Abstract  Protein tyrosine nitration is a prevalent post-translational modification which occurs as a result of oxidative and nitrative stress, it may be directly involved in the onset and/or progression of diseases. Considering the existence of nano titanium dioxide (TiO(2)) in environment and sunscreen products along with the high content of nitrite in sweat, the UV-exposed skin may be a significant target for the photosensitized damage. In this paper, tyrosine nitration of bovine serum albumin (BSA) was initiated in the UV-irradiated reaction mixture containing 0.2-3.0mg/ml of three commercially nano TiO(2) products and 0.25-1.0mM NO2-. It was found that anatase TiO(2) and Degussa P25 TiO(2) showed prominent photocatalytic activity on promoting the formation of protein tyrosine nitration, and the optimum condition for the reaction was around physiological pH. Meanwhile, the photocatalytic effect of rutile on protein tyrosine nitration was subtle. The potential physiological significance of nano TiO(2)-photocatalytic protein nitration was also demonstrated in mouse skin homogenate. Although the relationship between photocatalytic protein tyrosine nitration and chronic cutaneous diseases needs further study, the toxicity of nano TiO(2) to the skin disease should be paid more attention in the production and utilization process.

Abstract  [1] Field studies have been performed in Lindon, Utah (February 2003) and Rubidoux, California (July 2003) to determine if the Rupprecht and Patashnick (R&P) Filter Dynamic Measurement System (FDMS) determines total fine particulate mass, including the semivolatile ammonium nitrate and organic material. Collocated measurements were made with the FDMS, a conventional tapered element oscillating microbalance (TEOM) monitor with a heated filter, an R&P differential TEOM monitor, the Brigham Young University (BYU) Real-Time Total Ambient Mass Sampler (RAMS), the BYU particle concentrator-organic sampling system (PC-BOSS), a PM2.5 Federal Reference Method (FRM), a PM2.5 speciation sampler, an R&P continuous nitrate monitor, and two Sunset continuous carbon monitors (one to measure quartz filter-retained particulate carbon and one to measure particulate semivolatile carbonaceous material lost from the particles on a filter during sampling). The RAMS and PC-BOSS samplers have been shown to determine fine particulate material, including both the semivolatile and the nonvolatile components. Linear regression analysis at the Lindon site between the FDMS (X) and the PC-BOSS (Y), and the FDMS (X) and the RAMS (Y), resulted in zero-intercept slopes of 1.01 ± 0.06 (r2 = 0.63) and 1.00 ± 0.01 (r2 = 0.69), respectively. At the Rubidoux sampling site, linear regression analysis between the PC-BOSS (X) and the FDMS (Y) gave a zero-intercept slope of 0.96 ± 0.02 (r2 = 0.90). Linear regression analysis between the FDMS (X) and the RAMS (Y) resulted in a zero-intercept slope of 0.99 ± 0.01 (r2 = 0.80). Measurements made at the two sites indicate that the FDMS and the R&P differential TEOM monitors do measure total fine particulate mass, including the semivolatile ammonium nitrate and organic material. Both the heated TEOM monitor and PM2.5 FRM did not measure the semivolatile material. The difference between the FDMS and a heated TEOM monitor was explained by the semivolatile ammonium nitrate and organic material measured by the various chemical composition monitors.

Abstract  Global radiative forcing of nitrate and ammonium aerosols has mostly been estimated from aerosol concentrations calculated at thermodynamic equilibrium or using approximate treatments for their uptake by aerosols. In this study, a more accurate hybrid dynamical approach (HDYN) was used to simulate the uptake of nitrate and ammonium by aerosols and the interaction with tropospheric reactive nitrogen chemistry in a three-dimensional global aerosol and chemistry model, Umich/IMPACT, which also treats sulfate, sea salt and mineral dust aerosol. The calculated sulfate, ammonium and nitrate aerosol concentrations show good agreement with the available ground-based measurements over both ocean and land areas. The global annual average nitrate aerosol burden is 0.16 Tg N, with 43% (i.e., 0.079 Tg N) in the fine mode (D < 1.25 Ám) that scatters most efficiently. The global annual average ammonium burden is 0.29 Tg N with 92% in the fine mode. A sensitivity study with a thermodynamic equilibrium model underestimates the fine-mode nitrate aerosol burden by 25%, because of the excessive nitrate formation on coarse aerosols. These underpredictions are especially important in the remote continents or over the oceans, where the availability of the total nitrate is limited. We also examined two common approaches used to treat nitrate and ammonium aerosols in global models, including the first-order gas-to-particle approximation based on uptake coefficients (UPTAKE) and a simple hybrid method that combines the former with an equilibrium model (HYB). The two methods calculate higher nitrate aerosol burdens than HDYN by +106% and +47%, respectively. Both fine- and coarse-model nitrate aerosols are overestimated by UPTAKE, but the overestimation by HYB is mainly due to uptake of nitrate by the coarse aerosols. As a result, HYB calculates lower surface concentrations of the fine-mode nitrate aerosol by up to 50% over most continental areas, compared to HDYN. Surface HNO3 and NOx concentrations are underpredicted by HYB by up to 90% and 5%, respectively. Since the reaction of N2O5 on sulfate aerosols is not included in the UPTAKE method, the NOx burden and surface concentrations are overestimated by 56% and a factor of 2-5, respectively. These results suggest the importance of using the more accurate hybrid dynamical method in the estimates of both aerosol forcing and tropospheric ozone chemistry.

