these regions. The fast-response (1 s) HNO₃ measurements are acquired with a new autonomous CIMS instrument. In the LS, HNO₃ accounts for the majority of NO_y, and the sum of HNO₃ and NO_x accounts for approximately 90% of NO_y. In the UT, the sum of HNO₃ and NO_x varies between 40% and 100% of NO_y. Both HNO₃ and NO_y are strongly positively correlated with O₃, with larger correlation slopes in the UT than in the LS. In the UT at low values of the quantity (NO_y–NO_x–HNO₃), it is uncorrelated with O₃, while at higher values, a positive correlation with O₃ is found. Of these two air mass types, those with higher (NO_y–NO_x–HNO₃) mixing ratios are likely associated with the presence of peroxyacetyl nitrate (PAN) that is produced by NO_x-hydrocarbon chemistry.     

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.     

Simultaneous sampling of NH3, HNO3, HC1, SO2 and H2O2 in ambient air by a wet annular denuder system

A new sampling device is described for the simultaneous collection of NH₃, HNO₃, HCl, SO₂ and H₂O₂ in ambient air. The apparatus is based on air sampling by two parallel annular denuder tubes. The gases are collected by absorption in solutions present in the annulus of the denuder tubes. After a sampling time of 30 min at flow rate of 32 ℓ min⁻¹ the solutions are extracted from the denuders and analyzed off-line. The detection limits of NH₃, HNO₃, HCL and SO₂ are in the order of 0.1–0.5 μm⁻³. For H₂O₂ the detection limit is 0.01 μm⁻³. The reproducibility is 5–10% at the level of ambient air concentrations. Comparison of this novel technique with existing methods gives satisfactory results. The compact set-up offers the possibility of field experiments without the need of extensive equipment.     

The NOAA Twin Otter and its role in BRACE: A comparison of aircraft and surface trace gas measurements

A DeHavilland DHC-6 Twin Otter, operated by the National Oceanic and Atmospheric Administration, was deployed in Tampa, FL to measure aerosols and primary and secondary trace gases in support of the Bay Regional Atmospheric Chemistry Experiment (BRACE). The Twin Otter repeatedly overflew the surface chemistry monitoring super site near Sydney, FL to assess the comparability of surface and airborne datasets and the spatial representativeness of the surface measurements. Prior to comparing the chemical datasets, we evaluated the comparability of the standards used to calibrate surface and airborne detectors, as well as the uniformity of wind fields aloft and at the surface. Under easterly flow, when the dearth of significant upwind emission sources promoted chemical homogeneity at Sydney, trace gas concentrations at the surface and aloft were generally well correlated; R² ranged from 0.4396 for H₂O₂ to 0.9738 for O₃, and was typically better than 0.70 for NO, NO₂, NO_Y, HNO₃, HCHO, and SO₂. Mean ratios of aircraft-to-surface concentrations during 10 overflights of Sydney were as follows: 1.002±0.265 (NO), 0.948±0.183 (NO₂), 1.010±0.214 (NO_Y), 0.941±0.263 (HCHO), and 0.952±0.046 (O₃). Poorer agreement and larger variability in measured ratios were noted for SO₂ (1.764±0.559), HNO₃ (1.291±0.391), and H₂O₂ (1.200±0.657). Under easterly flow, surface measurements at Sydney were representative of conditions over horizontal scales as large as 50 km and agreed well with airborne values throughout the depth of the turbulently mixed boundary layer at mid-day. Westerly flow advected the Tampa urban plume over the site; under these conditions, as well as during transitional periods associated with the development of the land–sea breeze, surface conditions were representative of smaller spatial scales. Finally, we estimate possible errors in future measurement-model comparisons likely to arise from fine scale (or subgrid;<2 km) variability of trace gas concentrations. Large subgrid variations in concentration fields were observed downwind of large emission point sources, and persisted across multiple model grid cells (distances>4 km) in coherent plumes. Variability at the edges of the well-mixed urban plume, and at the interface of the land–sea breeze circulation, was significantly smaller. This suggests that even a failure of modeled wind fields to resolve the sea breeze return can induce moderate, but not overwhelming, errors in simulated concentration fields and dependent chemical processes.     

