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.     

Assessment of the Ozone-Nitrogen Oxide-Volatile Organic Compound Sensitivity of Mexico City through an Indicator-Based Approach: Measurements and Numerical Simulations Comparison

Abstract

The ozone (O₃) sensitivity to nitrogen oxides (NO_x, or nitric oxide [NO] + nitrogen dioxide [NO₂]) versus volatile organic compounds (VOCs) in the Mexico City metropolitan area (MCMA) is a current issue of scientific controversy. To shed light on this issue, we compared measurements of the indicator species O₃/NO_y (where NO_y represents the sum of NO + NO₂ + nitric acid [HNO₃] + peroxyacetyl nitrate [PAN] + others), NO_y, and the semiempirically derived O₃/NO_{z surrogate} (where NO_{z surrogate} is the derived surrogate NO_z, and NO_z represents NO_x reaction products, or NO_y – NO_x) with results of numerical predictions reproducing the transition regimes between NO_x and VOC sensitivities. Ambient air concentrations of O₃, NO_x, and NO_y were measured from April 14 to 25, 2004 in one downwind receptor site of photo-chemically aged air masses within Mexico City. MCMA-derived transition values for an episode day occurring during the same monitoring period were obtained through a series of photochemical simulations using the Multiscale Climate and Chemistry Model (MCCM). The comparison between the measured indicator species and the simulated spatial distribution of the indicators O₃/NO_y, O₃/NO_{z surrogate}, and NO_y in MCMA suggest that O₃ in this megacity is likely VOC-sensitive. This is in opposition to past studies that, on the basis of the observed morning VOC/NO_x ratios, have concluded that O₃ in Mexico City is NO_x-sensitive. Simulated MCMA-derived sensitive transition values for O₃/NO_y, hydrogen peroxide (H₂O₂)/HNO₃, and NO_y were found to be in agreement with threshold criteria proposed for other regions in North America and Europe, although the transition crossover for O₃/NO_z and O₃/HNO₃ was not consistent with values reported elsewhere. An additional empirical evaluation of weekend/weekday differences in average maximum O₃ concentrations and 6:00- to 9:00-a.m. NO_x and NO levels registered at the same site in April 2004 indirectly confirmed the above results. A preliminary conclusion is that additional reductions in NO_x emissions in MCMA might cause an increase in presently high O₃ levels.     

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.     

Measurements of trace gases in the tropical tropopause layer

A unique dataset of airborne in situ observations of HCl, O₃, HNO₃, H₂O, CO, CO₂ and CH₃Cl has been made in and near the tropical tropopause layer (TTL). A total of 16 profiles across the tropopause were obtained at latitudes between 10°N and 3°S from the NASA WB-57F high-altitude aircraft flying from Costa Rica. Few in situ measurements of these gases, particularly HCl and HNO₃, have been reported for the TTL. The general features of the trace gas vertical profiles are consistent with the concept of the TTL as distinct from the lower troposphere and lower stratosphere. A combination of the tracer profiles and correlations with O₃ is used to show that a measurable amount of stratospheric air is mixed into this region. The HCl measurements offer an important constraint on stratospheric mixing into the TTL because once the contribution from halocarbon decomposition is quantified, the remaining HCl (>60% in this study) must have a stratospheric source. Stratospheric HCl in the TTL brings with it a proportional amount of stratospheric O₃. Quantifying the sources of O₃ in the TTL is important because O₃ is particularly effective as a greenhouse gas in the tropopause region.     

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.     

Simple model for estimating dry deposition velocity of ozone and its destruction in a polluted nocturnal boundary layer