Abstract  A detailed chemical box model has been constructed based on a comprehensive chemical mechanism (the Master Chemical Mechanism) to investigate indoor air chemistry in a typical urban residence in the UK. Unlike previous modelling studies of indoor air chemistry, the mechanism adopted contains no simplifications such as lumping or the use of surrogate species, allowing more insight into indoor air chemistry than previously possible. The chemical mechanism, which has been modified to include the degradation reactions of key indoor air pollutants, contains around 15,400 reactions and 4700 species. The results show a predicted indoor OH radical concentration up to 4.0Î105 molecule cm-3, only a factor of 10û20 less than typically observed outdoors and sufficient for significant chemical cycling to take place. Concentrations of PAN-type species and organic nitrates are found to be important indoors, reaching concentrations of a few ppb. Sensitivity tests highlight that the most crucial parameters for modelling the concentration of OH are the light-intensity levels and the air exchange rate. Outdoor concentrations of O3 and NOX are also important in determining radical concentrations indoors. The reactions of ozone with alkenes and monoterpenes play a major role in producing new radicals, unlike outdoors where photolysis reactions are pivotal radical initiators. In terms of radical propagation, the reaction of HO2 with NO has the most profound influence on OH concentrations indoors. Cycling between OH and RO2 is dominated by reaction with the monoterpene species, whilst alcohols play a major role in converting OH to HO2. Surprisingly, the absolute reaction rates are similar to those observed outdoors in a suburban environment in the UK during the summer. The results from this study highlight the importance of tailoring a model for its particular location and the need for future indoor air measurements of radical species, nitrated species such as PANs and organic nitrates, photolysis rates of key species over the range of wavelengths observed indoors and concurrent measurements of outdoor air pollutant concentrations.

Abstract  #The production of nitric oxide (NO) in forest soils can indicate that the ecosystem is progressing toward a state of nitrogen (N) saturation. Soil NO emissions may also have impacts on local tropospheric ozone (03) levels. During 2000-2001, we made first-time measurements of NO emissions in two paired watershed studies. In each study, one watershed had been amended with aerial applications of 2.5-3.5 g N m(-2) per year above background atmospheric deposition rates since 1989, and an adjacent watershed served as a reference. In plots at the Fernow Experimental Forest (FEF) in West Virginia and the Bear Brook Watershed in Maine (BBWM), NO emissions in N-amended watersheds (0.61-6.8 mug NO-N m(-2) h(-1)) were higher than in the reference watersheds (0.21-1.4 NO-N m(-2) h(-1)). In the N-amended watershed at BBWM, NO fluxes in plots dominated by hardwood species were higher than in plots dominated by softwood species, in contrast to previous studies in other forests. Field NO fluxes were correlated with mineral soil nitrate (NO3-) Concentrations (r(2) = 0.65, P = 0.016) across all plots, suggesting that NO emissions may be a reliable indicator of NO3- leaching potential. Laboratory experiments indicated that nitrification was the dominant source of NO at both sites. At BBWM, increased NO emissions in N-amended soil appeared to result from more rapid nitrification. In contrast, reduced soil pH in N-amended soil at FEF may have caused increased protonation of nitrification-derived nitrite, and the subsequent abiotic formation of NO, even though nitrification rates were not significantly higher than in unamended soil. The results suggest that enhanced soil NO emissions are a characteristic response in forests subjected to elevated N inputs. One possible consequence of higher NO emissions is an increase in O-3-related phytotoxicity. This effect may mitigate the ability of forests to accumulate carbon in response to N inputs or increasing atmospheric CO2.

