The APCA Exhibition

On June 5 and 6 of 1980, two parallel plume oxidation studies were carried out in the vicinity of the Tennessee Valley Authority's Colbert Steam Plant. One study was performed in a smog chamber into which stack gases were injected and mixed with ambient air. The other study included direct airborne sampling of the power plant plume. Atmospheric oxidation rates for the conversion of SO₂ to SO₄ ^2- and the removal rates of NO _x (which is presumably the rate of NO₃ ^- formation) were estimated for both studies. The SO₂ to SO₄ ^2- rate coefficients were found to be 0.022 ± 0.009 h^-1 for both chamber experiments and the first airborne sampling day. For the second day, a rate constant of 0.041 ± 0.052 h^-1 was estimated from the aircraft data. The large deviation in this value is explained by the fact that the plume from the power plant combined and reacted with the urban plume from the city of Florence, AL. The formation of a very large "O3 bulge" on this day is also attributed to the mixed plumes. The first order rate coefficients for NO _x removal were estimated to be 0.27 ± 0.14 h^-1 for both chamber experiments and the first airborne sampling day. A NO _x removal rate could not be determined for the second airborne sampling day.     

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.     

Potential interferences in photolytic nitrogen dioxide converters for ambient air monitoring: Evaluation of a prototype

ABSTRACT

Mixing ratios of the criteria air contaminant nitrogen dioxide (NO₂) are commonly quantified by reduction to nitric oxide (NO) using a photolytic converter followed by NO-O₃ chemiluminescence (CL). In this work, the performance of a photolytic NO₂ converter prototype originally designed for continuous emission monitoring and emitting light at 395 nm was evaluated. Mixing ratios of NO₂ and NO_x (= NO + NO₂) entering and exiting the converter were monitored by blue diode laser cavity ring-down spectroscopy (CRDS). The NO₂ photolysis frequency was determined by measuring the rate of conversion to NO as a function of converter residence time and found to be 4.2 s^?1. A maximum 96% conversion of NO₂ to NO over a large dynamic range was achieved at a residence time of (1.5 ± 0.3) s, independent of relative humidity. Interferences from odd nitrogen (NO_y) species such as peroxyacyl nitrates (PAN; RC(O)O₂NO₂), alkyl nitrates (AN; RONO₂), nitrous acid (HONO), and nitric acid (HNO₃) were evaluated by operating the prototype converter outside its optimum operating range (i.e., at higher pressure and longer residence time) for easier quantification of interferences. Four mechanisms that generate artifacts and interferences were identified as follows: direct photolysis, foremost of HONO at a rate constant of 6% that of NO₂; thermal decomposition, primarily of PAN; surface promoted photochemistry; and secondary chemistry in the connecting tubing. These interferences are likely present to a certain degree in all photolytic converters currently in use but are rarely evaluated or reported. Recommendations for improved performance of photolytic converters include operating at lower cell pressure and higher flow rates, thermal management that ideally results in a match of photolysis cell temperature with ambient conditions, and minimization of connecting tubing length. When properly implemented, these interferences can be made negligibly small when measuring NO₂ in ambient air.

Implications: A new near-UV photolytic converter for measurement of the criteria pollutant nitrogen dioxide (NO₂) in ambient air by CL was characterized. Four mechanisms that generate interferences were identified and investigated experimentally: direct photolysis of HONO which occurred at a rate constant 6% that of NO₂, thermal decomposition of PAN and N₂O₅, surface promoted chemistry involving HNO₃, and secondary chemistry involving NO in the tubing connecting the converter and CL analyzer. These interferences are predicted to occur in all NO₂ P-CL systems but can be avoided by appropriate thermal management and operating at high flow rates.     

