HNO3 analyzer by scrubber difference and the NO–ozone chemiluminescence method

A fast response analyzer for HNO₃ in highly polluted air is described. The time resolution attainable was 12 s. The method is based on the difference in a technique for HNO₃-scrubbed and non-scrubbed air and the reduction of HNO₃ to NO with the use of a line of catalytic converters and a method for the subsequent NO-ozone chemiluminescence. A sample air stream, in which particulates are removed with a Teflon filter, is divided into two channels. CH-1 is directly connected to the converter line, and CH-2 contains a HNO₃ scrubber packed with a nylon fiber that goes to another converter line. Each converter line is composed of a hot quartz-bead converter (QBC) and a molybdenum converter (MC) in a series. A QBC reduces HNO₃ to (NO+NO₂), which is called NO_x. The MC reduces the NO_x to NO.For CH-1, the analyzer detects most compounds that typically comprise NO_y (J. Geophys. Res. 91 (1986) 9781). These CH-1 compounds are called NO_y′ hereafter (NO_y-particulate nitrate) because the particulates are removed by the filter. A difference in the detector signal for the two channels indicates HNO₃. For a blank test, atmospheric air in which HNO₃ was pre-scrubbed by an extra nylon fiber was introduced to the analyzer. Variations in the blank value were 0.38±0.42 and 0.34±0.55 ppb during the high readings (NO_y′-HNO₃ ) (called NO_y^* hereafter) (111±12 ppb, N=180), and low NO_y^* readings (62±8 ppb, N=180), respectively, indicating that the lowest detection limit of the analyzer is 1.1 ppb (2σ). When the data obtained with the analyzer is compared to the data using the denuder method, a linear correlation with the regression of Y=0.973X+0.077 (r²=0.916 (N=20)) in the range of 0–6.5 ppb HNO₃ is obtained, which is an excellent agreement. Atmospheric monitoring was carried out at Kobe. Although the average concentration of HNO₃ was 2.6±1.3 ppb, ca.10 ppb for a HNO₃ concentration was occasionally observed when the NO_y^* concentration was high, i.e., more than 100 ppb.     

Aircraft measurements of O3, NOx,CO, VOCs,and SO2 in the Yangtze River Delta region

In this study, air pollutants, including ozone (O₃), nitrogen oxides (NO_x = NO + NO₂), carbon monoxides (CO), sulfur dioxide (SO₂), and volatile organic compounds (VOCs) measured in the Yangtze River Delta (YRD) region during several air flights between September/30 and October/11 are analyzed. This measurement provides horizontal and vertical distributions of air pollutants in the YRD region. The analysis of the result shows that the measured O₃ concentrations range from 20 to 60 ppbv. These values are generally below the US national standard (84 ppbv), suggesting that at the present, the O₃ pollutions are modest in this region. The NO_x concentrations have strong spatial and temporal variations, ranging from 3 to 40 ppbv. The SO₂ concentrations also have large spatial and temporal variations, ranging from 1 to 35 ppbv. The high concentrations of CO are measured with small variations, ranging from 3 to 7 ppmv. The concentrations of VOCs are relatively low, with the total VOC concentrations of less than 6 ppbv. The relative small VOC concentrations and the relative large NO_x concentrations suggest that the O₃ chemical formation is under a strong VOC-limited regime in the YRD region. The measured O₃ and NO_x concentrations are strongly anti-correlated, indicating that enhancement in NO_x concentrations leads to decrease in O₃ concentrations. Moreover, the O₃ concentrations are more sensitive to NO_x concentrations in the rural region than in the city region. The ratios of Δ[O₃]/Δ[NO_x] are ?2.3 and ?0.25 in the rural and in the city region, respectively. In addition, the measured NO_x and SO₂ concentrations are strongly correlated, highlighting that the NO_x and SO₂ are probably originated from same emission sources. Because SO₂ emissions are significantly originated from coal burnings, the strong correlation between SO₂ and NO_x concentrations suggests that the NO_x emission sources are mostly from coal burned sources. As a result, the future automobile increases could lead to rapid enhancements in O₃ concentrations in the YRD region.     