Determining the destructions of both ozone and odd oxygen, O_x, in the nocturnal boundary layer (NBL) is important to evaluate the regional ozone budget and overnight ozone accumulation. This work develops a simple method to determine the dry deposition velocity of ozone and its destruction at a polluted nocturnal boundary layer. The destruction of O_x can also be determined simultaneously. The method is based on O₃ and NO₂ profiles and their surface measurements. Linkages between the dry deposition velocities of O₃ and NO₂ and between the dry deposition loss of O_x and its chemical loss are constructed and used. Field measurements are made at an agricultural site to demonstrate the application of the model. The model estimated nocturnal O₃ dry deposition velocities from 0.13 to 0.19 cm s^?1, very close to those previously obtained for similar land types. Additionally, dry deposition and chemical reactions account for 60 and 40% of the overall nocturnal ozone loss, respectively; ozone dry deposition accounts for 50% of the overall nocturnal loss of O_x, dry deposition of NO₂ accounts for another 20%, and chemical reactions account for the remaining 30%. The proposed method enables the use of measurements made in typical ozone field studies to evaluate various nocturnal destructions of O₃ and O_x in a polluted environment.     

Conversion of nitrogen oxides on commercial photocatalytic dispersion paints

In the present study, photocatalytic reactions of nitrogen oxides (NO_x = NO + NO₂) were studied on commercial TiO₂ doped facade paints in a flow tube photoreactor under simulated atmospheric conditions. Fast photocatalytic conversion of NO and NO₂ was observed only for the photocatalytic paints and not for non-catalytic reference paints. Nitrous acid (HONO) was formed in the dark on all paints studied, however, it efficiently decomposes under irradiation only on the photocatalytic samples. Thus, it is concluded that photocatalytic paint surfaces do not represent a daytime source of HONO, in contrast to other recent studies on pure TiO₂ surfaces. As main final product, the formation of adsorbed nitric acid/nitrate anion (HNO₃/NO₃^?) was observed with near to unity yield. In addition, traces of H₂O₂ were observed in the gas phase only in the presence of O₂. Formation of the greenhouse gas nitrous oxide (N₂O) could be excluded. The uptake kinetics of NO, NO₂ and HONO was very fast under atmospheric conditions (e.g. γ(NO + TiO₂) > 10^?5). Thus, the uptake on urban surfaces (painted houses, etc.) will be limited by transport. For a hypothetically painted street canyon, an average reduction of nitrogen oxide levels of ca. 5% is estimated. Since the harmful HNO₃/NO₃^? is formed on the surface of the photoactive paints, whereas it is formed in the gas phase in the atmosphere, the use of photocatalytic paints may also help to reduce acid deposition, e.g. on plants, or nitric acid related health issues.     

Seasonal variations in atmospheric SOx and NOy species in the adirondacks

This paper reports the results of over 2 years of measurements of several of the species comprising atmospheric SO_x (=SO₂+SO₄²⁻) and NO_y (=NO+NO₂ + PAN + HNO₃+NO₃⁻+ organicnitrates + HONO + 2N₂O₅ …) at Whiteface Mountain, New York. Continuous real-time measurements of SO₂ and total gaseous NO_y provided data for about 50% and 65% of the period, respectively, and 122 filter pack samples were obtained for HNO₃, SO₂ and aerosol SO₄²⁻, NO₃⁻, H⁺ and NH₄⁺. Concentrations of SO₂ and NO_y were greatest in winter, whereas concentrations of the reaction products SO₄²⁻ and HNO₃were greatest in summer. The seasonal variation in SO₄²⁻ was considerably more pronounced than that of HNO₃and the high concentrations of SO₄²⁻ aerosol present in summer were also relatively more acidic than SO₄²⁻ aerosol in other seasons. As a result, SO₄²⁻ aerosol was the predominant acidic species present in summer, HNO₃was predominant in other seasons. Aerosol NO₃⁻ concentrations were low in all seasons and appeared unrelated to simultaneous NO_y and HNO₃concentrations. These data are consistent with seasonal variations in photochemical oxidation rates and with existing data on seasonal variations in precipitation composition. The results of this study suggest that emission reductions targeted at the summer season might be a cost-effective way to reduce deposition of S species, but would not be similarly cost-effective in reducing deposition of N species. kwAcid deposition, seasonal variation, sulfate, nitrate, nitric acid, sulfur dioxide, oxides of nitrogen, hydrogen peroxide, ozone, air pollution, Adirondack Mountains     

