Particulate NO Research Articles

Evaluation of dry deposition of acidifying N compounds to vegetation.

This papers deals with direct measurements of dry deposition flux of total acidifying N species (gaseous HNO3 and particulate NO3(-)) and theoretically by parameterization method to vegetation (leaf surfaces) over a semiarid region in India. Annual average experimental dry deposition of NO3(-) to leaf of Cassia is 0.65 ± 0.61 mg m(-2) day(-1). Ambient concentrations of HNO3 vapor and particulate NO3(-) are 1.53 and 1.24 μg m(-3), respectively. Theoretically obtained dry deposition velocity of HNO3 and NO3(-) are 0.74 cm s(-1) for both while calculated dry deposition flux of total NO3(-) is found to be 1.3 ± 0.33 mg m(-2) day(-1). The measured dry deposition flux of NO3(-) to Cassia leaf is in the range of theoretically obtained flux. The annual input of N as nitrate is 3.8 mmol m(-2) year(-1) which is lower than the other forest site in China which is probably because of low pollution than China.

Photodegradation of methylmercury in the water body of the Three Gorges Reservoir

Biogeochemical cycling of mercury in the young Three Gorges Reservoir (TGR), China, is strongly considered. Although methylmercury (MMHg) photodegradation (PD) is an important process involved in mercury cycling in this zone, little is known about this process. In situ incubation experiments were therefore performed to quantify the effect of different wavelength radiations and environmental factors on the PD process of MMHg in the water bodies of TGR. It was found that the effect of solar radiation on MMHg PD was highly dependent on wavelength and water depth. All PD-rate constants resulting from each wavelength range decreased rapidly with water depth. For surface water, UV-A radiation (320–400 nm) was the key driver, accounting for 49%–62% of MMHg PD. For the entire water column, both photosynthetically active radiation (PAR, 400–700 nm) and UV-A were responsible for MMHg PD. MMHg PD fluxes peaked in summer (7.5−18 ng m−2 d−1), followed by spring (3.3–8.0 ng m−2 d−1), autumn (1.0–2.7 ng m−2 d−1), and winter (0.060–0.15 ng m−2 d−1). The annual fluxes of MMHg PD were estimated to be 1.1–2.8 μg m−2 a−1. Filtering the reservoir water and amending it with chemicals (i.e., Cl−, NO3−, and dissolved organic matter (DOM)) showed significant effects on MMHg PD rate constants. Stepwise regression analysis showed that intensity of solar radiation, suspended particulate matter (SPM), DOM, Cl−, and NO3− were involved in the PD process. Path analysis clarified the relationship between MMHg PD rate constants and environmental variables, as well as the comparative strength of direct and indirect relationships among variables. The results are of great importance for understanding MMHg cycling characteristics in TGR and also facilitate the understanding of the underlying process, MMHg PD, in natural waters.

Spatial and temporal variations of chemicals in the TSP aerosols simultaneously collected at three islands in Okinawa, Japan

East Asia’s rapid economic growth has led to concerns about the emission of air pollutants. We collected total suspended particle (TSP) aerosol samples simultaneously at three islands in Okinawa, Japan, which are downwind of East Asia, during the Asian dust season, to examine the spatial and temporal variations and chemical transformations of major chemicals in the aerosols. Weekly samples were collected from July 2008 to June 2010, and the concentrations of water-soluble cations, anions, and organic carbon (WSOC) were determined (n = 303). Spatial distribution analysis showed that monthly mean concentrations of non-sea-salt (nss)-SO42− in the spring (Asian dust season) decreased with increasing distance from Asia, while the trend for NO3− was less evident, suggesting that chemical transformation affected the long-range transport of certain chemicals. Temporal variation analysis showed that concentrations of nss-SO42−, NO3−, and WSOC during the spring were about 2.0, 2.4, and 1.8 times those in the summer (cleaner air mass from the Pacific Ocean), respectively. This study demonstrated that air pollutants were transported from the Asian continent to the Okinawa islands and affected the air quality in the region. There may also be impacts on ecosystems, because increased concentrations of particulate NO3− could increase nutrient levels around the Okinawa islands.

Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends and interrelationships

This study characterizes the spatial and temporal patterns of aerosol and precipitation composition at six sites across the United States Southwest between 1995 and 2010. Precipitation accumulation occurs mostly during the wintertime (December-February) and during the monsoon season (July-September). Rain and snow pH levels are usually between 5-6, with crustal-derived species playing a major role in acid neutralization. These species (Ca2+, Mg2+, K+, Na+) exhibit their highest concentrations between March and June in both PM2.5 and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with [Formula: see text], [Formula: see text], and Cl-, suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate [Formula: see text] show a statistically significant correlation with rain [Formula: see text] unlike snow [Formula: see text], which may be related to some combination of the vertical distribution of [Formula: see text] (and precursors) and the varying degree to which [Formula: see text]-enriched particles act as cloud condensation nuclei versus ice nuclei in the region. The coarse : fine aerosol mass ratio was correlated with crustal species concentrations in snow unlike rain, suggestive of a preferential role of coarse particles (mainly dust) as ice nuclei in the region. Precipitation [Formula: see text] : [Formula: see text] ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM2.5; (ii) they exhibit the opposite annual cycle compared to particulate [Formula: see text] : [Formula: see text] ratios; and (iii) they are higher in snow relative to rain during the wintertime. Long-term trend analysis for the monsoon season shows that the [Formula: see text] : [Formula: see text] ratio in rain increased at the majority of sites due mostly to air pollution regulations of [Formula: see text] precursors.

Occurrence of atmospheric nitrous acid in the urban area of Beijing (China)

The atmospheric concentrations of nitrous acid (HONO) have been measured during two field campaigns in the winter and summer of 2007 at Beijing (China). The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. The daily average HONO concentrations were in the range of 0.03–2.91ppb and didn't show temporal variation between the winter and summer periods. The temporal trends seemed to be largely affected by meteorological conditions. HONO concentrations showed very typical diurnal variations during intensive winter and summer periods. Nitrogen oxides were key precursors of HONO formation and the HONO/NO2 values were higher than those reported for direct emission (<1%), indicating the prevalence of secondary chemical HONO formation on direct emission during both periods.We used a pseudo steady state approach (PSS), which included homogeneous and heterogeneous reactions and direct emission, explaining on average about 83% and 48% of the observed HONO levels during the intensive winter and summer periods, respectively. The daytime unknown HONO production was on average 2.58ppbh−1 during the summer period. The HNO3 and fine particulate NO3− photolysis contributed weakly as HONO source. Including these sources in the PSS calculation, we explained about 53% of the observed HONO levels. The results showed that heterogeneous JNO2 dependent processes on aerosol and ground surfaces, involving NO2 as HONO precursor, were HONO sources during the summer measurements.

Chemical characteristics of inorganic ammonium salts in PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; in the atmosphere of Beijing (China)

Abstract. The atmospheric concentrations of gaseous HNO3, HCl and NH3 and their relative salts have been measured during two field campaigns in the winter and in the summer of 2007 at Beijing (China), as part of CAREBEIJING (Campaigns of Air Quality Research in Beijing and Surrounding Region). In this study, annular denuder technique used with integration times of 2 and 24h to collect inorganic and soluble PM2.5 without interferences from gas–particle and particle–particle interactions. The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. Fine particulate Cl−, NH4+ and SO42− exhibited distinct temporal variations, while fine particulate NO3− did not show much variation with respect to season. Daily mean concentrations of fine particulate NH4+ and SO42− were higher during summer (12.30 μg m−3 and 18.24 μg m−3, respectively) than during winter (6.51 μg m−3 and 7.50 μg m−3, respectively). Daily mean concentrations of fine particulate Cl− were higher during winter (2.94 μg m−3) than during summer (0.79 μg m−3), while fine particulate NO3− showed similar both in winter (8.38 μg m−3) and in summer (9.62 μg m−3) periods. The presence of large amounts of fine particulate NO3− even in summer are due to higher local and regional concentrations of NH3 in the atmosphere available to neutralize H2SO4 and HNO3, which is consistent with the observation that the measured particulate species were neutralized. The composition of fine particulate matter indicated the domination of (NH4)2SO4 during winter and summer periods. In addition, the high relative humidity conditions in summer period seemed to dissolve a significant fraction of HNO3 and NH3 enhancing fine particulate NO3− and NH4+ in the atmosphere. All measured particulate species showed diurnal similar patterns during the winter and summer periods with higher peaks in the early morning, especially in summer, when humid and stable atmospheric conditions occurred. These diurnal variations were affected by wind direction suggesting regional and local source influences. The fine particulate species were correlated with NOx and PM2.5, supporting the hypothesis that traffic may be also an important source of secondary particles.