Field observations of regional and urban impacts on NO2, ozone,UVB, and nitrate radical production rates in the Phoenix air basin

In the May and June of 1998, field measurements were taken at a site near the Usery Pass Recreation Area, ∼27 miles from the downtown Phoenix area, overlooking Phoenix and Mesa, Arizona. This site was selected to examine the impacts of the Phoenix urban plume on the Usery Pass Recreation Area and surrounding regions. Data were obtained for ultraviolet-B (UVB) radiation, nitrogen dioxide (NO₂), peroxyacetyl nitrate (PAN), ozone (O₃), and carbon monoxide (CO). Nocturnal plumes of NO₂ (in tens of ppb), observed near midnight, were correlated with CO and anti-correlated with O₃. This behavior was consistent with the titration of locally generated NO by boundary layer O₃ to form the nighttime NO₂ plumes that were subsequently transported into the Usery Pass Recreation area. Nitrate radical (NO₃) production rates were calculated to be very high on the edges of these nocturnal plumes. Examination of O₃ and PAN data also indicates that Phoenix is being affected by long-range transport of pollutants from the Los Angeles to San Diego areas. A regional smoke episode was observed in May, accompanied by a decrease in UVB of factor of two and a decrease in O₃ and an increase in methyl chloride. Low level back trajectories and chemical evidence confirm that the smoke event originated in northern Mexico and that the reduced O₃ levels observed at Usery Pass could be partially due to reduced photolysis rates caused by carbonaceous soot aerosols transported in the smoke plume. The results are discussed with regard to potential effects of local pollution transport from the Phoenix air basin as well as an assessment of the contributions from long-range transport of pollutants to the background levels in the Phoenix-Usery Pass area.     

Modeling and monitoring of CO,NO and NO2 in a modern townhouse

Mass-balance models for indoor concentrations of CO, NO and NO₂ were applied to an energy-efficient townhouse. Model parameters included source emission rates, infiltrating airflows, and, for NO₂, loss rate coefficients. Emission factors for CO, NO and NO₂ were estimated for each of the gas-fired appliances in the house. Airflows were estimated using sulfur hexafluoride (SF₆) decay techniques. Loss rates for NO₂ were calculated as the difference between NO₂ removal rates and estimated air exchange rates; CO and NO concentrations decayed at a rate not significantly different than that for SF₆. Comparing model predictions with measured concentrations yielded differences averaging 17% for CO and NO, and 28% for NO₂.     

Partitioning, sources and variability of regional and local oxidant (OX = O3 + NO2) in a coastal rural area in the southwest of Iberian Peninsula

The purpose of this work is to investigate the behaviour and variability of oxidant levels (OX?=?NO₂?+?O₃), for the first time, in a rural coastal area in the southwest of the Iberian Peninsula, affected by several air masses types. Detailed database (built-up over the years 2008 to 2011, and containing around 500,000 data) from the Atmospheric Sounding Station “El Arenosillo” was used. The observed daily cycles of NO _x and OX were influenced by air masses coming from industrial and urban area. It can be seen that the concentration of OX is made up of a NO _x -independent ‘regional’ contribution (i.e. the O₃ background), and a linearly NO _x -dependent ‘local’ contribution from primary emissions, such as traffic. The local emission is very low in this area. Also, the regional contribution is similar to unpolluted sites and presents seasonal variation, being higher in May. However, our measurements showed that the proportion of OX in the form of NO₂ increases with the increase in NO _x concentration during the day. The higher proportion of NO₂ observed at night must be due to the conversion of NO to NO₂ by the NO?+?O₃ reaction. With regards to the source of the local NO _x -dependent contribution, it may be attributed to industrial emission, or the termolecular reaction 2NO?+?O₂?=?2NO₂, at high-NO _x levels and stagnant air during several days. Finally, we estimated the photolysis rate of NO₂, J _NO2, an important key atmospheric reaction coupled with ozone. We also present surface plots of annual variation of the daily mean NO _x and OX levels, which indicate that oxidants come from transport processes instead of local emissions associated as local photochemistry.     

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.     

