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.     

Vertical distribution of gases and aerosols: The behaviour of ammonia and related components in the lower atmosphere

Vertical concentration profiles for NH₃, HNO₃ and HCl-gas and for NH₄⁺, NO₃⁻, SO²⁻₄, Cl⁻ and Na⁺ aerosol were obtained from a meteorological tower in the central part of the Netherlands. An upward NH₃ flux of 0.12 μgm⁻² s⁻¹ was calculated from the NH₃ profiles and meteorological data. From the HNO₃ profiles a maximum HNO₃ dry deposition velocity of 4 cm s⁻¹ was calculated. Good agreement was found between the measured concentration products [NH₃]_(g) × [HNO₃]_(g) and the theoretical values at temperatures above 0°C and relative humidities below 80%. In other cases, higher NH₃ and/or HNO concentrations in the gas phase were measured than theoretically predicted.     

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.     

Numerical studies of acidification processes within and below clouds with a flow-through chemical reactor model

A flow-through chemical reactor model has been exercised to assess the importance of various oxidation reactions and cloud processes on wet removal and redistribution of atmospheric pollutants and to investigate the effect of in-cloud acidification on precipitation chemistry at the surface. Preliminary results indicate that in-cloud acidification accounts for more than 60% of the wet deposited acids derived from acidification of initial SO₂, that 42–57% of water-soluble, non-reactive NH₃ and HNO₃ are removed by wet deposition. The pseudo-first-order conversion rate of SO₂ to SO₄²⁻ ranges from 3 to 25% h ⁻¹ depending on initial and boundary conditions.Sensitivity studies have been carried out to test the importance of time evolution of clouds on partitioning of pollutants in the atmosphere and to investigate the variability of precipitation chemistry due to changes in rate constants. The distributions of NH₃ and HNO₃ are found to be dependent largely on the cloud microphysical parameters, while the distributions of H₂O₂ and SO₂ depend largely on initial conditions of both species. Individual physical and chemical mechanisms can determine the overall rate of sulfate wet deposition at different stages of cloud evolution.     

Atmospheric concentrations of ammonia and ammonium at an agricultural site in the southeast United States

In this study, we present ∼1 yr (October 1998–September 1999) of 12-hour mean ammonia (NH₃), ammonium (NH₄⁺), hydrochloric acid (HCl), chloride (Cl⁻), nitrate (NO₃⁻), nitric acid (HNO₃), nitrous acid (HONO), sulfate (SO₄²⁻), and sulfur dioxide (SO₂) concentrations measured at an agricultural site in North Carolina's Coastal Plain region. Mean gas concentrations were 0.46, 1.21, 0.54, 5.55, and 4.15 μg m⁻³ for HCl, HNO₃, HONO, NH₃, and SO₂, respectively. Mean aerosol concentrations were 1.44, 1.23, 0.08, and 3.37 μg m⁻³ for NH₄⁺, NO₃⁻, Cl⁻, and SO₄²⁻, respectively. Ammonia, NH₄⁺, HNO₃, and SO₄²⁻ exhibit higher concentrations during the summer, while higher SO₂ concentrations occur during winter. A meteorology-based multivariate regression model using temperature, wind speed, and wind direction explains 76% of the variation in 12-hour mean NH₃ concentrations (n=601). Ammonia concentration increases exponentially with temperature, which explains the majority of variation (54%) in 12-hour mean NH₃ concentrations. Dependence of NH₃ concentration on wind direction suggests a local source influence. Ammonia accounts for >70% of NH_x (NH_x=NH₃+NH₄⁺) during all seasons. Ammonium nitrate and sulfate aerosol formation does not appear to be NH₃ limited. Sulfate is primarily associated ammonium sulfate, rather than bisulfate, except during the winter when the ratio of NO₃⁻–NH₄⁺ is ∼0.66. The annual average NO₃⁻–NH₄⁺ ratio is ∼0.25.     

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.     

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.     

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.     

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.     

Ammonia concentrations and fluxes over a forest in the midwestern USA

We present measurements of ammonia (NH₃) over a deciduous forest in southern Indiana collected during four field campaigns; two in the spring during the transition to leaf-out and two during the winter. Above canopy NH₃ concentrations measured continuously using two Wet Effluent Diffusion Denuders indicate mean concentrations of 0.6–1.2 μg m⁻³ during the spring and 0.3 μg m⁻³ during the winter. Measurements suggest that on average the forest act as a sink of NH₃, with a representative daily deposition flux of 1.8 mg-NH₃ m⁻² during the spring. However, on some days during the spring inverted concentration gradients of NH₃ were observed resulting in an apparent upward flux of nearly 0.2 mg-NH₃ m⁻² h⁻¹. Analyses suggest that this apparent emission flux may be due to canopy emission but evaporation of ammonium nitrate particles may also be partly responsible for the observed inverted concentration gradients.     