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.     

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.     

Chemistry of HOx radicals in the upper troposphere

Aircraft observations from three recent missions (STRAT, SUCCESS, SONEX) are synthesized into a theoretical analysis of the factors controlling the concentrations of HO_x radicals (HO_x=OH+peroxy) and the larger reservoir family HO_y (HO_y=HO_x+2H₂O₂+2CH₃OOH+HNO₂+HNO₄) in the upper troposphere. Photochemical model calculations capture 66% of the variance of observed HO_x concentrations. Two master variables are found to determine the variance of the 24 h average HO_x concentrations: the primary HO_x production rate, P(HO_x), and the concentration of nitrogen oxide radicals (NO_x=NO+NO₂). We use these two variables as a coordinate system to diagnose the photochemistry of the upper troposphere and map the different chemical regimes. Primary HO_x production is dominated by the O(¹D)+H₂O reaction when [H₂O]>100 ppmv, and by photolysis of acetone (and possibly other convected HO_x precursors) under drier conditions. For the principally northern midlatitude conditions sampled by the aircraft missions, the HO_x yield from acetone photolysis ranges from 2 to 3. Methane oxidation amplifies the primary HO_x source by a factor of 1.1–1.9. Chemical cycling within the HO_x family has a chain length of 2.5–7, while cycling between the HO_x family and its HO_y reservoirs has a chain length of 1.6–2.2. The number of ozone molecules produced per HO_y molecule consumed ranges from 4 to 12, such that ozone production rates vary between 0.3 and 5 ppbv d⁻¹ in the upper troposphere. Three chemical regimes (NO_x-limited, transition, NO_x-saturated) are identified to describe the dependence of HO_x concentrations and ozone production rates on the two master variables P(HO_x) and [NO_x]. Simplified analytical expressions are derived to express these dependences as power laws for each regime. By applying an eigenlifetime analysis to the HO_x–NO_x–O₃ chemical system, we find that the decay of a perturbation to HO_y in the upper troposphere (as from deep convection) is represented by four dominant modes with the longest time scale being factors of 2–3 times longer than the steady-state lifetime of HO_y.     

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.     

Ozone response to precursor controls: comparison of data analysis methods with the predictions of photochemical air quality simulation models

Comparisons were made between the predictions of six photochemical air quality simulation models (PAQSMs) and three indicators of ozone response to emission reductions: the ratios of O₃/NO_z and O₃/NO_y and the extent of reaction. The values of the two indicator ratios and the extent of reaction were computed from the model-predicted mixing ratios of ozone and oxidized nitrogen species and were compared to the changes in peak 1 and 8 h ozone mixing ratios predicted by the PAQSMs. The ozone changes were determined from the ozone levels predicted for base-case emission levels and for reduced emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NO_x). For all simulations, the model-predicted responses of peak 1 and 8 h ozone mixing ratios to VOC or NO_x emission reductions were correlated with the base-case extent of reaction and ratios of O₃/NO_z and O₃/NO_y. Peak ozone values increased following NO_x control in 95% (median over all simulations) of the high-ozone (>80 ppbv hourly mixing ratio in the base-case) grid cells having mean afternoon O₃/NO_z ratios less than 5 : 1, O₃/NO_y less than 4 : 1, or extent less than 0.6. Peak ozone levels decreased in response to NO_x reductions in 95% (median over all simulations) of the grid cells having peak hourly ozone mixing ratios greater than 80 ppbv and where mean afternoon O₃/NO_z exceeded 10 : 1, O₃/NO_y was greater than 8 : 1, or extent exceeded 0.8. Ozone responses varied in grid cells where O₃/NO_z was between 5 : 1 and 10 : 1, O₃/NO_y was between 4 : 1 and 8 : 1, or extent was between 0.6 and 0.8. The responses in such grid cells were affected by ozone responses in upwind grid cells and by the changes in ozone levels along the upwind boundaries of the modeling domains.     

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.     