Abstract  #We describe a new laboratory-based method for in situ detection of nitrous acid (HONO) using a combination of thermal dissociation (TD) and chemiluminescent (CL) detection of nitric oxide. A prototype was built using a commercial NO sensor. Laboratory tests for possible chemical interferences show that measurements are affected in predictable ways by NO2, peroxy nitrates, alkyl nitrates, HNO3, O3 and H2O.

Abstract  #We investigated N cycling and denitrification rates following five years of N and dolomite amendments to whole-tree harvested forest plots at the long-term soil productivity experiment in the Fernow Experimental Forest in West Virginia, USA. We hypothesized that changes in soil chemistry and nutrient cycling induced by N fertilization would increase denitrification rates and the N2O:N2 ratio. Soils from the fertilized plots had a lower pH (2.96) than control plots (3.22) and plots that received fertilizer and dolomite (3.41). There were no significant differences in soil %C or %N between treatments. Chloroform-labile microbial biomass carbon was lower in fertilized plots compared to control plots, though this trend was not significant. Extractable soil NO3- was elevated in fertilized plots on each sample date. Soil-extractable NH4+, NO3-, pH, microbial biomass carbon, and %C varied significantly by sample date suggesting important seasonal patterns in soil chemistry and N cycling. In particular, the steep decline in extractable NH4+ during the growing season is consistent with the high N demands of a regenerating forest. Net N mineralization and nitrification also varied by date but were not affected by the fertilization and dolomite treatments. In a laboratory experiment, denitrification was stimulated by NO3- additions in soils collected from all field plots, but this effect was stronger in soils from the unfertilized control plots, suggesting that chronic N fertilization has partially alleviated a NO3- limitation on denitrification rates. Dextrose stimulated denitrification only in the whole-tree-harvest soils. Denitrification enzyme activity varied by sample date and was elevated in fertilized plots for soil collected in July 2000 and June 2001. There were no detectable treatment effects on N2O or N2 flux from soils under anaerobic conditions, though there was strong temporal variation. These results suggest that whole-tree harvesting has altered the N status of these soils so they are less prone to N saturation than more mature forests. It is likely that N losses associated with the initial harvest and high N demand by aggrading vegetation is minimizing, at least temporarily, the amount of inorganic N available for nitrification and denitrification, even in the fertilized plots in this experiment.

Abstract  The aim of the investigation was to assess the relations between pairs of personal, indoor, and outdoor levels of fine particles and their components with respect to effects for older subjects with cardiovascular disease. In the framework of a study funded by the European Union (Exposure and Risk Assessment for Fine and Ultrafine Particles in Ambient Air; referred to as ULTRA)*, panel studies were conducted in Amsterdam (The Netherlands) and Helsinki (Finland). Concentrations of outdoor particulate matter 2.5 pm or smaller in aerodynamic diameter (PM2.5) were measured at a fixed site in each location. With HEI funding, each subject's personal and indoor PM2.5 exposure was measured every other week for 6 months during the 24-hour period preceding intensive health measurements. Particle reflectance was measured as a marker for diesel exhaust. Elemental content of more than 50% of the personal and indoor samples and all corresponding outdoor samples was measured using x-ray fluorescence (XRF). Ion content (sulfate, nitrate) was measured using chromatography. For Amsterdam, 337 personal and 409 indoor measurements were collected from 37 subjects; for Helsinki, 336 personal and 503 indoor measurements were collected from 47 subjects. Median personal, indoor, and outdoor PM2.5 concentrations were 13.6, 13.6, and 16.5 microg/m3 in Amsterdam and 9.2, 9.2, and 11.1 microg/m3 in Helsinki. In both cities, personal and indoor PM2.5 concentrations were lower than and highly correlated with outdoor concentrations (median correlation coefficient [R] 0.7-0.8). For most elements, personal and indoor concentrations were also highly correlated with outdoor concentrations. The highest correlations (median R > 0.9) were found for sulfur (S), sulfate, and particle reflectance (reported as the absorption coefficient). Reflectance was a useful proxy for elemental carbon (EC), but site-specific calibration with EC data is necessary. The findings of this study support using fixed-site measurements as a measure of exposure to PM in time-series studies linking the day-to-day variations in PM to the day-to-day variations in health endpoints, especially for components of PM that are generally associated with fine particles and have few indoor sources.