Deposition of sulphur,nitrogen and acidity in precipitation over Ireland: chemistry,spatial distribution and long-term trends

The chemical composition of pollutant species in precipitation sampled daily or weekly at 10 sites in Ireland for the five-year period, 1994–1998, is presented. Sea salts accounted for 81% of the total ionic concentration. Approximately 50% of the SO₄²⁻ in precipitation was from sea-salt sources. The proportion of sea salts in precipitation decreased sharply eastwards. In contrast, the concentration of NO₃⁻ and the proportion of non-sea-salt SO₄²⁻ increased eastwards reflecting the closer proximity to major emission sources. The mean (mol_c) ratio of SO₄²⁻:NO₃⁻ was 1.6 for all sites, indicating that SO₄²⁻ was the major acid anion.The spatial correlation between SO₄²⁻, NO₃⁻ and NH₄⁺ concentrations in precipitation was statistically significant. The regional trend in NO₃⁻ concentration was best described by linear regression against easting. SO₄²⁻ concentration followed a similar pattern. However, the regression was improved by inclusion of elevation. Inclusion of northing in the regression did not significantly improve any of the relationships except for NH₄⁺, indicating a significant increase in concentrations from northwest to southeast.The spatial distribution of deposition fluxes showed similar gradients increasing from west and southwest to east and northeast. However, the pattern of deposition shows the influence of precipitation volume in determining the overall input. Mean depositions of sulphur and nitrogen in precipitation were ≈30 ktonnes S yr⁻¹ and 48 ktonnes N yr⁻¹ over the five-year period, 1994–1998, for Ireland.Least-squares linear regression analysis indicated a slight decreasing trend in precipitation concentrations for SO₄²⁻ (20%), NO₃⁻ (13%) and H⁺ (24%) and a slight increasing trend for NH₄⁺ (15%), over the period 1991–1998.     

Nitric acid-air diffusion coefficient: Experimental determination

Trace gaseous HNO₃ in air is removed in a laminar flow nylon tube. The HNO₃ deposition pattern was obtained by sectioning the tube, extracting with an aqueous solution, and measuring the concentration by ion chromatography. Mass transport analysis of the deposition pattern demonstrated the HNO₃ was removed from the air stream at a rate controlled by gaseous diffusion. The HNO₃-air diffusion coefficient = 0.118 ± 0.003 cm² s⁻¹ (n = 7) for T = 298 K and P = 1 atm. It exhibited no dependence on relative humidity over the range 5–95 %.     

Concentration-dependent NH3 deposition processes for mixed moorland semi-natural vegetation

Dry deposition modelling typically assumes that canopy resistance (R_c) is independent of ammonia (NH₃) concentration. An innovative flux chamber system was used to provide accurate continuous measurements of NH₃ deposition to a moorland composed of a mixture of Calluna vulgaris (L.) Hull, Eriophorum vaginatum L. and Sphagnum spp. Ammonia was applied at a wide range of concentrations (1–100 μg m⁻³). The physical and environmental properties and the testing of the chamber are described, as well as results for the moorland vegetation using the ‘canopy resistance’ and ‘canopy compensation point’ interpretations of the data.Results for moorland plant species demonstrate that NH₃ concentration directly affects the rate of NH₃ deposition to the vegetation canopy, with R_c and cuticular resistance (R_w) increasing with increasing NH₃ concentrations. Differences in R_c were found between night and day: during the night R_c increases from 17 s m⁻¹ at 10 μg m⁻³ to 95 s m⁻¹ at 80 μg m⁻³, whereas during the day R_c increases from 17 s m⁻¹ at 10 μg m⁻³ to 48 s m⁻¹ at 80 μg m⁻³. The lower resistance during the day is caused by the stomata being open and available as a deposition route to the plant. R_w increased with increasing NH₃ concentrations and was not significantly different between day and night (at 80 μg m⁻³ NH₃ day R_w=88 s m⁻¹ and night R_w=95 s m⁻¹). The results demonstrate that assessments using fixed R_c will over-estimate NH₃ deposition at high concentrations (over ∼15 μg m⁻³).     