Distribution of reactive nitrogen species in the remote free troposphere: data and model comparisons

The available reactive nitrogen measurements from the global free troposphere obtained during the period of 1985–1995 have been compiled and analyzed. The species of interest are NO, NO_x (NO+NO₂), NO_y, PAN, HNO₃ and O₃. Data extending to 13 km have been gridded with a 5°×5° horizontal and 1 km vertical resolution. The data have been divided into two seasons, namely “Winter” and “Summer” depending upon the time and location of the observations. Data described here as well as additional analysis have also been archived and are accessible on-line through the World Wide Web at: http://george.arc.nasa.gov/∼athakur. Global maps of the reactive nitrogen species distribution are produced in a form that would be most useful for the test and evaluation of models of tropospheric transport and chemistry. Limited comparisons of the observed reactive nitrogen species data with predictions by 3-D global models were performed using three selected models. Significant model to model as well as data to model differences were frequently observed. During summer, models tended to underpredict NO (−25 to −60%) while significantly overpredicting HNO₃ (+250 to +400%) especially in the upper troposphere. Similarly, the seasonal HNO₃ variations predicted by some models were opposite to those observed. PAN was generally overpredicted, especially in the upper troposphere, while NO_y was underpredicted. Ozone on average was better simulated but significant deviations at specific locations were evident. By comparing model predictions with observations, an overall quantitative assessment of the accuracy with which these three models describe the global distribution of measured reactive nitrogen species is provided. No reliable trend information for any of the reactive nitrogen species was possible based on the presently available data set. The reactive nitrogen data currently offer only a limited spatial and temporal coverage for the validation of global models.     

The impact of an 8 h ozone air quality standard on ROG and NOx controls in Southern California

The new National Ambient Air Quality Standard for ozone in the US uses 8 h averaging for the concentration. Based on the 1993 ambient data for Southern California, 8 h averaging has a moderate tendency to move the location of the peak ozone concentration east of the location of the peak 1 h ozone concentration. Reducing the area-wide peak 8 h ozone concentration to 80 ppb would require an effective reduction of the area-wide peak 1 h ozone concentration to around 90 ppb. The Urban Airshed Model with improved numerical solvers, meteorological input based on a mesoscale model and an adjusted emissions inventory was used to study the effect of reactive organic gases (ROG) and NO_x controls on daily-maximum and peak 8 h ozone concentrations under the 26–28 August 1987 ozone episodic conditions in Southern California. The NO_x disbenefit remains prominent for the case of 8 h ozone concentration but is somewhat less prominent, especially when areal ozone exposure is considered, than the case for 1 h ozone concentration. The role of two indicators – O₃/NO_y and H₂O₂/HNO₃ – for NO_x- and ROG-sensitivity for 1 and 8 h ozone concentrations were also studied. In general, the indicator trends are consistent with model predictions, but the discriminating power of the indicators is rather limited.     

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.     

Characterizations of chemical oxidants in Mexico City: A regional chemical dynamical model (WRF-Chem) study

The formation of chemical oxidants, particularly ozone, in Mexico City were studied using a newly developed regional chemical/dynamical model (WRF-Chem). The magnitude and timing of simulated diurnal cycles of ozone (O₃), carbon monoxide (CO) and nitrogen oxides (NO_x)_, and the maximum and minimum O₃ concentrations are generally consistent with surface measurements. Our analysis shows that the strong diurnal cycle in O₃ is mainly attributable to photochemical variations, while diurnal cycles of CO and NO_x mainly result from variations of emissions and boundary layer height. In a sensitivity study, oxidation reactions of aromatic hydrocarbons (HCs) and alkenes yield highest peak O₃ production rates (20 and 18 ppbv h⁻¹, respectively). Alkene oxidations, which are generally faster, dominate in early morning. By late morning, alkene concentrations drop, and oxidations of aromatics dominate, with lesser contributions from alkanes and CO. The sensitivity of O₃ concentrations to NO_x and HC emissions was assessed. Our results show that daytime O₃ production is HC-limited in the Mexico City metropolitan area, so that increases in HC emissions increase O₃ chemical production, while increases in NO_x emissions decrease O₃ concentrations. However, increases in both NO_x and HC emissions yield even greater O₃ increases than increases in HCs alone. Uncertainties in HC emissions estimates give large uncertainties in calculated daytime O₃, while NO_x emissions uncertainties are less influential. However, NO_x emissions are important in controlling O₃ at night.     