Planktonic dinitrogen fixation along a longitudinal gradient across the Mediterranean Sea during the stratified period (BOUM cruise)

Abstract. This study provides extensive data on planktonic N2 fixation rates across the whole Mediterranean Sea. They show that N2 fixation occurs in Mediterranean waters during the stratification period, with a clear decreasing trend from the oligotrophic western basin (10–76 μmol m−2 d−1) to the ultra oligotrophic eastern basin (0–0.4 μmol m−2 d−1). Highest rates are measured in the less oligotrophic western basin, between the surface and 75 m-depth, where 45 to 75 % of N2 fixation are found within the picoplanktonic fraction (&lt;3 μm). While the biogeochemical impact of N2 fixation in the eastern basin seems negligible, N2 fixation is able to sustain up to 35 % of new primary production during the stratified period in the western basin. These data disagree with indirect estimates of N2 fixation based on geochemical tracers and nutrient budgets, which indicates that N2 fixation increases with increasing N:P ratios and decreasing stable N isotopic signature of particulate organic nitrogen and NO3− from west to east. These results finally point out the need to assess N2 fixation at a higher temporal resolution in order to better understand the diazotrophs' dynamic under contrasted biogeochemical conditions.

Source of surface ozone and reactive nitrogen speciation at Mount Waliguan in western China: New insights from the 2006 summer study

measurement period of 2006 (55%) than that of 2003 (25%). The abnormally high values of NOy observed in 2003 were suspected to be due to the positive interference from ammonia (NH3) to the particular catalytic converter used in that study. Varied diurnal patterns were observed for the various NOy components. The ozone production efficiencies (DO3/DNOz), which were estimated from the slope of the O3‐NOz scatterplot, were 7.7–11.3 for the polluted plumes from central and eastern China. The speciation of reactive nitrogen was investigated for the first time in the remote free troposphere in western China. PAN and particulate NO3 − were the most abundant reactive nitrogen species at WLG, with average proportions of 32% and 31%, followed by NOx (24%) and HNO3 (20%). The relatively large contribution of particulate NO3 − to NOy was due to the presence of high concentrations of NH3 and crustal particles, which favor the formation of particulate nitrate. An analysis of backward trajectories for the recent 10 years revealed that air masses from central and eastern China dominated the airflow at WLG in summer, suggesting strong impact of anthropogenic forcing on the surface ozone and other trace constituents on the Plateau.

Regional characteristics of dry deposition of sulfur and nitrogen compounds at EANET sites in Japan from 2003 to 2008

Abstract The regional characteristics of estimated dry deposition and total atmospheric deposition, including dry and wet deposition, at 10 EANET sites in Japan from April 2003 to March 2008 are discussed. The components examined for dry deposition were sulfur compounds (SO2 and particulate SO42−) and nitrogen compounds (HNO3, NH3, particulate NO3−, and NH4+). Dry deposition was calculated by the product of the deposition velocity estimated by the inferential method for forest and grass surfaces and the air concentration of each compound. The 5-year mean annual dry deposition amounts for sulfur and nitrogen compounds were in the range of 5–37 and 7–50 mmol m−2 year−1, respectively. The regional characteristics of dry deposition amounts were similar between sulfur and nitrogen compounds, which showed higher deposition in the Sea of Japan side and in the Western Japan. The 5-year mean annual total deposition amounts for sulfur and nitrogen compounds were in the range of 28–77 and 22–130 mmol m−2 year−1, respectively. The contribution of dry deposition to the total deposition amounts was 10–55% and 13–56% for sulfur and nitrogen compounds, respectively. The regional characteristic of total deposition was different between sulfur and nitrogen compounds. Total deposition amounts in Japan were larger than those in CASTNET and EMEP because of high wet deposition, which implied that the increasing emissions of air pollutants in East Asia caused high atmospheric depositions.