An investigation of the formation of ambient NH4NO3 aerosol

The aerosol equilibrium formulation of Stelson and Seinfeld (1982a, b, Atmospheric Environment16, 983–992, 993–1000) is incorporated into the STEM-II transport/chemistry model and is evaluated against NH₃, HNO₃ and aerosol NH₄⁺ and NO₃⁻ measured at Nagano Prefecture, Japan on 29 and 30 July 1983. These results indicate that this modeling approach is useful in analyzing field data.     

A theoretical estimate of O3 and H2O2 dry deposition over the northeast United States

Estimates of short-term, regional-scale spatial distributions of ozone (O₃) and hydrogen peroxide (H₂O₂) dry deposition over the northeast U.S. are presented. Dry deposition fluxes to surfaces are computed using a regional tropospheric chemistry model with deposition velocities which vary with local meteorology, land type, insolation, seasonal factors and surface wetness. A compilation of O₃ surface resistances is presented based on a survey of O₃ dry deposition measurements. The surface resistance for H₂O₂ is assumed to be small under most conditions, causing H₂O₂ to dry deposit at a rate which is frequently limited by surface-layer turbulence. Regional patterns of dry deposition velocities for these oxidants over the northeast U.S. are computed using landuse data and meteorological information predicted using a mesoscale meteorology model. Domain-averaged O₃ deposition velocities during a spring period reach a mid-day peak of 0.7–0.8 cm s⁻¹ and drop to 0.1–0.2 cm s⁻¹ at night. Domain-averaged H₂O₂ deposition velocities at a height of approximately 80 m are predicted to reach a mid-day peak of 1.6–2.0cm s⁻¹, and fall to 0.6–0.9 cm s⁻¹ at night. Time-averaged surface-layer H₂O₂ concentrations show a latitude dependence, with higher concentrations in the south. H₂O₂ concentrations are significantly reduced due to efficient wet removal and chemical destruction during the passage of a cyclonic frontal system. In contrast, O₃ concentrations are predicted to rise during the passage of a frontal system due to efficient vertical exchange of midtropospheric air into the boundary layer during convective conditions, followed by synoptic-scale subsidence occurring in the high pressure airmass following a cyclone. Maximum O₃ deposition during this 3-day springtime period occurs in polluted agricultural areas. In contrast, H₂O₂ dry deposition exhibits a latitude dependence with maximum 3-day accumulations occurring in the south. Domain-averaged mid-day deposition rates for O₃ and H₂O₂ were 45–50 μmol m⁻² h⁻¹ and 4–5 μmol m⁻² h⁻¹. At night, deposition rates were approximately 5–10 μmol m⁻² h⁻¹ and 1.5–2.5 μmol m⁻² h⁻¹ for O₃ and H₂O₂. These model results show that regional patterns of oxidant dry deposition are strongly influenced by oxidant concentrations, atmospheric stability, surface roughness and numerous other surface and meteorological factors. Each of these factors must be well-characterized before regional patterns of biological damage associated with oxidant dry deposition can be quantified.     

Characterization of atmospheric air pollutants at two sites in northern Kyushu,Japan – chemical form,and chemical reaction

Airborne gaseous and particulate matter in winter was measured over for 37 days in January and December 1997 at 2 sampling sites in northern Kyushu, Japan. One sampling site, Goto Island (an isolated island in the East China Sea), was about 200 km southwest of the other sampling site, Dazaifu city. In winter, acidic sulfates generated over the East Asian continent were transported to northwest Kyushu, to places such as Goto Island and the inland Kyushu area, and high sulfate concentrations were observed at the 2 sampling sites when strong NW winds blew. Acidity around Goto was mainly influenced by particulate NH₄HSO₄. The concentrations of NH₃ at Goto Island were lower than at Dazaifu city. The difference in NH₃ levels at the 2 sampling sites plays an important role in the chemical forms and sizes of the particulate matter. Nitrates at Goto Island were mostly present as NaNO₃ and Ca(NO₃)₂ in coarse-size particles. During the process of long-range transport of air pollutants from the Asian continent to Goto, gaseous HNO₃ was produced by a photochemical reactions of nitrogen oxides in the atmosphere, and particulate NaNO₃ and gaseous HCl were formed by a chlorine-loss reaction between NaCl and gaseous HNO₃. When strong NW winds blew, acidic sulfates together with some of the NaNO₃ and/or Ca(NO₃)₂ and some of gaseous HCl and HNO₃, which exist in the sea to the west of Kyushu and Goto Island, were transported to inland Kyushu such as Dazaifu city. During the process of transport, most of the acidic sulfates and acidic gases were mixed with regional air pollutants such as chlorides and nitrates existing around Dazaifu city, and neutralized forming (NH₄)₂SO₄, NH₄Cl and NH₄NO₃ in an environment of excess NH₃. Therefore, the main chemical forms of NO₃⁻ at Dazaifu city varied day-by-day from fine-sized NH₄NO₃ to coarse-sized NaNO₃ and/or Ca(NO₃)₂. The appearance of NO₃⁻ in coarse-size particles at Dazaifu city was due to the transport of NO₃⁻ from around the sea to the west of Kyushu.     