Modelling NOx from lightning and its impact on global chemical fields

We have used a three-dimensional off-line chemical transport model (CTM) to assess the impact of lightning emissions in the free troposphere both on NO_x itself and on other chemical species such as O₃ and OH. We have investigated these effects using two lightning emission scenarios. In the first, lightning emissions are coupled in space and time to the convective cloud top height calculated every 6 h by the CTM's moist convection scheme. In the second, lightning emissions are calculated as a constant, monthly mean field. The model's performance against observed profiles of NO_x and O₃ in the Atlantic and Pacific ocean improves significantly when lightning emissions are included. With the inclusion of these emissions, the CTM produces a significant increase in the NO_x concentrations in the upper troposphere, where the NO_x lifetime is long, and a smaller increase in the lower free troposphere, where the surface NO_x sources dominate. These changes cause a significant increase in the O₃ production in the upper troposphere and hence higher calculated O₃ there. The model indicates that lightning emissions cause local increases of over 50 parts per 10¹² by volume (pptv) in NO_x, 200 pptv in HNO₃ and 20 parts per 10⁹ by volume (ppbv) (>40%) in O₃. In addition, a smaller increase of O₃ in the lower troposphere occurs due to an increase in the downward transport of O₃. The O₃ change is accompanied by an increase in OH which is more pronounced in the upper troposphere with a corresponding reduction in CO. The method of emission employed in the model does not appear to have a significant effect globally. In the upper troposphere (above about 300 hPa) NO_x concentrations are generally lower with monthly mean emissions, because of the de-coupling of emissions from the model's convection scheme, which vents NO_x aloft more efficiently in the coupled scheme. Below the local convective outflow altitude, NO_x concentrations are larger when using the monthly mean emissions than when coupled to the convection scheme, because the more dilute emissions, and nighttime emissions, lead to a slower NO_x destruction rate. Only minor changes are predicted in the monthly average fields of O₃ if we emit lightning as a monthly constant field. However, the method of emission becomes important when we make a direct comparison of model results with time varying data. These differences should be taken into account when a direct comparison of O₃ with measurements collected at particular times and locations is attempted.     

Summertime photochemical ozone formation in Santiago,Chile

The city of Santiago, Chile experiences frequent high pollution episodes and as a consequence very high ozone concentrations, which are associated with health problems including increasing daily mortality and hospital admissions for respiratory illnesses. The development of ozone abatement strategies requires the determination of the potential of each pollutant to produce ozone, taking into account known mechanisms and chemical kinetics in addition to ambient atmospheric conditions. In this study, the photochemical formation of ozone during a summer campaign carried out from March 8–20, 2005 has been investigated using an urban photochemical box model based on the Master Chemical Mechanism (MCMv3.1). The MCM box model has been constrained with 10 min averages of simultaneous measurements of HONO, HCHO, CO, NO, j(O¹D), j(NO₂), 31 volatile organic compounds (VOCs) and meteorological parameters. The O₃–NO_x–VOC sensitivities have been determined by simulating ozone formation at different VOC and NO_x concentrations. Ozone sensitivity analyses showed that photochemical ozone formation is VOC-limited under average summertime conditions in Santiago. The results of the model simulations have been compared with a set of potential empirical indicator relationships including H₂O₂/HNO₃, HCHO/NO_y and O₃/NO_z. The ozone forming potential of each measured VOC has been determined using the MCM box model. The impacts of the above study on possible summertime ozone control strategies in Santiago are discussed.     