Nitrogen Dioxide and Nitric Oxide in Non-Urban Air

Measurement of NO₂ and NO has been carried out in Piedmont, N. C. and in the southern Appalachian Mountains. Average values for the Piedmont were: continuous NO₂ measured 30 ft above surface, 0.76 pphm (14.3 μg/m³), 1 20 ft, 0.61 pphm (11.5 μg/m³); simultaneous values (also Piedmont) (2-hr discrete samples) taken at an earlier time at 4 ft, NO₂, 0.56 pphm (10.6 μg/m³), NO, 0.19 pphm (2.34 μg/m³). The mountain top values (5120 ft, 1573 m) were: NO₂, 0.46 pphm (6.4 μg/m³), NO, 0.26 pphm (2.72 μg/m³). The results of this study furnish further proof that tropospheric NO and NO₂ are produced at the surface of the earth. Data obtained are consistent with the belief that a major sink for NO_x is reaction with O3 and ultimate conversion to nitrate.

Ozone values frequently increased and NO_x values decreased ahead of cold fronts, probably as a result of deep vertical mixing. Also, in small scale turbulence the changes in NO_x values and in the O₃ values tended to be “out of phase,” i.e., as O₃ concentration increased, NO_x concentration decreased and vice versa. Values of NO_x from Green Knob, N. C. (mountain top) also tended to be higher at times when O₃ values were lowest.

The NO₂ hourly average values in Piedmont, N. C, demonstrated a diurnal cycle reminiscent of diurnal urban changes. An early morning peak was followed by a minimum in mid-afternoon. Next, the values rose to a broad evening peak and then decreased slowly during the night. Reported urban concentrations are usually about ten times those found in Piedmont, N. C.     

Nitrogen oxide fluxes between corn (Zea mays L.) leaves and the atmosphere

In the United States, fertilized corn fields, which make up approximately 5% of the total land area, account for approximately 45% of total soil NO_x emissions. Leaf chamber measurements were conducted of NO and NO₂ fluxes between individual corn leaves and the atmosphere in (1) field-grown plants near Champaign, IL (USA) in order to assess the potential role of corn canopies in mitigating soil–NO_x emissions to the atmosphere, and (2) greenhouse-grown plants in order to study the influence of various environmental variables and physiological factors on the dynamics of NO₂ flux. In field-grown plants, fluxes of NO were small and inconsistent from plant to plant. At ambient NO concentrations between 0.1 and 0.3 ppbv, average fluxes were zero. At ambient NO concentrations above 1 ppbv, NO uptake occurred, but fluxes were so small (14.3±0.0 pmol m⁻² s⁻¹) as to be insignificant in the NO_x inventory for this site. In field-grown plants, NO₂ was emitted to the atmosphere at ambient NO₂ concentrations below 0.9 ppbv (the NO₂ compensation point), with the highest rate of emission being 50 pmol m⁻² s⁻¹ at 0.2 ppbv. NO₂ was assimilated by corn leaves at ambient NO₂ concentrations above 0.9 ppbv, with the maximum observed uptake rate being 643 pmol m⁻² s⁻¹ at 6 ppbv. When fluxes above 0.9 ppbv are standardized for ambient NO₂ concentration, the resultant deposition velocity was 1.2±0.1 mm s⁻¹. When scaled to the entire corn canopy, NO₂ uptake rates can be estimated to be as much as 27% of the soil-emitted NO_x. In greenhouse-grown and field-grown leaves, NO₂ deposition velocity was dependent on incident photosynthetic photon flux density (PPFD; 400–700 nm), whether measured above or below the NO₂ compensation point. The shape of the PPFD dependence, and its response to ambient humidity in an experiment with greenhouse-grown plants, led to the conclusion that stomatal conductance is a primary determinant of the PPFD response. However, in field-grown leaves, measured NO₂ deposition velocities were always lower than those predicted by a model solely dependent on stomatal conductance. It is concluded that NO₂ uptake rate is highest when N availability is highest, not when the leaf deficit for N is highest. It is also concluded that the primary limitations to leaf-level NO₂ uptake concern both stomatal and mesophyll components.     