Airborne observations of ammonia and ammonium nitrate formation over Houston, Texas

Anthropogenic emissions of NOx (nitric oxide (NO) + nitrogen dioxide (NO2)), which in sunlight can be oxidized to form nitric acid (HNO3), can react with ammonia (NH3) to form ammonium nitrate particles. Ammonium nitrate formation was observed from the NOAA WP‐3D aircraft over Houston during the 2006 Texas Air Quality Study with fast‐response measurements of NH3, HNO3, particle composition, and particle size distribution. Typically, NH3 mixing ratios over the urban area ranged from 0.2 to 3 ppbv and were predominantly from area sources. No NH3 enhancements were observed in emission plumes from power plants. The few plumes with high NH3 levels from point source emissions that were sampled are analyzed in detail. While the paucity of NH3 data in emission inventories made point source identification difficult, one plume was traced to NH3 release from an industrial accident. NH3 mixing ratios in these plumes ranged from 5 to 80 ppbv. In these plumes, the NH3 enhancement correlated with a decrease in HNO3 mixing ratio and an increase in particulate NO3− concentration indicating ammonium nitrate formation. The ammonium nitrate aerosol mass budget in the plumes was analyzed to assess the quantitative agreement between the gas and aerosol phase measurements. The thermodynamic equilibrium between the gas and aerosol phase was examined for one flight by comparing the modeled dissociation constant for ammonium nitrate with NH3 and HNO3 measurements. The high levels of NH3 in these plumes shifted the equilibrium toward favorable thermodynamic conditions for the condensation of ammonium nitrate onto particles.

Influence of ozone and humidity on the formation of sulfate and nitrate in airborne fine particles

Ambient concentrations of HNO3 and SO2, and particulate NO3 − and SO4 2 − were simultaneously measured in daytime and nighttime in southern Taiwan, to investigate the conversion effect of these inorganic species into airborne particulate. During the episode days, the average particulate nitrate mass of accumulation mode (0.18–1.8 μ m) measured over daytime and nighttime were about 3.9 and 7.6 times higher than those measured during non-episode days, respectively. The mean value of gaseous nitric acid was always higher during episode daytime than that during non-episode daytime. In addition, the SO4 2 − mass of accumulation mode during episode days was about 2.6 and 2.0 times higher than those during the daytime and nighttime of the non-episode days, respectively. Both of (1) the extent of SO2 oxidation to sulfate and NO2 oxidation to nitrate and (2) conversion ratios for sulfur (Fs) and nitrogen (Fn) were defined and calculated using field measurements. The nighttime Fn and Fs during the episode days were about 4 and 1.6 times higher, respectively, than those during the non-episode days. Furthermore, the Fs and Fn increased with the increase of relative humidity during both of the episode daytime and nighttime. A positive correlation coefficient that the Fn and Fs increases with increasing ozone concentration was found during the non-episode daytime. These results might be attributed to high NO2, SO2 and ozone concentrations in a humid atmosphere, and also the fact that the gas-to-particle conversion plays an important role during episode days.