Abstract  Global 3-D tropospheric chemistry models in the literature show large differences in global budget terms for tropospheric ozone. The ozone production rate in the troposphere, P(Ox ), varies from 2300 to 5300 Tg yr-1 across models describing the present-day atmosphere. The ensemble mean of P(Ox ) in models from the post-2000 literature is 35% higher than that compiled in the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR). Simulations conducted with the GEOS-Chem model using two different assimilated meteorological data sets for 2001 (GEOS-3 and GEOS-4), as well as 3 years of GISS GCM meteorology, show P(Ox ) values in the range 4250-4700 Tg yr-1; the differences appear mostly because of clouds. Examination of the evolution of P(Ox ) over the GEOS-Chem model history shows major effects from changes in heterogeneous chemistry, the lightning NOx source, and the yield of organic nitrates from isoprene oxidation. Multivariate statistical analysis of model budgets in the literature indicates that 74% of the variance in P(Ox ) across models can be explained by differences in NOx emissions, inclusion of nonmethane volatile organic compounds (NMVOCs, mostly biogenic isoprene), and ozone influx from stratosphere-troposphere exchange (STE). Higher NOx emissions, more widespread inclusion of NMVOC chemistry, and weaker STE in the more recent models increase ozone production; however, the effect of NMVOCs does not appear generally sensitive to the magnitude of emissions within the range typically used in models (500-900 Tg C yr-1). We find in GEOS-Chem that P(Ox ) saturates when NMVOC emissions exceed 200 Tg C yr-1 because of formation of organic nitrates from isoprene oxidation, providing an important sink for NOx.

Abstract  Nitric oxide (NO.) is a physiological messenger formed by several cell types. Reaction with O2 forms oxides that nitrosate amines at pH values near 7. We now report experiments in which NO. was added to intact human cells and to aerobic solutions of DNA, RNA, guanine, or adenine. TK6 human lymphoblastoid cells were mutated 15- to 18-fold above background levels at both the HPRT and TK gene loci. Xanthine and hypoxanthine, from deamination of guanine and adenine, respectively, were formed in all cases. NO. induced dose-responsive DNA strand breakage. Yields of xanthine ranged from nearly equal to about 80-fold higher than those of hypoxanthine. Yields of xanthine and hypoxanthine from nucleic acids were higher than those from free guanine and adenine. This was most pronounced for xanthine; 0.3 nmol/mg was formed from free guanine vs. 550 nmol/mg from calf thymus RNA. Nitric oxide added to TK6 cells produced a 40- to 50-fold increase in hypoxanthine and xanthine in cellular DNA. We believe that these results, plus the expected deaminations of cytosine to uracil and 5-methylcytosine to thymine, account for the mutagenicity of nitric oxide toward bacteria and mammalian cells.

Abstract  Male CD-1 mice were exposed to an nominal concentration of 20 p.p.m. of 15N-nitrogen dioxide (15NO2) for 6 h/day for 4 days and for 2 h on the day 5, and to 1 g morpholine/kg body wt by gavage daily for five consecutive days. N-Nitrosomorpholine (NMOR) was found in whole mice, stomachs, skins with hair, and remains. The sum of individual tissue concentrations measured separately was 3421 ng/tissue, where the average skin weighed 4.3 g, the average stomach weighed 1.0 g and the average remains weighed 22.2 g. The average whole mouse weighed 27.7 g and contained a total of 3903 ng of NMOR. The concentration of NMOR was highest in the skin, next highest in the stomach, and lowest in the remains. However, the total quantity of NMOR per tissue, while highest in the skin (83%), was next highest in the remains (14.8%) and lowest in the stomach (2.2%). GC-MS analysis served to distinguish between the NMOR of 15NO2 origin and that of other origin. All of the NMOR in the whole mouse homogenates was identified as 15NMOR. In the stomach 73% was identified as 14NMOR, representing 1.6% of the total NMOR in the mouse, and 27% as 15NMOR, representing 0.6% of the total NMOR in the mouse. N-Nitrosamine formation in vivo is discussed as a possibly ongoing mammalian process.

Abstract  The experiments reported here identify nitric oxide as a molecular effector of activated macrophage induced cytotoxicity. Cytotoxic activated macrophages synthesize nitric oxide from a terminal guanidino nitrogen atom of L-arginine which is converted to L-citrulline without loss of the guanidino carbon atom. In addition, authentic nitric oxide gas causes the same pattern of cytotoxicity in L10 hepatoma cells as is induced by cytotoxic activated macrophages (iron loss as well as inhibition of DNA synthesis, mitochondrial respiration, and aconitase activity). The results suggest that nitric oxide is the precursor of nitrite/nitrate synthesized by cytotoxic activated macrophages and, via formation of iron-nitric oxide complexes and subsequent degradation of iron-sulfur prosthetic groups, an effector molecule.

