Measurements of nitric acid and ammonium salts in lower Bavaria

Nitric acid and ammonium-containing particulate species were measured by the annular denuder-filter pack technique at Manndorf, a rural site in South Germany, in July 1990. The analyses of filter packs indicated that nitrate was present as ammonium salt which mostly dissociated during sampling. Moreover, due to the NH⁺₄/NO⁻₃ ratios higher than unity found in back-up filters, NH₄Cl was assumed to represent an appreciable fraction of the total particulate ammonium. Finally, the molar ratios NH⁺₄/SO²⁻₄ found on front (Teflon) filters, suggested a large predominance of (NH₄)₂SO₄ among the different forms of sulphate. The concentration levels of gaseous HNO₃ observed in the daytime were characterised by a maximum after midday, whereas particulate nitrate showed five times out of eight days an opposite trend with early afternoon minima. The total nitrate (HNO₃+NH₄NO₃) showed in turn a diurnal pattern similar to that of sulphate. These findings led to the conclusion that a significant HNO₃ production pathway involved the thermal dissociation of NH₄NO₃ rather than the reaction of NO₂ with OH radical.     

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.     

Harmful effects of atmospheric nitrous acid on the physiological status of Scots pine trees

An open top chamber experiment was carried out in the summer of 2003 to examine the effect of nitrous acid (HONO) gas on the physiological status of Scots pine saplings (Pinus sylvestris). Four-year-old pine trees were exposed to two different levels of HONO gas (at ca. 2.5 ppb and 5.0 ppb) and a control (filtered air) from early evening to early morning (18:00-6:00), in duplicate open top chambers. Significant decreases in the ratios of chlorophylls a to b, an increase in the carbon to nitrogen (C/N) ratio, and a reduction of maximum yield of PS II (F(v)/F(m)) in pine needles were also observed after the 2 months' fumigation. Cation contents of pine needles were also decreased by the fumigation with HONO gas. The results could be explained by the harmful effects of OH radicals, generated from photolysis of HONO gas, and/or aqueous phase HONO (NO(2)(-)/HONO), on the photosynthetic capacity of pine needles.     

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

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

Ammonia and nitric acid concentrations in equilibrium with atmospheric aerosols: Experiment vs theory

The equilibrium between gaseous ammonia, nitric acid, and aerosol nitrate is discussed on the basis of a recent field experiment in southern California. Comparison is drawn between theoretical equilibrium calculations and simultaneous measurements of nitric acid, ammonia, ammonium ion, nitrate ion, sulfate ion, other ionic species, temperature and dewpoint. Particulate and gaseous pollutant concentrations at some inland sampling sites are readily explained if the aerosol is assumed to exist as an external mixture with all particulate nitrate and ammonium available to form pure NH₄NO₃. At other monitoring sites, especially near the coast, aerosol nitrate is found in the presence of NH₃ and HNO₃ concentrations that thermodynamic calculations show are too low to produce pure NH₄NO₃. This can be explained when the amount of aerosol nitrate that can be derived from reaction of nitric acid with sea salt and soil dust is taken into account. A calculation approach that accounts for the presence of mixed sulfate and nitrate salts improves the agreement between predicted and observed pollutant concentrations in the majority of cases studied. Uncertainties in these calculations arise from a number of sources including the thermodynamic quantities, and the effect of these uncertainties on the comparison between theory and experiment is discussed.     

A comparison of three methods for measurement of atmospheric nitric acid and aerosol nitrate and ammonium

Three methods for measuring gaseous ambient nitric acid in the low concentration range 0–15 μ m⁻³ were compared under field conditions in Southwestern Ontario during 1–14 June 1982. The methods employed were (1) tunable diode laser absorption, (2) a tungstic acid denuder tube and (3) a filter pack containing a Teflon-nylon-W41 filter combination operated over 3-h sampling periods. In general, the three methods measured nitric acid with adequate sensitivity (sub ppb) and they correlated reasonably over the low ambient concentrations considered here. However, there were some notable differences (up to a factor of 2 at night) by the tungstic acid and filter pack methods. During the field comparison, particulate nitrate and ammonium were also measured (0–6 μg m⁻³ range) by the tungstic acid and filter pack methods. These correlated well but some differences (less than 20%) were observed in the particulate nitrate measurements. In view of the low concentrations measured and the resultant larger experimental error, specific loss or interfering mechanisms could not be unambiguously identified.     