Influence of NO2 and dissolved iron on the S(IV) oxidation in synthetic aqueous solution

The influence of dissolved NO₂ and iron on the oxidation rate of S(IV) species in the presence of dissolved oxygen is presented. To match the conditions in the real environment, the concentration of iron in the reaction solution and trace gases in the gas mixture was typical for a polluted atmosphere. The time dependence of HSO₃⁻, SO₄²⁻, NO₂⁻ and NO₃⁻ and the concentration ratio between Fe(II) and total dissolved iron were monitored. Sulphate formation was the most intensive in the presence of an SO₂/NO₂/air gas mixture and Fe(III) in solution. The highest contribution to the overall oxidation was from Fe-catalysed S(IV) autoxidation. The reaction rate in the presence of both components was equal to the sum of the reaction rates when NO₂ and Fe(III) were present separately, indicating that under selected experimental conditions there exist two systems: SO₂/NO₂/air and SO₂/NO₂/air/Fe(III), which are unlikely to interact with each other. The radical chain mechanism can be initiated via reactions Fe(III)–HSO₃⁻ and NO₂–SO₃²⁻/HSO₃⁻.     

A new measurement method for nitrogen oxides in the air using an annular diffusion scrubber coated with titanium dioxide

A new convenient measurement method of nitrogen oxides (NO_x) in the ambient air was developed. The collection of NO_x is performed by an annular diffusion scrubber coated with a mixture of titanium dioxide (TiO₂) and hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) and the analysis is carried out by ion chromatography with conductivity detection. Under ultraviolet light (UV) illumination, TiO₂ produces reactive oxygen species such as super oxide (O₂⁻), hydroxyl radical (OH·) and peroxyhydroxyl radical (HO₂·), by which nitric oxide (NO) is oxidized to nitrogen dioxide (NO₂), and is further oxidized to nitric acid (HNO₃). The yielded HNO₃ and NO₂ are effectively adsorbed on the surface of TiO₂ and hydroxyapatite. The collection efficiencies of NO and NO₂ by the annular diffusion scrubber coated with the catalysts under UV illumination are higher than 98%, respectively, at the air flow rate of 0.2–1.0 l min⁻¹. After the collection of NO_x, by feeding deionized water into the annular diffusion scrubber, HNO₃ and NO₂ which adsorbed on the catalysts are extracted as forms of nitrite ion (NO₂⁻) and nitrate ion (NO₃⁻). The extraction efficiencies of NO and NO₂ are almost 100%. The activity of the washed catalysts can be completely recovered by drying with the purified air. Further, a simultaneous separated measurement of NO and NO₂ can be performed by utilizing the UV illumination dependence. This method was applied to the measurement of NO_x in the ambient air. The NO_x concentration measured by this method was in good agreement with that obtained using the chemiluminescence NO_x analyzer.     

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.     

Transformations,Lifetimes, and Sources of NO2, HONO,and HNO3 in Indoor Environments

Recent research has demonstrated that nitrogen oxides are transformed to nitrogen acids in indoor environments, and that significant concentrations of nitrous acid are present in indoor air. The purpose of the study reported in this paper has been to investigate the sources, chemical transformations and lifetimes of nitrogen oxides and nitrogen acids under the conditions existing in buildings. An unoccupied single family residence was instrumented for monitoring of NO, NO₂, NO_y, MONO, HNO₃, CO, temperature, relative humidity, and air exchange rate. For some experiments, NO₂ and HONO were injected into the house to determine their removal rates and lifetimes. Other experiments investigated the emissions and transformations of nitrogen species from unvented natural gas appliances. We determined that HONO is formed by both direct emissions from combustion processes and reaction of NO₂ with surfaces present indoors. Equilibrium considerations influence the relative contributions of these two sources to the indoor burden of HONO. We determined that the lifetimes of trace nitrogen species varied in the order NO ~ HONO > NO₂ >HNO₃. The lifetimes with respect to reactive processes are on the order of hours for NO and HONO, about an hour for NO₂, and 30 minutes or less for HNO3. The rapid removal of NO₂ and long lifetime of HONO suggest that HONO may represent a significant fraction of the oxidized nitrogen burden in indoor air.     