Measurements and modeling of reactive nitrogen deposition in southeast Brazil

Increased reactive nitrogen (N_r) deposition due to expansion of agro-industry was investigated considering emission sources, atmospheric transport and chemical reactions. Measurements of the main inorganic nitrogen species (NO₂, NH₃, HNO₃, and aerosol nitrate and ammonium) were made over a period of one year at six sites distributed across an area of ∼130,000 km² in southeast Brazil. Oxidized species were estimated to account for ∼90% of dry deposited N_r, due to the region’s large emissions of nitrogen oxides from biomass burning and road transport. NO₂-N was important closer to urban areas, however overall HNO₃-N represented the largest component of dry deposited N_r. A simple mathematical modeling procedure was developed to enable estimates of total N_r dry deposition to be made from knowledge of NO₂ concentrations. The technique, whose accuracy here ranged from <1% to 29%, provides a useful new tool for the mapping of reactive nitrogen deposition.     

Land-surface exchange in a chemically-reactive system; surface fluxes of HNO3, HCl and NH3

Vertical gradients from 0.25 to 2 m of NH₃, HNO₃ and HCl and associated aerosol components have been measured in the field above various surfaces in eastern England. The data have been examined to identify the effect, if any, of chemical reaction processes upon the observed vertical profiles. It is concluded that chemical transformations are too slow to influence concentration gradients and thus the surface exchange process. Assuming chemically conservative behaviour, deposition velocities for HNO₃ and HCl have been calculated; these lie within the range 0.4–7.7 cms⁻¹ and 0.4–6.9 cms⁻¹ for HNO₃ and HCl, respectively. Estimation of resistances to deposition indicates a negligible surface resistance for both species. Fluxes of ammonia were predominantly upward from the ground with a mean value of 0.031 μg m⁻²s⁻¹ which is consistent both with an emission inventory of the U.K. and with the measured atmospheric concentration of NH₃.     

Differences between Weekday and Weekend Air Pollutant Levels in Southern California

Abstract

Ambient air quality data were analyzed to empirically evaluate the effects of reductions of volatile organic compounds (VOCs) and oxides of nitrogen (NO_x) emissions on weekday and weekend levels of ozone (O₃; 1991–1998) and particulate NO₃ ^- (1980–1999) in southern California. Despite significantly lower O₃ precursor levels on weekends, 20 of 28 South Coast Air Basin (SoCAB) sites (28 of all 78 southern California sites) showed statistically significant higher mean O₃ levels on Sundays than on weekdays (p < 0.01); 49 of the remaining 50 sites showed no significant differences between mean weekday and Sunday peak O₃ levels. We also observed no statistically significant differences between mean weekday and weekend concentrations of particulate NO₃ ^- or nitric acid (HNO₃, the precursor of particulate NO₃ ^-). Averaged over sites, the mean Sunday NO_x and nonmethane hydrocarbon concentrations were 25–41% and 16–30% lower, respectively, than on weekdays. Site-to-site differences between weekend and weekday mean peak hourly O₃ levels were related to whether O₃ formation was limited by the availability of NO_x. A thermodynamic equilibrium model predicts that particulate NO₃ ^- levels would decrease in response to a reduction of HNO₃, and that particulate ammonium NO₃ ^- formation was not limited by the availability of ammonia. The similarity of mean weekday and weekend levels of NO₃ ^- therefore did not result from limitations on the formation of particulate NO₃ ^- from its precursor, HNO₃.     

The Clean Air Act Amendments and Motor Vehicle Inspection/Maintenance

Seventeen days of detailed measurements of NO, NO₂, O₃, HNO₃ and the frequency of NO₂ photolysis—j(NO₂)—were carried out in Claremont, CA, in September 1980. Under conditions when the rate of change of NO concentration is small, there must be a balance between formation and loss processes. In the classical photostationary state this balance is between NO₂ photolysis: NO₂ + hv → NO + O and reaction with ozone: NO + O₃ → NO₂ + O₂. The results show that the latter reaction with ozone is inadequate to balance the formation step; a significant contribution is required from another NO oxidation process, possibly peroxy radical oxidation. If so, the inferred concentration of peroxy radicals shows a diurnal variation, peaking around solar noon.     