Abstract  For a period of almost 3 years, sampling of size-fractionated ambient particulate matter with diameter below 10 Ám (PM10) was performed at urban source sites (Downey and University of Southern California) and inland receptor sites (Claremont and Riverside) in the Los Angeles Basin as part of the Southern California Particle Center and Supersite. Results for size-resolved PM10 mass, inorganic ions (sulfate and nitrate), metals, elemental carbon, and organic carbon were obtained. Three collocated micro-orifice uniform deposit impactors (MOUDIs) were deployed to collect 24-hour samples roughly once a week. Ultrafine particle concentrations (particle diameter d p < 0.1 Ám) were found to be the highest at the source sites resulting from fresh vehicular emissions. Mass concentrations in the accumulation mode (0.1 < d p < 2.5 Ám) were lower in winter than in summer, especially at the receptor sites. PM concentrations in the coarse mode (2.5 < d p < 10 Ám) were lower in winter and were composed mostly of nitrate and crustal elements (iron, calcium, potassium, silicon, and aluminum). Consistent relative levels of these elements indicate a common source of soil and/or road dust. In the accumulation mode, nitrate and organic carbon were predominant with higher nitrate levels found at the receptor sites. The ultrafine mode PM consisted of mostly organic carbon, with higher wintertime levels at the source sites due to increased organic vapor condensation from vehicles at lower temperatures. Conversely, higher ultrafine organic carbon levels at the receptor areas are due to secondary organic aerosol formation by photochemical reactions as well as increased advection of polluted air masses from upwind.

Abstract  Agricultural watersheds in the upper Midwest are the major source of nutrients to the Mississippi River and Gulf of Mexico, but temporal patterns in nutrient export and the role of hydrology in controlling export remain unclear. Here we report on NO3- -N, dissolved reactive phosphorus (DRP), and total P export from three watersheds in Illinois during the past 8-12 years. Our program of intensive, long-term monitoring allowed us to assess how nutrient export was distributed across the range of discharge that occurred at each site and to examine mechanistic differences between NO3- -N and DRP export from the watersheds. Last, we used simple simulations to evaluate how nutrient load reductions might affect NO3- -N and P export to the Mississippi River from the Illinois watersheds. Artificial drainage through under-field tiles was the primary mechanism for NO3- -N export from the watersheds. Tile drainage and overland flow contributed to DRP export, whereas export of particulate P was almost exclusively from overland flow. The analyses revealed that nearly all nutrient export occurred when discharge was median discharge, and extreme discharges ( 90th percentile) were responsible for >50% of the NO3- -N export and >80% of the P export. Additionally, the export occurred annually during a period beginning in mid-January and continuing through June. These patterns characterized all sites, which spanned a 4-fold range in watershed area. The simulations showed that reducing in-stream nutrient loads by as much as 50% during periods of low discharge would not affect annual nutrient export from the watersheds.

Abstract  After implementation of legislative measures for the reduction of environmental hazards from nitrate leaching and ammonia volatilisation when using organic manures and fertilizers in Europe, much attention is now paid to the specific effects of these fertilizers on the dynamics of global warming-relevant trace gases in soil. Particularly nitrogen fertilizers and slurry from animal husbandry are known to play a key role for the CH[4] and N[2]O fluxes from soils. Here we report on a short-term evaluation of trace gas fluxes in grassland as affected by single or combined application of mineral fertilizer and organic manure in early spring. Methane fluxes were characterised by a short methane emission event immediately after application of cattle slurry. Within the same day methane fluxes returned to negative, and on average over the 4-day period after slurry application, only a small but insignificant trend to reduced methane oxidation was found. Nitrous oxide emissions showed a pronounced effect of combined slurry and mineral fertilizer application. In particular fresh cattle slurry combined with calcium ammonium nitrate (CAN) mineral fertilizer induced an increase in mean N[2]O flux during the first 4 days after application from 10 to 300 ?g N[2]O-N m - 2 h[-1]. [15]N analysis of emitted N[2]O from ' [15]N-labelled fertilizer or manure indicated that easily decomposable slurry C compounds induced a pronounced promotion of N[2]O-N emission derived from mineral CAN fertilizer. Fluxes after application of either mineral fertilizer or slurry alone showed an increase of less than 5-fold. The NO[x] sink strength of the soil was in the range of -6 to - 10 ?g NO[x]-N m[-2] h[-1] and after fertilization it showed a tendency to be reduced by no more than 2 ?g NO[x]-N m[-2] h[-1], which was a result of both, increased NO emission and slightly increased NO[2] deposition. Associated determination of the N[2]O:N[2] emission ratio revealed that after mineral N application (CAN) a large proportion (c. 50%) was emitted as N[2]O, while after application of slurry with easily decomposable C and predominantly NH[+][4]-N serving as N-source, the N[2]O:N[2] emission ratio was 1:14, i.e. was changed in favour of N[2]. Our work provides evidence that particularly the combination of slurry and nitrate-containing N fertilizers gives rise to considerable N[2]O emissions from mineral fertilizer N pool.