Estimation of night-time N2O5 concentrations from ambient NO2 and NO3 radical concentrations and the role of N2O5 in night-time chemistry

Dinitrogen pentoxide (N₂O₅), which is present in equilibrium with NO₃ radicals and NO₂, has been recognized for some time as an intermediate in the NO_x chemistry of night-time atmospheres. However, until the advent of long pathlength spectroscopic techniques for the measurement of atmospheric NO₃ radical concentrations, no reliable method for estimating N₂O₅ concentrations has been available. We have calculated maximum night-time N₂O₅ concentrations from the available experimentally determined concentrations of the NO₃ radical and NO₂ in the U.S. and Germany, and find that N₂O₅ concentrations as high as ~ 15 ppb can occur. We have also estimated removal rates for N₂O₅ and for NO₃ radicals during these nights. From data obtained under conditions devoid of point sources of NO_x, upper limit estimates of the homogeneous rate constant for the reaction of N₂O₅ with water vapor are obtained, leading to the conclusion that the homogeneous gas phase rate constant for this reaction is ⩽ 1 × 10⁻²¹ cm³ molecule⁻¹ s⁻¹ at 298 K, consistent with recent environmental chamber data.     

Rates of Conversion of Sulfur Dioxide to Sulfate in a Scrubbed Power Plant Plume

ABSTRACT

The rate of conversion of SO₂ to SO₄ ^2- was re-estimated from measurements made in the plume of the Cumberland power plant, located on the Cumberland River in north-central Tennessee, after installation of flue gas desulfurization (FGD) scrubbers for SO₂ removal in 1994. The ratio of SO₂ to NO_y emissions into the plume has been reduced to ~0.1, compared with a prescrubber value of ~2. To determine whether the SO₂ emissions reduction has correspondingly reduced plume-generated particulate SO₄ ^2- production, we have compared the rates of conversion before and after scrubber installation. The prescrubber estimates were developed from measurements made during the Tennessee Plume Study conducted in the late 1970s. The post-scrubber estimates are based upon two series of research flights in the summers of 1998 and 1999. During two of these flights, the Cumberland plume did not mix with adjacent power plant plumes, enabling rate constants for conversion to be estimated from samples taken in the plume at three downwind distances. Dry deposition losses and the fact the fact that SO₂ is no longer in large excess compared with SO₄ ^2- have been taken into account, and an upper limit for the conversion rate constant was re-estimated based on plume excess aerosol volume. The estimated upper limit values are 0.069 hr^-1 and 0.034 hr^-1 for the 1998 and 1999 data, respectively. The 1999 rate is comparable with earlier values for nonscrubbed plumes, and although the 1998 upper limit value is higher than expected, these estimates do not provide strong evidence for deviation from a linear relationship between SO₂ emissions and SO₄ ^2- formation.     

Measurements of nitrogen oxides from Hudson Bay: Implications for NOx release from snow and ice covered surfaces

Measurements of NO and NO₂ were made at a surface site (55.28 °N, 77.77 °W) near Kuujjuarapik, Canada during February and March 2008. NO_x mixing ratios ranged from near zero to 350 pptv with emission from snow believed to be the dominant source. The amount of NO_x was observed to be dependent on the terrain over which the airmass has passed before reaching the measurement site. The 24 h average NO_x emission rates necessary to reproduce observations were calculated using a zero-dimensional box model giving rates ranging from 6.9 × 10⁸ molecule cm^?2 s^?1 to 1.2 × 10⁹ molecule cm^?2 s^?1 for trajectories over land and from 3.8 × 10⁸ molecule cm^?2 s^?1 to 6.6 × 10⁸ molecule cm^?2 s^?1 for trajectories over sea ice. These emissions are higher than those suggested by previous studies and indicate the importance of lower latitude snowpack emissions. The difference in emission rate for the two types of snow cover shows the importance of snow depth and underlying surface type for the emission potential of snow-covered areas.     