The characteristics of ozone and related compounds in the boundary layer of the South China coast: temporal and vertical variations during autumn season

We present measurements of several trace gases made at a subtropical coastal site in Hong Kong in October and November 1997. The gases include O₃, CO, SO₂, and NO_x. The surface measurement data are compared with those from an aircraft study [Kok et al. J. Geophys. Res. 102 (D15) (1997) 19043–19057], and a subset of the latter is used to show the vertical distribution of the trace gases in the boundary layer. During the study period, averaged concentrations at the surface site for O₃, CO, NO_x, and SO₂ were 50, 298, 2.75, and 1.65 ppbv, respectively. Their atmospheric abundance and diurnal pattern are similar to those found in the “polluted” rural areas in North America. The measured trace gases are fairly well mixed in the coastal boundary layer in the warm South China region. Large variability is indicated from the data. Examination of 10-day, isentropic back trajectories shows that the measured trace gases are influenced by maritime air masses, outflow of pollution-laden continental air, and the mixing of the two. The trajectories capture the contrasting chemical features of the large-scale air masses impacting on the study area. CO, NO_x and SO₂ all show higher concentrations in the strong outflow of continental air, as expected, than those in the marine category. Compared with previously reported values for the western Pacific, the much higher levels found in the marine trajectories in our study suggest the impacts of regional and/or sub-regional emissions on the measured trace gases at the study site. The presence of abundant O₃ and other chemically active trace gases in the autumn season, coupled with high solar radiation and warm weather, suggests that the South China Sea is a photochemically active region important for studying the chemical transformation of pollutants emitted from the Asian continent.     

Factors influencing ozone chemistry in subsonic aircraft plumes

This study investigates several factors that could influence ozone chemistry occurring in subsonic aircraft plumes in the upper troposphere. The study focuses on uncertainties in gas-phase rate parameters, but also examines the influence of selected heterogeneous reactions, the rate of expansion of the plume, ambient and initial plume concentrations, and the time of emissions. Monte Carlo analysis with Latin hypercube sampling was applied to an expanding box model of an aircraft plume, in order to estimate the sensitivities of O₃ perturbations (ΔO₃) to uncertainties in rate constants in the RADM2 chemical mechanism. The resulting coefficient of variation in ΔO₃ at the end of a 36 h simulation was about 50%. Influential uncertainties in gas-phase rate parameters include those for photolysis of NO₂ and HCHO, O₃+NO, HO₂+NO, and formation of PAN and HNO₃. With high background concentrations of non-methane hydrocarbons, uncertainties in rate parameters of reactions involving peroxy radicals from ethene and propene oxidation were also influential. The coefficient of variation for ΔO₃ due to uncertainties in emission indices of NO_x, CO, and organic compounds was less than 15%. The effects of the heterogeneous reaction of N₂O₅ leading to HNO₃ formation, and hypothesized reactions of HNO₃ and NO₂ on soot, were also investigated. The results suggest that the latter two reactions could be influential for ΔO₃ if published estimates of reaction probabilities and high estimates of soot concentrations in plumes are realistic.     

Description and evaluation of a model of deposition velocities for routine estimates of dry deposition over North America. Part II: review of past measurements and model results