Factors controlling sea salt modification and dry deposition of nonsea‐salt components to the ocean

Modification of sea salt aerosol particles (SSA particles) by HNO3 and SO2 is an important process for changing phase partitioning of acidic gases from industrial regions to the ocean. During 12–29 September 2005, size‐segregated aerosol particles and acidic gases were sampled around the western part of the Japanese Islands to elucidate controlling factors of modification of SSA particles by acidic gases and to estimate dry deposition flux over the ocean. For coarse (&gt;2 μm diameter) SSA particles, the amount of Cl− deficiency from the seawater ratio was comparable to the sum of the equivalent concentrations of NO3− and nonsea‐salt (nss) SO42−, suggesting Cl− displacement of SSA particles by acidic gases such as HNO3 and SO2. The Cl− deficiency of SSA particles varied according to the size range and wind speed. Decreasing modification occurred with increasing wind speed for particles of 2–8 μm. Under high (low) wind conditions, the NO3− concentration per unit surface area of coarse SSA particles was lower (higher) for particles &gt;8 μm than for those of 2–8 μm diameter. The respective dry deposition fluxes (Fdry) of NO3−, nss‐SO42−, HNO3, and SO2 were estimated according to the wind speed and size of aerosol particles. On average, the Fdry of particulate NO3− was 10 times larger than that of HNO3, but Fdry of nss‐SO42− was almost equal to that of SO2. Phase partitioning of dry deposition for NO3− and nss‐SO42− over the ocean differs from that of coastal marine areas.

Transport and transformation of total reactive nitrogen over the East China Sea

Ground‐based measurement of total reactive nitrogen (NOy), NOy(g) (gas phase NOx + HNO3), and particulate NO3− was carried out at the Cape Hedo Atmosphere and Aerosol Monitoring Station (CHAAMS) in Okinawa, Japan, from spring to winter in 2006. The concentrations of NOy, NOy(g), and particulate NO3− were simultaneously high in spring but low in summer. This difference was mainly caused by air mass history, which was strongly associated with the typical weather pattern observed in the east Asian region for each season. The chemical transformation process of particulate NO3− during transport was examined using the data measured at Qingdao, China, in spring 2006. As the transport time of air masses increased, particulate NO3− continuously shifted from fine mode to coarse mode. It was found that the chemical transformation of particulate NO3− was mainly associated with the transport time of air masses, the geographical position of CHAAMS, and the transition from NH4NO3 to gas phase HNO3. In air masses from Qingdao, China, the ratio of NOy concentration observed at CHAAMS to that at Qingdao was about 0.1, which was lower than that of SOy (SO2 + nss‐SO42−). Sulfate was found in fine particles at CHAAMS in contrast to particulate NO3−. As the lifetimes of NOy and SOy depend on the particle size, the difference in chemical transformation process during transport largely influences the abundance of transported NOy. The variations of NOy(g) and particulate NO3− were analyzed when dust plumes reached CHAAMS. The presence of dust causes the formation of particulate NO3− in coarse mode from NOy(g) and an increase of its fraction in NOy. The effect of volcanic activity on particulate NO3− concentration was also analyzed. It is suggested that particulate NO3− escaped to gas phase HNO3 through the uptake of abundant volcanic H2SO4 by aerosols.

Nitric acid phase partitioning and cycling in the New England coastal atmosphere

During summer 2004, HNO3 and size‐resolved aerosols were measured in parallel, and corresponding dry‐deposition fluxes were modeled at Appledore Island, Maine, as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field program. HNO3 concentrations varied widely on the timescale of hours; however, all days were characterized by a minimum near sunrise. Mixing ratios normally peaked in the early afternoon; maximum and median concentrations of HNO3 during the campaign were 337 and 22.8 nmol m−3, respectively. Aerosol NO3− exhibited a bimodal size distribution with a primary peak associated with sea salt at ∼4 μm and a secondary sub‐μm peak. The median NO3− concentrations in sub‐ and super‐μm diameter size fractions were 3.3 and 7.7 nmol m−3, respectively. Peak HNO3 and super‐μm NO3− concentrations were associated with westerly and southwesterly flow regimes respectively. The multiphase cycling of HNO3 was evaluated as a function of transport sector. Although median total nitrate (HNO3 + NO3−) concentrations were higher under westerly flow, higher median dry‐deposition rates of total nitrate were associated with southwesterly flow. Sea‐salt concentrations were ∼3 times greater during southwesterly flow, which shifted the phase partitioning toward particulate NO3−. Consequently, under westerly flow, HNO3 deposited more quickly than aerosol NO3−, while for southwesterly flow, the fluxes from the two phases were comparable. On the basis of all data, the median dry‐deposition fluxes for HNO3 and aerosol NO3− were 8.2 and 5.6 μmol m−2 d−1; super‐μm size fractions dominated the NO3− flux.