NOy removal from the Cumberland Power Plant Plume

Airborne measurements were performed in the plume of the Cumberland Power Plant during August 1998 using a highly sensitive SO₂ instrument. The measurements confirmed previous suggestions that NO_y species are removed from the plume at a faster rate than SO₂. The differential removal rate (the difference between loss rate of NO_y and that of SO₂) was estimated to be 0.06 h⁻¹. This value implies that the NO_y loss rate is in the range of 0.09–0.14 h⁻¹. The application of a mathematical argument, based on the convolution integral, enabled improved synchronization of the data from the SO₂ and NO_y instruments. Examination of the synchronized data revealed that the concentration ratio of SO₂ and NO_y varies across the plume. Near the source it is higher at the wings of the plume, while in the core of the plume it is similar to the ratio at the release point. Two possible explanations of the observations are discussed: conversion to non-measurable NO_y species, and in-plume loss of NO_y (as HNO₃) via dry deposition.     

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.     

Sensitivity of ozone concentrations to diurnal variations of surface emissions in Mexico City: A WRF/Chem modeling study

Sensitivity of ozone (O₃) concentrations in the Mexico City area to diurnal variations of surface air pollutant emissions is investigated using the WRF/Chem model. Our analysis shows that diurnal variations of nitrogen oxides (NO_x = NO + NO₂) and volatile organic compound (VOC) emissions play an important role in controlling the O₃ concentrations in the Mexico City area. The contributions of NO_x and VOC emissions to daytime O₃ concentrations are very sensitive to the morning emissions of NO_x and VOCs. Increase in morning NO_x emissions leads to decrease in daytime O₃ concentrations as well as the afternoon O₃ maximum, while increase in morning VOC emissions tends to increase in O₃ concentrations in late morning and early afternoon, indicating that O₃ production in Mexico City is under VOC-limited regime. It is also found that the nighttime O₃ is independent of VOCs, but is sensitive to NO_x. The emissions of VOCs during other periods (early morning, evening, and night) have only small impacts on O₃ concentrations, while the emissions of NO_x have important impacts on O₃ concentrations in the evening and the early morning.This study suggests that shifting emission pattern, while keeping the total emissions unchanged, has important impacts on air quality. For example, delaying the morning emission peak from 8 am to 10 am significantly reduced the morning peaks of NO_x and VOCs, as well as the afternoon O₃ maxima. It suggests that without reduction of total emission, the daytime O₃ concentrations can be significantly reduced by changing the diurnal variations of the emissions of O₃ precursors.     

NMOC,ozone, and organic aerosol in the southeastern United States, 1999–2007: 2. Ozone trends and sensitivity to NMOC emissions in Atlanta,Georgia

Non-methane organic carbon (NMOC) measurements made in Atlanta, Georgia from 1999–2007 are used with nitrogen oxide (NO_x or NO_y) and ozone (O₃) data to investigate relationships between O₃ precursors and peak 8-hour O₃ concentrations in the city. Data from a WNW-to-ENE transect of sites illustrate that the mean urban peak 8-hour O₃ excess constitutes about 20% of the peak 8-hour O₃ measured at the area-wide maximum O₃ site when air-mass movement is from the northwest quadrant; local influence is potentially greater on days with more stagnation or recirculation. The peak 8-hour O₃ concentrations in Atlanta increase as (1) surface temperature (T), ambient NMOC and NO_y concentrations, and previous-day peak O₃ concentrations increase, and as (2) relative humidity, surface wind speeds, and ratios of NMOC-to-NO_y decrease. An observation-based statistical model is introduced to relate area-wide peak 8-hour O₃ concentrations to ambient NMOC and NO_y concentrations, while accounting for the non-linear dependences of peak 8-hour O₃ concentrations on meteorological factors. On the majority of days when the area-wide peak 8-hour O₃ exceeds 75 ppbv, meteorologically-adjusted peak 8-hour O₃ concentrations increase as ambient NMOC concentrations increase (NMOC sensitive) and ambient NO_y concentrations decrease. This result contrasts with regional conditions in which O₃ formation appears to be NO_x-sensitive in character. The results offer observationally-based information of relevance to O₃ management strategies in the Atlanta area, potentially contributing to “weight-of-evidence” assessments.