Numerical sensitivity tests and four months of complete model runs have been conducted for the Routine Deposition Model (RDM). The influence of individual model inputs on dry deposition velocity as a function of land-use category (LUC) and pollutant (SO₂, O₃, SO²⁻₄ and HNO₃) were examined over a realistic range of values for solar radiation, stability and wind speed. Spatial and temporal variations in RDM deposition velocity (V_d) during June – September 1996 time period generated using meteorological input from a mesoscale model run at 35 km resolution over north-eastern North America were also examined. Comparison of RDM V_d values to a variety of measurements of dry deposition velocities of SO₂, O₃, SO²⁻₄ and NHO₃ that have been reported in the literature demonstrated that RDM produces realistic results. Over northeastern NA RDM monthly averaged dry deposition velocities for SO₂ vary from 0.2 to 3.0 cm s⁻¹ with the highest deposition velocities over water surfaces. For O₃, the monthly averaged dry deposition velocities are from 0.05 to 1.0 cm s⁻¹ with the lowest values over water surfaces and the highest over forested areas. For HNO₃, the monthly averaged dry deposition velocities have the range of 0.5 to 6 cm s⁻¹, with the highest values for forested areas. For SO²⁻₄, they range from 0.05–1.5 cm s⁻¹, with the lowest values over water and the highest over forest. The monthly averaged dry deposition velocities for SO₂ and O₃ are higher in the growing season compared to the fall, but this behaviour is not apparent for HNO₃ and sulphate. In the daytime, the hourly averaged dry deposition velocities for SO₂, O₃, SO²⁻₄ and HNO₃ are higher than that in the nighttime over most of the vegetated area. The diurnal variation is most evident for surfaces with large values for leaf area index (LAI), such as forests. Based on the results presented in this paper, it is concluded that RDM V_d values can be combined with measured air concentrations over hourly, daily or weekly periods to determine dry deposition amounts and with wet deposition measurements to provide seasonal estimates of total deposition and estimates of the relative importance of dry deposition.     

Precision of atmospheric dry deposition data from the Clean Air Status and Trends Network

A collocated, dry deposition sampling program was begun in January 1987 by the US Environmental Protection Agency to provide ongoing estimates of the overall precision of dry deposition and supporting data entering the Clean Air Status and Trends Network (CASTNet) archive. Duplicate sets of dry deposition sampling instruments were installed adjacent to existing instruments and have been operated for various periods at 11 collocated field sites. All sampling and operations were performed using standard CASTNet procedures. The current study documents the bias-corrected precision of CASTNet data based on collocated measurements made at paired sampling sites representative of sites across the network. These precision estimates include the variability for all operations from sampling to data storage in the archive. Precision estimates are provided for hourly, instrumental ozone (O₃) concentration and meteorological measurements, hourly model estimates of deposition velocity (V_d) from collocated measurements of model inputs, hourly O₃ deposition estimates, weekly filter pack determinations of selected atmospheric chemical species, and weekly estimates of V_d and deposition for each monitored filter pack chemical species and O₃.Estimates of variability of weekly pollutant concentrations, expressed as coefficients of variation, depend on chemical species: NO₃⁻∼8.1%; HNO₃∼6.4%; SO₂∼4.3%; NH₄⁺∼3.7%; SO₄²⁻∼2.3%; and O₃∼1.3%. Precision of estimates of weekly V_d from collocated measurements of model inputs also depends on the chemical species: aerosols ∼2.8%; HNO₃∼2.6%; SO₂∼3.0%; and O₃∼2.0%. Corresponding precision of weekly deposition estimates are: NO₃⁻∼8.6%; HNO₃⁻∼5.2%; SO₂∼5.6%; NH₄⁺∼3.9%; SO₄²⁻∼3.5%; and O₃∼3.3%. Precision of weekly concentration, V_d estimates, and deposition estimates are comparable in magnitude and slightly smaller than the corresponding hourly values. Annual precision estimates, although uncertain due to their small sample size in the current study, are consistent with the corresponding weekly values.     

Variability of indicator values for ozone production sensitivity: a model study in Switzerland and San Joaquin Valley (California)

The threshold values of indicator species and ratios delineating the transition between NO_x and VOC sensitivity of ozone formation are assumed to be universal by various investigators. However, our previous studies suggested that threshold values might vary according to the locations and conditions. In this study, threshold values derived from various model simulations at two different locations (the area of Switzerland by UAM Model and San Joaquin Valley of Central California by SAQM Model) are examined using a new approach for defining NO_x and VOC sensitive regimes. Possible definitions for the distinction of NO_x and VOC sensitive ozone production regimes are given. The dependence of the threshold values for indicators and indicator ratios such as NO_y, O₃/NO_z, HCHO/NO_y, and H₂O₂/HNO₃ on the definition of NO_x and VOC sensitivity is discussed. Then the variations of threshold values under low emission conditions and in two different days are examined in both areas to check whether the models respond consistently to changes in environmental conditions. In both cases, threshold values are shifted similarly when emissions are reduced. Changes in the wind fields and aging of the photochemical oxidants seem to cause the day-to-day variation of the threshold values. O₃/NO_z and HCHO/NO_y indicators are predicted to be unsatisfactory to separate the NO_x and VOC sensitive regimes. Although NO_y and H₂O₂/HNO₃ provide a good separation of the two regimes, threshold values are affected by changes in the environmental conditions studied in this work.     