Partitioning of HNO3 and particulate nitrate over Tokyo: Effect of vertical mixing

Ground‐based measurements of gas‐phase nitric acid (HNO3) and particulate nitrate (NO3−) were performed in Tokyo during 2003–2004. These measurements provide a comprehensive data set for investigating the diurnal and seasonal variations of gas‐phase HNO3 and particulate NO3− and the thermodynamic equilibrium of these compounds. HNO3 and NO3− have distinct diurnal and seasonal variations, especially in summer. This study shows that the thermodynamic equilibrium of HNO3 and NO3− and the production rate of total nitrate (TNO3 = HNO3 + NO3−) are the major controlling factors affecting these seasonal and diurnal variations. A thermodynamic equilibrium model (ISORROPIA) is newly coupled with a one‐dimensional (1‐D) model to take into account the effect of vertical mixing during daytime on the partitioning of HNO3 and NO3− by constraining the TNO3 concentrations to the observations. The 1‐D model reproduces the NO3−/TNO3 ratios observed during daytime, whereas the equilibrium model significantly underestimates these ratios. The agreement between the observed and calculated NO3−/TNO3 ratios is improved over the observed temperature range (1°–34°C) and relative humidity (18–95%) by the 1‐D model. These results suggest the importance of vertical mixing in determining HNO3‐NO3− partitioning in the boundary layer. It is also found that the mass accommodation coefficient for HNO3 needs to be approximately 0.1 to explain the observed HNO3‐NO3− partitioning at the surface.

Comparison of PM2.5 and PM10 in A Suburban Area in Korea During April, 2003

Airborne particulate matter (PM) concentrations were measured in Iksan, a suburban area in South Korea during April, 2003. PM2.5 (particles with an aerodynamic diameter less than 2.5 μm) and PM10 (particles with an aerodynamic diameter less than 10 μm) samples were collected, and the chemical characteristics of particles were examined for diurnal patterns, yellow dust/rainfall influences, and scavenging effects. Average concentrations of PM2.5 and PM10 mass measured were 37.3 ± 16.2 μg m−3 and 60.8 ± 29.5 μg m−3, respectively. The sum of ionic chemical species concentrations for PM2.5 and PM10 was 16.9 ± 7.3 and 23.1 ± 10.1 μg/m3, respectively. A significant reduction in PM mass concentrations during rainfall days was observed for coarse mode (PM2.5 − 10) particles, but less reduction was found for fine (PM2.5) mass concentration. SO42−, NH4+, and K+ predominated in fine particulate mode, NO3− and Cl− predominated in fine particle mode and coarse particle mode, but Na+, Mg2+, and Ca2+ mostly existed in coarse mode. The high concentration of ammonium due to local emissions and long-range transport neutralized sulfate and nitrate to ammonium sulfate and ammonium nitrate, which were major forms of airborne PM in Iksan. Average mass concentrations of PM10 in daytime and at night were 57.6 and 70.0 μg m−3, and those of PM2.5 were 35.4 and 42.5 μg m−3, respectively. NO3− and Cl− in both PM2.5 and PM10 were about double at night than in the daytime, while the rest of the chemical species were equal or a little higher at night than in the daytime. The results suggest the formation of ammonium nitrate and chloride when high ammonia concentration and low air temperature are allowed. Backward air trajectory analyses showed that air masses arriving at the site during yellow dust period were transported from arid Chinese regions, which resulted in high concentrations of airborne PM mass concentrations. In the meantime, air mass trajectories during a rainfall period were mostly from the Pacific Ocean or the East China Sea, along with a relatively low PM concentration.