Abstract  #Soil net nitrogen mineralization and nitrification rates were studied on nine undisturbed, forested watersheds in an effort to explain large variations in nitrate export in streamflow within the mid-Appalachian region. Rates of soil net nitrogen mineralization and net nitrification were measured in the upper 10 cm of mineral soil over a 5-week summer incubation period (June-July) using nine buried bags in each of the three major soil types on each watershed. Watersheds with high, medium, and low nitrate export rates exhibited high, medium, and low mean net nitrogen mineralization and net nitrification rates, respectively. Exchangeable calcium (an index to site fertility), C/N ratios, and soil moisture content together explained 63% of the variation in soil nitrogen mineralization rates, and exchangeable calcium and soil moisture content explained 61% of the variation in soil nitrification rates using multiple regression analysis. The variation in watershed nitrate export was best explained by total nitrogen in the upper 10 cm of mineral soil (explained 46%) and the percentage of mineralization due to nitrification (explained 42%). Estimated rates of wet and dry atmospheric deposition of nitrogen were not significantly correlated with watershed nitrate export. Results from this study demonstrate that soil nitrogen pools and dynamics are the most critical factors controlling nitrate export from forested watersheds in the mid-Appalachians. Long-term changes in site fertility, C/N ratios, and soil moisture, which largely control microbial nitrogen cycling, should have a significant effect on long-term trends in nitrate leaching.

Abstract  Direct measurements of the dry deposition velocity of peroxyacetyl nitrate (PAN) were made during the daytime between the months of July and October above a grassland surface in northern Illinois by a modified Bowen ratio technique. Differences in the air temperature, water vapor content, and PAN concentration were measured between the heights of 3.0 m and 0.92 m. Although the measurement uncertainties were large, the cumulative data indicate a slight downward flux of PAN, with an average and standard error of 0.13 +/- 0.13 cm s(-1) for the dry deposition velocity. Theoretical calculations showed that thermochemical decomposition of PAN on leaf and soil surfaces heated to temperatures above the ambient air levels would contribute less than 15% of the total PAN flux at the elevations of the PAN measurements. A theoretical evaluation of the transfer of PAN through leaf stomata and the plant cuticular membrane indicated that uptake of PAN by vegetation during the daytime is controlled by transfer through the leaf stomata rather than the cuticular membrane. The stomatal resistance for PAN is greater by a factor of 1.6 than the value for O-3. The mesophyll resistance for O-3 is also expected to be less than the value for PAN, because O-3 has more reaction sites within plant cells and reacts faster than PAN with protein thiols of the cell membranes. Measurements from other studies indicate that the dry deposition velocity for PAN above a vegetated surface during the daytime is lower by a factor of 0.5-0.3 than for O-3. Our measurements of the PAN deposition velocity agree with the results of previous studies and with theoretical calculations based on the physicochemical properties of PAN and the grassland surface. These measurements imply that removal of PAN from the daytime atmospheric boundary layer by thermochemical decomposition is more rapid than dry deposition to a grassland surface.

Abstract  The heterogeneous reaction of NO2 with water on the surface of laboratory systems has been known for decades to generate HONO, a major source of OH that drives the formation of ozone and other air pollutants in urban areas and possibly in snowpacks. Previous studies have shown that the reaction is first order in NO2 and in water vapor, and the formation of a complex between NO2 and water at the airûwater interface has been hypothesized as being the key step in the mechanism. We report data from long path FTIR studies in borosilicate glass reaction chambers of the loss of gaseous NO2 and the formation of the products HONO, NO and N2O. Further FTIR studies were carried out to measure species generated on the surface during the reaction, including HNO3, N2O4 and NO2+. We propose a new reaction mechanism in which we hypothesize that the symmetric form of the NO2 dimer, N2O4, is taken up on the surface and isomerizes to the asymmetric form, ONONO2. The latter autoionizes to NO+NO3-, and it is this intermediate that reacts with water to generate HONO and surface-adsorbed HNO3. Nitric oxide is then generated by secondary reactions of HONO on the highly acidic surface. This new mechanism is discussed in the context of our experimental data and those of previous studies, as well as the chemistry of such intermediates as NO+ and NO2+ that is known to occur in solution. Implications for the formation of HONO both outdoors and indoors in real and simulated polluted atmospheres, as well as on airborne particles and in snowpacks, are discussed. A key aspect of this chemistry is that in the atmospheric boundary layer where human exposure occurs and many measurements of HONO and related atmospheric constituents such as ozone are made, a major substrate for this heterogeneous chemistry is the surface of buildings, roads, soils, vegetation and other materials. This area of reactions in thin films on surfaces (SURFACE=Surfaces, Urban and Remote: Films As a Chemical Environment) has received relatively little attention compared to reactions in the gas and liquid phases, but in fact may be quite important in the chemistry of the boundary layer in urban areas.