Uptake of nitrous acid and nitrogen oxides by nylon surfaces: Implications for nitric acid measurement

The interference in HNO₃ determination due to HNO₂ and NO_x retention on nylon filters has been evaluated in laboratory and field conditions. Nitrous acid is retained on nylon filters with efficiencies varying from 25% at 12ℓ min⁻¹ to 80% at 2ℓ min⁻¹, yielding NO₂⁻ ion. In ambient sampling performed during photochemical smog episodes, NO₂⁻ is oxidized to NO₃⁻ with conversion factors up to 100%, resulting in a positive bias in HNO₃ determination.NO₂ reacts heterogeneously with H₂O on nylon surfaces according to the reaction 2NO₂ + H₂O → HNO₂ + HNO₃ with a removal constant of about 1 × 10⁻⁴ ms⁻¹ at a H₂O concentration of 20,000 ppm. The resulting nitrite and nitrate are independent of the sampling flow rate, while NO₂ concentration, sampling time and exposed nylon surface area play a directly proportional role. Accordingly, the relative interference of NO₂ with respect to HNO₃ determination is almost negligible for nylon filters, usually run at relatively high flow rates, while it may be significant for nylon denuders, which are characterized by larger exposed surfaces and lower operating flow rates.     

An assessment of the polar HOx photochemical budget based on 2003 Summit Greenland field observations

An interpretative modeling analysis is conducted to simulate the diurnal variations in OH and HO₂+RO₂ observed at Summit, Greenland in 2003. The main goal is to assess the HO_x budget and to quantify the impact of snow emissions on ambient HO_x as well as on CH₂O and H₂O₂. This analysis is based on composite diurnal profiles of HO_x precursors recorded during a 3-day period (July 7–9), which were generally compatible with values reported in earlier studies. The model simulations can reproduce the observed diurnal variation in HO₂+RO₂ when they are constrained by observations of H₂O₂ and CH₂O. By contrast, model predictions of OH were about factor of 2 higher than the observed values. Modeling analysis of H₂O₂ suggests that its distinct diurnal variation is likely controlled by snow emissions and loss by deposition and/or scavenging. Similarly, deposition and/or scavenging sinks are needed to reproduce the observed diel profile in CH₂O. This study suggests that for the Summit 2003 period snow emissions contribute ∼25% of the total CH₂O production, while photochemical oxidation of hydrocarbon appears to be the dominant source. A budget assessment of HO_x radicals shows that primary production from O(¹D)+H₂O and photolysis of snow emitted precursors (i.e., H₂O₂ and CH₂O) are the largest primary HO_x sources at Summit, contributing 41% and 40%, respectively. The snow contribution to the HO_x budget is mostly in the form of emissions of H₂O₂. The dominant HO_x sink involves the HO₂+HO₂ reaction forming H₂O₂, followed by its deposition to snow. These results differ from those previously reported for the South Pole (SP), in that primary production of HO_x was shown to be largely driven by both the photolysis of CH₂O and H₂O₂ emissions (46%) with smaller contributions coming from the oxidation of CH₄ and the O(¹D)+H₂O reaction (i.e., 27% each). In sharp contrast to the findings at Summit in 2003, due to the much higher levels of NO_x, the SP HO_x sinks are dominated by HO_x–NO_x reactions, leading to the formation and deposition of HNO₃ and HO₂NO₂. Thus, a comparison between SP and Summit studies suggests that snow emissions appear to play a prominent role in controlling primary HO_x production in both environments. However, as regards to maintaining highly elevated levels of OH, the two environments differ substantially. At Summit the elevated rate for primary production of HO_x is most important; whereas, at SP it is the rapid recycling of the more prevalent HO₂ radical, through reaction with NO, back to OH that is primarily responsible.