Contribution of particulate nitrate to airborne measurements of total reactive nitrogen

Simultaneous measurements of speciated, total reactive nitrogen (NOy) and particulate NO3− (particle diameter &lt;1.3 μm) were made on board the NASA P‐3B aircraft over the western Pacific in February–April 2001 during the Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment. Gas‐phase and particulate NOy was measured using a gold tube catalytic converter. For the interpretation of particulate NOy, conversion efficiencies of particulate NH4NO3, KNO3, NaNO3, and Ca(NO3)2 were measured in the laboratory. Only NH4NO3 showed quantitative conversion, and its conversion efficiency was as high as that for HNO3. NOy measured on board the aircraft was found to be systematically higher by 10–30% than the sum of the individual NOy gas components (∑(NOy)i) at 0–4 km. Particulate NO3− concentrations measured by a particle‐into‐liquid sampler (PILS) were nearly equal to NOy − ∑(NOy)i under low‐dust‐loading conditions. The PILS data showed that the majority of the particulate NO3− was in the form of NH4NO3 under these conditions, suggesting that NH4NO3 particles were quantitatively converted to detectable NO by the NOy converter, consistent with the laboratory experiments. The contribution of particulate NO3− to NOy was most important at 0–2 km, where NO3− constituted 10–30% of NOy during TRACE‐P. On average, the amounts of particulate NO3− and gas‐phase HNO3 were comparable in this region.

The impact of changing nitrogen oxide emissions on wet and dry nitrogen deposition in the northeastern USA

This study is an attempt to quantify the relation between changes in NOx emissions and nitric acid (HNO3) in the northeastern USA. From this relation, and previous work relating NOx emission changes and wet NO3− deposition, we can estimate how changing NOx emissions may impact total (wet+dry) measured nitrogen (N) deposition. Electric utility emissions account for 14, and vehicle emissions account for over 12 of the total NOx emissions in the eastern USA. Canadian NOx emissions from the seven easternmost provinces (Manitoba and east) represent less than 10% (1.2 teragrams (Tg) NOx) of the NOx emissions compared with those from the eastern USA. Emissions from eastern Canada are dominated by vehicle NOx emissions, which account for 23 of the total NOx emissions from eastern Canada. Data from the EPA National Emissions Inventory show, for the period 1991–2001, that nitrogen oxide (NOx) emissions in the eastern USA have declined by 17% to from 16.1 to 13.1 Tg. Large declines in vehicle emissions in 2001 may be questionable. If 2001 data are excluded the decline in total NOx is only 7%. A recent assessment of EPA's emissions estimates suggest that vehicle NOx emissions may be underestimated, and total NOx emissions reductions may be less than what is reported by the EPA. The CASTNet (Clean Air Status and Trends Network) measurements of N dry deposition include HNO3, particulate NO3− and NH4+. The dominant N dry deposition product measured is HNO3, which represents 80% of measured N dry deposition for the sites used in this study. Amounts of NH3, NO2, organic nitrate and PAN dry deposition are not measured by CASTNet. The NH3 and NO2 deposition are probably significant, and may be major N dry deposition components in some areas. Random coefficient models with total NOx emissions as the independent variable, and HNO3 concentrations as the dependent variable, show that reducing total NOx emissions by 50% should reduce HNO3 concentrations by 36%. The average efficiency (the ratio of % change in HNO3 to % change in NOx emissions) is 72%. Random coefficient models with non-vehicle NOx emissions as the independent variable, and HNO3 concentrations as the dependent variable, show a 50% decline in non-vehicle NOx emissions (which is a 23% decline in total NOx emissions) should reduce HNO3 by 17–20%. The average efficiency in this case is 81%. Because non-vehicle NOx emissions data are more reliable than vehicle NOx emissions, non-vehicle NOx models are likely more accurate than the total NOx models. Combining the results of this study with previous work, which examined the relation between NOx emissions and wet NO3 concentrations, show that reducing total NOx emissions by 50% should reduce total NO3− deposition by 37% (wet+dry combined efficiency is 74%), and total N deposition (as measured by CASTNet sites in the northeastern USA) by 25%. A decline in total NOx emissions of 23%, from a 50% reduction in non-vehicle NOx emissions should, on average, reduce total NO3− deposition by 20% (wet+dry combined efficiency is 87%), and total N deposition, as measured by CASTNet sites, by 15%.