Concentration-dependent NH3 deposition processes for moorland plant species with and without stomata

Currently, in operational modelling of NH₃ deposition a fixed value of canopy resistance (R_c) is generally applied, irrespective of the plant species and NH₃ concentration. This study determined the effect of NH₃ concentration on deposition processes to individual moorland species. An innovative flux chamber system was used to provide accurate continuous measurements of NH₃ deposition to Deschampsia cespitosa (L.) Beauv., Calluna vulgaris (L.) Hull, Eriophorum vaginatum L., Cladonia spp., Sphagnum spp., and Pleurozium schreberi (Brid.) Mitt. Measurements were conducted across a wide range of NH₃ concentrations (1–140 μg m⁻³).NH₃ concentration directly affects the deposition processes to the vegetation canopy, with R_c, and cuticular resistance (R_w) increasing with increasing NH₃ concentration, for all the species and vegetation communities tested. For example, the R_c for C. vulgaris increased from 14 s m⁻¹ at 2 μg m⁻³ to 112 s m⁻¹ at 80 μg m⁻³. Diurnal variations in NH₃ uptake were observed for higher plants, due to stomatal uptake; however, no diurnal variations were shown for non-stomatal plants. R_c for C. vulgaris at 80 μg m⁻³ was 66 and 112 s m⁻¹ during day and night, respectively. Differences were found in NH₃ deposition between plant species and vegetation communities: Sphagnum had the lowest R_c (3 s m⁻¹ at 2 μg m⁻³ to 23 at 80 μg m⁻³), and D. cespitosa had the highest nighttime value (18 s m⁻¹ at 2 μg m⁻³ to 197 s m⁻¹ at 80 μg m⁻³).     

Simultaneous measurements of ammonia and nitric acid in ambient air at Agra (27°10′N and 78°05′E) (India)

Simultaneous measurements of ammonia and nitric acid in ambient air were conducted at Dayalbagh, Agra using the mist chamber technique. The sampling site is located near a cattle shed. A total of 120 samples were collected during the period July–September and November–February (1997–1998). Sampling was performed during six different times a day. Gas-phase HNO₃ was estimated as NO₃⁻ using ion chromatographic technique while ammonia was determined colorimetrically as NH₄⁺ using indophenol blue method. The mean levels of NH₃ and HNO₃ for the entire data set were 16.3±2.8 and 1.6±1.4 ppbv, respectively. In the monsoon, mean values for NH₃ and HNO₃ averaged to 16.4±3.5 and 0.9±0.7 ppbv while the winter means were 11.8±4.4 and 2.1±1.2 ppbv, respectively. Concentration of both the species (NH₃ and HNO₃) did not show any significant diurnal behaviour in both the seasons. However, concentration of both NH₃ and HNO₃ were lower at dawn than the previous night's value. This has been ascribed to their removal through dew. Concentrations of HNO₃ are observed to increase during the daytime, consistent with its formation by photochemical reactions. Nitric acid and ammonia concentrations show a significant seasonal variation. Levels of HNO₃ are higher in winter but lower in monsoon, while ammonia shows a reverse trend with higher monsoon and lower winter values. Observed trends in nitric acid and ammonia concentration are due to seasonal variation in emission sources, chemistry and meteorology. Gaseous ammonia and nitric acid are in equilibrium with NH₄NO₃ (solid or aqueous) in the atmosphere. The existence of this equilibrium was examined from simultaneous measurements of NH₃ and HNO₃ in the ambient air. It is found that for the monsoon data, measured concentrations are qualitatively below the predicted equilibrium value, while in the winter, concentration product ([NH₃] [HNO₃]) lies consistently above the predicted values. These deviations may be explained due to local sources of both [NH₃] and [HNO₃], presence of coarse nitrate particles and low-temperature and high-humidity conditions.     

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.     

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.