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

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

Performance evaluation of an in situ nitrous acid measurement system and continuous measurement of nitrous acid in an indoor environment

Nitrous acid (HONO) may cause adverse effects to mucous membranes and lung function when people are exposed to higher HONO concentrations than those present in typical indoor residential environments. Therefore, determination of HONO concentration in indoor environments is required to investigate occurrences of high HONO levels. In this work, a high-time-resolution measurement system was utilized to better understand the levels and dynamic behavior of HONO in an indoor environment. The performance of the in situ HONO analyzer applied to this work was evaluated using a 12-hr integrated annular denuder technique under ambient conditions. Both methods for the measurements of HONO were in good agreement, with a regression slope of 0.84, an intercept of 0.09, and correlation coefficient (r2) of 0.67. Indoor HONO and nitrogen oxide concentrations were also observed for approximately 5 days in winter in the living room of an apartment that had a gas range for cooking in the kitchen. Investigation of the relationships among nitric oxide (NO), nitrite (NO2), and HONO concentrations suggests that HONO production during combustion could be the result of direct emission, whereas the heterogeneous NO2 chemistry during the background period and after combustion was the possible pathway of HONO production. Controlled combustion experiments, performed at a burning rate of 50% valve setting, show peak HONO concentrations during the unvented combustion to be approximately 8-10 times higher than background levels depending on the time of day. At a burning rate setting of 50%, the peak concentration of HONO during unvented combustion was found to be 33-37% higher than those from "weak" (airflow = 340 m3/hr) and "strong" (airflow = 540 m3/hr) vented combustions. The decay rate of the HONO concentrations for the unvented combustion conditions was approximately 2-fold higher in the daytime than in the nighttime and significantly less than those of NO and NO2.     

Identification and measurement of nitrous acid in an indoor environment

We report here direct observation by differential optical absorption spectroscopy (DOAS) of the formation of ppb levels of gaseous nitrous acid (MONO) from the reaction of ppm levels of nitrogen dioxide (NO₂) with water vapor, in an indoor environment. The rate of formation of HONO displayed first order kinetics with respect to NO₂ with a rate of (0.25 ±0.04) ppb min⁻¹ per ppm of NO₂ present. Assuming a lifetime of l h with respect to both physical and chemical removal processes for HONO, this leads to an estimated steady state concentration of ~ 15 ppb of HONO per ppm of NO₂ present. This relatively high level of HONO associated with NO₂-air mixtures raises new questions concerning the health implications of elevated NO₂ concentrations in indoor environments e.g. HONO is a respirable nitrite known to convert secondary amines in vitro to carcinogenic nitrosamines.     

Nitric oxide and nitrous oxide emission from Hungarian forest soils; linked with atmospheric N-deposition

Studies of forest nitrogen (N) budgets generally measure inputs from the atmosphere in wet and dry deposition and outputs via hydrologic export. Although denitrification has been shown to be important in many wetland ecosystems, emission of N oxides from forest soils is an important, and often overlooked, component of an ecosystem N budget. During 1 year (2002–03), emissions of nitric oxide (NO) and nitrous oxide (N₂O) were measured from Sessile oak and Norway spruce forest soils in northeast Hungary. Accumulation in small static chambers followed by gas chromatography-mass spectrometry detection was used for the estimation of N₂O emission flux. Because there are rapid chemical reactions of NO and ozone, small dynamic chambers were used for in situ NO flux measurements. Average soil emissions of NO were 1.2 and 2.1 μg N m⁻² h⁻¹, and for N₂O were 15 and 20 μg N m⁻² h⁻¹, for spruce and oak soils, respectively. Due to the relatively high soil water content, and low C/N ratio in soil, denitrification processes dominate, resulting in an order of magnitude greater N₂O emission rate compared to NO. The previously determined N balance between the atmosphere and the forest ecosystem was re-calculated using these soil emission figures. The total (dry+wet) atmospheric N-deposition to the soil was 1.42 and 1.59 g N m⁻² yr⁻¹ for spruce and oak, respectively, while the soil emissions are 0.14 and 0.20 g N m⁻² yr⁻¹. Thus, about 10–13% of N compounds deposited to the soil, mostly as and , were transformed in the soil and emitted back to the atmosphere, mostly as greenhouse gas (N₂O).     

Humic acid in ice: Photo-enhanced conversion of nitrogen dioxide into nitrous acid

Laboratory studies on the heterogeneous conversion of nitrogen dioxide into nitrous acid on irradiated ice films containing humic acid are described. It was found that the presence of light in the visible range of the solar spectrum significantly enhances the rate of nitrous acid release from a humic acid doped ice film. This process might contribute to observed HONO production in snow, where the NO₂ is thought to originate from nitrate photolysis. Analysis of the experimental data based on the Langmuir Hinshelwood model framework allowed quantification of the observed dependencies of the nitrous acid production rate on nitrogen dioxide concentration. The observed dependencies on the humic acid concentration as well as on the irradiation intensity were used to estimate light-driven HONO fluxes for environmental snow covers.     