Abstract  Measurements of atmospheric organic nitrates derived from isoprene, i.e., "isoprene nitrates", were conducted from July 14 to August 19, 1998, as part of the 1998 summer intensive measurement campaign of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) at the University of Michigan Biological Station in Pellston, Michigan. The measurements were conducted using on-line chromatography in conjunction with a nitrate-selective detection scheme. Measured concentrations of isoprene nitrates ranged from 0.5 parts per trillion (ppt), the detection limit of the method employed, to 35 ppt. In this paper we discuss the contribution of the isoprene nitrates to NOy which was typically 0.5 - 1.5% of total odd nitrogen, but up to similar to4% for well-aged air. Concentrations of isoprene nitrates exhibited a strong diurnal variation consistent with their expected chemical and physical removal rates. In this work we also discuss the chemistry of the precursor peroxy radicals and the NOx dependence of isoprene nitrate formation.

Abstract  [ 1] We employed a fast response thermal dissociation-chemical ionization mass spectrometer (TD-CIMS) system to measure eddy covariance fluxes of peroxyacetyl nitrate ( PAN), peroxypropionyl nitrate (PPN) and peroxymethacryloyl nitrate (MPAN). Fluxes were measured for eight consecutive days in July 2003 at a Loblolly pine forest in North Carolina along with eddy covariance NOy fluxes. Covariances between PAN concentration and vertical wind velocity indicated consistent deposition fluxes that ranged up to approximately - 14 ng N m(-2) s(-1). The average daytime flux peaked at - 6.0 ng N m(-2) s(-1) and accounted for similar to 20% of the daytime NOy flux. Calculations suggest minimum daytime surface resistances for PAN in the range of 70 - 130 s m(-1). It was estimated that approximately half of daytime uptake was through plant stomates. Average PAN deposition velocities, V-d(PAN), showed a daytime maximum of similar to 10.0 mm s(-1); however, deposition did not cease during nighttime periods. V-d(PAN) was highly variable at night and increased when canopy elements were wet from either precipitation or dew formation. Diel patterns of deposition velocity of MPAN and PPN were similar to that of PAN. These results suggest that deposition of PAN, at least to coniferous forest canopies, is much faster than predicted with current deposition algorithms. Although deposition of PAN is unlikely to compete with thermal dissociation during warm summer periods, it will likely play an important role in removing PAN from the atmosphere in colder regions or during winter. The fate of PAN at the surface and within the plants remains unknown, but may present a previously ignored source of nitrogen to ecosystems.

Abstract  Nitric acid (HNO3) is the dominant end product of NOx (= NO + NO2) oxidation in the troposphere, and its dry deposition is considered to be a major removal pathway for the atmospheric reactive nitrogen. Here we present both field and laboratory results to demonstrate that HNO3 deposited on ground and vegetation surfaces may undergo effective photolysis to form HONO and NOx, 1-2 orders of magnitude faster than in the gas phase and aqueous phase. With this enhanced rate, HNO3 photolysis on surfaces may significantly impact the chemistry of the overlying atmospheric boundary layer in remote low-NOx regions via the emission of HONO as a radical precursor and the recycling of HNO3 deposited on ground surfaces back to NOx.