Effects of SO2 and NOx Emission Reductions on PM2.5 Mass Concentrations in the Southeastern United States

Measurements from sites of the Southeastern Aerosol Research and Characterization (SEARCH) program, made from 1998 to 2001, are used with a thermodynamic equilibrium model, Simulating Composition of Atmospheric Particles at Equilbrium (SCAPE2), to extend an earlier investigation of the responses of fine particulate nitrate (NO3 − and fine particulate matter (PM2.5) mass concentrations to changes in concentrations of nitric acid (HNO3) and sulfate (SO4 2− ). The responses were determined for a projected range of variations of SO4 2− and HNO3 concentrations resulting from adopted and proposed regulatory initiatives. The predicted PM2.5 mass concentration decreases averaged 1.8– 3.9 g/m3 for SO4 2− decreases of 46– 63% from current concentrations. Combining the SO4 2− decrease with a 40% HNO3 decrease from current concentrations (approximating expected mobile-source oxides of nitrogen [NOx] reductions by 2020) yielded additional incremental reductions of mean predicted PM2.5 mass concentration of 0.2 g/m3 for three nonurban sites and 0.8– 1 g/m3 for one nonurban and two urban sites. Increasing the HNO3 reduction to 55% (an estimate of adding Clear Skies Phase II NOx reductions) yielded additional incremental reductions of mean predicted PM2.5 mass concentration of 0–0.4 μg/m3. Because of the well-documented losses of particulate NO3 − from Federal Reference Method (FRM) filters, only a fraction of these incremental changes would be observed.

NITROGEN DEPOSITION ONTO THE UNITED STATES AND WESTERN EUROPE: SYNTHESIS OF OBSERVATIONS AND MODELS

The documented acceleration of NH3 and NOx (NO NO2) emissions over the last 150 years has accelerated N deposition, compromising air and water quality and altering the functioning of terrestrial and aquatic ecosystems worldwide. To construct con- tinental-scale N budgets, we produced maps of N deposition fluxes from site-network observations for the United States and Western Europe. Increases in the rates of N cycling for these two regions of the world are large, and they have undergone profound modification of biospheric-atmospheric N exchanges, and ecosystem function. The maps are necessarily restricted to the network measured quantities and consist of statistically interpolated fields of aqueous NO3 and NH4, gaseous HNO3 and NO2 (in Europe), and particulate NO3 and NH4. There remain a number of gaps in the budgets, including organic N and NH3 de- position. The interpolated spatially continuous fields allow estimation of regionally inte- grated budget terms. Dry-deposition fluxes were the most problematic because of low station density and uncertainties associated with exchange mechanisms. We estimated dry N de- position fluxes by multiplying interpolated surface-air concentrations for each chemical species by model-calculated, spatially explicit deposition velocities. Deposition of the ox- idized N species, by-products of fossil-fuel combustion, dominate the U.S. N deposition budget with 2.5 Tg of NOy-N out of a total of 3.7-4.5 Tg of N deposited annually onto the conterminous United States. Deposition of the reduced species, which are by-products of farming and animal husbandry, dominate the Western European N-deposition budget with a total of 4.3-6.3 Tg N deposited each year out of a total of 8.4-10.8 Tg N. Western Europe receives five times more N in precipitation than does the conterminous United States. Estimated N emissions exceed measured deposition in the United States by 5.3- 7.81 Tg N, suggesting significant N export or under-sampling of urban influence. In Europe, estimated emissions better balance measured deposition, with an imbalance of between 0.63 and 2.88 Tg N, suggesting that much of the N emitted in Europe is deposited there, with possible N import from the United States. The sampling network in Europe includes urban influences because of the greater population density of Western Europe. Our analysis of N deposition for both regions was limited by sampling density. The framework we present for quantification of patterns of N deposition provides a constraint on our under- standing of continental biospheric-atmospheric N cycles. These spatially explicit wet and dry N fluxes also provide a tool for verifying regional and global models of atmospheric chemistry and transport, and they represent critical inputs into terrestrial models of bio- geochemistry.