Measurements of primary trace gases and NOY composition in Houston,Texas

Concentrations of CO, SO₂, NO, NO₂, and NO_Y were measured atop the University of Houston's Moody Tower supersite during the 2006 TexAQS-II Radical and Aerosol Measurement Project (TRAMP). The lowest concentrations of all primary and secondary species were observed in clean marine air in southerly flow. SO₂ concentrations were usually low, but increased dramatically in sporadic midday plumes advected from sources in the Houston Ship Channel (HSC), located NE of the site. Concentrations of CO and NO_x displayed large diurnal variations in keeping with their co-emission by mobile sources in the Houston Metropolitan Area (HMA). CO/NO_x emission ratios of 5.81 ± 0.94 were observed in the morning rush hour. Nighttime concentrations of NO_x (NO_x = NO + NO₂) and NO_Y (NO_Y = NO + NO₂ + NO₃ + HNO₃ + HONO + 21N₂O₅ + HO₂NO₂ + PANs + RONO₂ + p-NO₃^? + …) were highest in winds from the NNW-NE due to emission from mobile sources. Median ratios of NO_x/NO_Y were approximately 0.9 overnight, reflecting the persistence and/or generation of NO_Z (NO_Z = NO_Y ? NO_x) species in the nighttime Houston boundary layer, and approached unity in the morning rush hour. Daytime concentrations of NO_x and NO_Y were highest in winds from the HSC. NO_x/NO_Y ratios reached their minimum values (median ca 0.63) from 1300 to 1500 CST, near local solar noon, and air masses often retained enough NO_x to sustain additional O₃ formation farther downwind. HNO₃ and PANs comprised the dominant NO_Z species in the HMA, and on a median basis represented 17–20% and 12–15% of NO_Y, respectively, at midday. Concentrations of HNO₃, PANs, and NO_Z, and fractional contributions of these species to NO_Y, were at a maximum in NE flow, reflecting the source strength and reactivity of precursor emissions in the HSC. As a result, daytime O₃ concentrations were highest in air masses with HSC influence. Overall, our findings confirm the impact of the HSC as a dominant source region within the HMA. A comparison of total NO_Y measurements with the sum of measured NO_Y species (NO_Yi = NO_x + HNO₃ + PANs + HONO + p-NO₃^?) yielded excellent overall agreement during both day ([NO_Y](ppb) = ([NO_Yi](ppb)11.03 ± 0.16) ? 0.42; r² = 0.9933) and night ([NO_Y](ppb) = ([NO_Yi](ppb)11.01 ± 0.16) + 0.18; r² = 0.9975). A similar comparison between NO_Y–NO_x concentrations and the sum of NO_Zi (NO_Zi = HNO₃ + PANs + HONO + p-NO₃^?) yielded good overall agreement during the day ([NO_Z](ppb) = ([NO_Zi](ppb)11.01 ± 0.30) + 0.044 ppb; r² = 0.8527) and at night ([NO_Z](ppb) = ([NO_Zi](ppb)11.12 ± 0.69) + 0.16 ppb; r² = 0.6899). Median ratios of NO_Z/NO_Zi were near unity during daylight hours but increased to approximately 1.2 overnight, a difference of 0.15–0.50 ppb. Differences between NO_Z and NO_Zi rarely exceeded combined measurement uncertainties, and variations in NO_Z/NO_Zi ratios may have resulted solely from errors in conversion efficiencies of NO_Y species and changes in NO_Y composition. However, nighttime NO_Z/NO_Zi ratios and the magnitude of NO_Z ? NO_Zi differences were generally consistent with recent observations of ClNO₂ in the nocturnal Houston boundary layer.