Abstract  We report concentrations of atmospheric NOx, nitric acid (HNO3), peroxyacetyl nitrate (PAN), and NOy; eddy covariance fluxes of NOx and NOy; inferred fluxes of HNO3 at the mixed deciduous Harvard Forest field site, June-November 2000. A novel Tunable Diode Laser Absorption Spectrometer (TDLAS) produced sensitive, hourly HNO3 concentration data, which were used to evaluate systematic error in the Dry Deposition Inferential Method (DDIM), often employed to estimate weekly HNO3 flux at deposition monitoring network sites. Due to the weak diurnal variation in HNO3 concentration at Harvard Forest, no systematic bias was found in the application of this method to compute daily and weekly average fluxes. The sum of individually measured reactive nitrogen species concentrations and fluxes were approximately equal to total NOy concentrations and fluxes for clean Northwesterly flows, but fell short of the total NOy values for the more polluted Southwesterly transport regime. The concentration and deposition velocity of the unmeasured reactive nitrogen compounds were consistent with prior estimates and recent measurements of alkyl- and hydroxyalkyl nitrates, suggesting that these compounds play an important role in reactive nitrogen deposition processes where anthropogenic NOx emissions and natural hydrocarbons are present.

Abstract  The nitrogen isotopic composition (15N/14N) of forested ecosystems varies systematically worldwide. In tropical forests, which are elevated in 15N relative to temperate biomes, a decrease in ecosystem 15N/14N with increasing rainfall has been reported. This trend is seen in a set of well characterized Hawaiian rainforests, across which we have measured the 15N/14N of inputs and hydrologic losses. We report that the two most widely purported mechanisms, an isotopic shift in N inputs or isotopic discrimination by leaching, fail to explain this climate-dependent trend in 15N/14N. Rather, isotopic discrimination by microbial denitrification appears to be the major determinant of N isotopic variations across differences in rainfall. In the driest climates, the 15N/14N of total dissolved outputs is higher than that of inputs, which can only be explained by a 14N-rich gas loss. In contrast, in the wettest climates, denitrification completely consumes nitrate in local soil environments, thus preventing the expression of its isotope effect at the ecosystem scale. Under these conditions, the 15N/14N of bulk soils and stream outputs decrease to converge on the low 15N/14N of N inputs. N isotope budgets that account for such local isotopic underexpression suggest that denitrification is responsible for a large fraction (24-53%) of total ecosystem N loss across the sampled range in rainfall.

Abstract  Exposure to silo gas is a recognized agricultural hazard. Silo gas produced from corn fermentation may consist of oxides of nitrogen and carbon dioxide. The presence of potentially lethal concentrations of nitrogen dioxide (NO2) within vertical silos has been well documented. The risk of silo gas exposure from other silage storage methodologies--including horizontal "ag-bags" and concrete bunkers--has been less well characterized. A dry growing season is known to be a factor for elevating nitrate levels in corn plants and can result in increased NO2 production. Farms in the northeastern United States faced drought conditions during the 1995 growing season. The New York State (NYS) Department of Health (DOH) and the New York Center for Agricultural Medicine and Health (NYCAMH) investigated four exposure incidents involving six farmworkers during September/October 1995. Four of these workers were hospitalized for multiple days, with two workers receiving treatment in intensive care units. The remaining two workers were treated in hospital emergency departments; one refused admission and left against medical advice. We monitored NO2 levels from "ag-bags" at several New York farms. For four days, outdoor concentrations of NO2 at one site remained in excess of the National Institute for Occupational Safety and Health's (NIOSH) immediately dangerous to life and health value (IDLH) of 20 ppm. As a result of the clinical and industrial hygiene data, and the growing season's abnormal weather conditions, DOH and NYCAMH issued statewide health hazard alerts and conducted educational activities to warn farmers and their families. The findings of this study reinforce the potential hazards associated with silo gas exposure and identify the use of ag-bags as a relatively new avenue for significant worker exposure.

Abstract  #In this study, we present approximately two years (January 1999-December 2000) of atmospheric NH3, NH4+, HCl, Cl-, HNO3, NO3-, SO2, and SO4= concentrations measured by the annular denuder/filter pack method at an agricultural site in eastern North Carolina. This site is influenced by high NH3 emissions from animal production and fertilizer use in the surrounding area and neighboring counties. The two-year mean NH3 concentration is 5.6 (+/-5.13) microg m(-3). The mean concentration of total inorganic PM2.5, which includes SO4=, NO3-, NH4+, and Cl-, is 8.0 (+/-5.84) microg m(-3). SO4=, NO3-, NH4+, and Cl- represent, respectively, 53, 24, 22, and 1% of measured inorganic PM2.5. NH3 contributes 72% of total NH3 + NH4+, on an average. Equilibrium modeling of the gas+aerosol NH3/H2SO4/HNO3 system shows that inorganic PM2.5 is more sensitive to reductions in gas + aerosol concentrations of sulfate and nitrate relative to NH3.