Spectroscopic identification and measurement of gaseous nitrous acid in dilute auto exhaust

We report here the direct spectroscopic detection of gaseous nitrous acid (HONO) in exhaust emissions from certain light duty motor vehicles (LDMV). Co-pollutants such as nitrogen dioxide (NO₂), formaldehyde (HCHO), benzaldehyde (C₆H₅CHO) and sulfur dioxide (SO₂) were also readily determined. Nitric oxide (NO) was measured too, but with reduced accuracy. To avoid possible artifactual formation of HONO on the surfaces of conventional dilution and sampling systems (e.g. Federal and California constant volume sampling systems), an instrument was developed consisting of a multiple reflection cell without walls coupled to a u.v. differential optical absorption spectrometer (DOAS), the entire system being placed in the open air ~ 2 m from the tailpipe of the LDMV. At an optical path of 31.2 m, detection limits (in parts per 10⁹, ppb) were: HONO-12; HCHO-78; C₆H₅CHO-13; NO₂-57; and SO₂-11. With this instrument, HONO levels observed in diluted exhaust ranged from nondetectable (< 12 ppb) for a 1982 California car with an effective 3-way catalyst (and associated low NO_x emissions), to ~ 300 ppb for a heavily used 1974 station wagon having high NO_x emissions and run on leaded gasoline. While the number of LDMV tested was too small for statistical treatment, our results show that the older portion of the total LDMV population (i.e. without current emission control devices) may be a significant primary source of gaseous HONO, a key precursor to photochemical air pollution and an inhalable nitrite.     

Preliminary measurements of summer nitric acid and ammonia concentrations in the Lake Tahoe Basin air-shed: implications for dry deposition of atmospheric nitrogen

Over the past 50 years, Lake Tahoe, an alpine lake located in the Sierra Nevada mountains on the border between California and Nevada, has seen a decline in water clarity. With significant urbanization within its borders and major urban areas 130 km upwind of the prevailing synoptic airflow, it is believed the Lake Tahoe Basin is receiving substantial nitrogen (N) input via atmospheric deposition during summer and fall. We present preliminary inferential flux estimates to both lake surface and forest canopy based on empirical measurements of ambient nitric acid (HNO3), ammonia (NH3), and ammonium nitrate (NH4NO3) concentrations, in an effort to identify the major contributors to and ranges of atmospheric dry N deposition to the Lake Tahoe Basin. Total flux from dry deposition ranges from 1.2 to 8.6 kg N ha-1 for the summer and fall dry season and is significantly higher than wet deposition, which ranges from 1.7 to 2.9 kg N ha-1 year-1. These preliminary results suggest that dry deposition of HNO3 is the major source of atmospheric N deposition for the Lake Tahoe Basin, and that overall N deposition is similar in magnitude to deposition reported for sites exposed to moderate N pollution in the southern California mountains.     

Acid precipitation: The importance of nitric acid

Acid precipitation is caused by H₂SO₄ and HNO₃. Currently, in the north-eastern United States on an annual basis, the maximum contributions of H₂SO₄ and HNO₃ to the acidity of precipitation in summer are 73 and 31% and in winter are 59 and 61%, respectively. Based on statistical analyses of the 15-y record of precipitation composition from the Hubbard Brook Experimental Forest during 1964–1979, the importance of H₂SO₄ has decreased by about 30% relative to HNO₃ and HNO₃ has increased by about 50% relative to H₂SO₄.     

A study of atmospheric acid formation in different environments

The mathematical model presented in this paper describes in detail the gas-phase chemistry (80 reactions), gas-phase/liquid-phase equilibrium (12 equilibria) and liquid-phase chemistry (27 reactions and equilibria) of acid formation. The model is used to simulate acid formation under various conditions:

• 1.(1) nonprecipitating clouds in the Adirondacks,
• 2.(2) raining clouds in the Ohio River valley,
• 3.(3) night-time fog in the Los Angeles basin and
• 4.(4) night-time nonprecipitating clouds in the Los Angeles basin. Model calculations are compared with observed values of sulfate and nitrate formation. The model is used to analyze the relative importance of sulfate and nitrate formation pathways under these various conditions.