Simulation of Sulfate and Nitrate Chemistry in Power Plant Plumes

ABSTRACT

The rate of formation of secondary particulate matter (PM) in power plant plumes varies as the plume material mixes with the background air. Consequently, the rate of oxidation of sulfur dioxide (SO₂) and nitrogen dioxide (NO₂) to sulfate and nitric acid, respectively, can be very different in plumes and in the background air (i.e., air outside the plume). In addition, the formation of sulfate and nitric acid in a power plant plume is a strong function of the chemical composition of the background air and the prevailing meteorological conditions.

We describe the use of a reactive plume model, the Reactive and Optics Model of Emissions, to simulate sulfate and nitrate formation in a power plant plume for a variety of background conditions. We show that SO₂ and NO₂ oxidation rates are maximum in the background air for volatile organic compound (VOC)-limited airsheds but are maximum at some downwind distance in the plume when the background air is nitrogen oxide (NO_x)-limited. Our analysis also shows that it is essential to obtain measurements of background concentrations of ozone, aldehydes, peroxyacetyl nitrate, and other VOCs to properly describe plume chemistry.     

Formation of chlorinated species through reaction of SO2 with NaClO2 powder and their role in the oxidation of NO and Hg0

This study examines gaseous chlorinated species generated from the reaction of sulfur dioxide (SO₂) with sodium chlorite powder (NaClO_2(s)) to obtain insight into the propensity of this process to enhance NO and Hg⁰ oxidation. A packed bed reactor containing NaClO_2(s) was used and the reaction temperature was set to 130 °C. Initially, we determined that the presence of SO₂ enhances the oxidation of NO and Hg⁰ by reaction with NaClO_2(s). We then introduced NO₂ into the gas mixture as a radical scavenger and determined that the chlorinated species generated by the reaction of SO₂ with NaClO_2(s) are OClO, Cl, ClO, and Cl₂. Based on these results, we suggest that such gaseous chlorinated ones are responsible for the enhancement of NO and Hg⁰ oxidation.     

Nitrogen Oxide Flux from an Agricultural Soil During Winter Fallow in the Upper Coastal Plain of North Carolina,U.S.A.

Abstract

Incorporation of the remaining crop residue, including the root system, of grain (soybean and corn) and fiber (cotton) crops into the soil following harvest is a common agricultural practice. The crop residue represents a substantial portion of nitrogen initially applied as fertilizer, and thus is a potential source of nitrogen for NO emissions during the winter fallow period. Fluxes of NO and NO₂ were measured from fallow fields from February 7 to March 23, 1994, using a dynamic chamber technique (ambient air as the carrier gas). Average NO flux rates, as a function of previous crop residue, were 9.2 (range –4.2 to 76) ng–N m^–2 s^–1 for soybean, 6.1 (range –11.7 to 110) ng–N m^–2 s^–1 for cotton, and 4.7 (range –0.2 to 40) ng–N m^–2 s^–1 for corn. Maximum NO fluxes were observed in mid–morning when soil temperatures were lowest. Minimum NO flux occurred after mid–afternoon when soil temperature reached a maximum. The decrease in NO flux with increase in soil temperature (5 cm depth) reflected the existence of a NO compensation concentration (i.e., the rate for the NO consumption reactions continued to increase with increase in temperature). NO₂ deposition was calculated for 92% of the data points, with no trend in deposition between the three fields and their corresponding crop residue. These results indicate that significant fluxes of NO are generated from fallow agricultural fields following incorporation of the residue from the previous crop.     

The Plume Volume Molar Ratio Method for Determining NO2/NOx Ratios in Modeling—Part II: Evaluation Studies

ABSTRACT

A previous paper¹ discusses the methodology for a new method for deriving the nitrogen dioxide/nitrogen oxide (NO₂/NO_x) ratio in plumes that originally are composed mainly of (NO_x). It is called the Plume Volume Molar Ratio Method (PVMRM). This paper documents its performance against six different data sets. These performance evaluations show that the PVMRM can realistically predict the NO₂ fraction at close-in receptors yet still provide conservative estimates so that the air quality standards can be protected.