NO2 Trends Research Articles

Water Quality Trends and Changing Agricultural Practices in a Midwest U.S. Watershed, 1994–2006

Sediment and nutrient concentrations in surface water in agricultural regions are strongly influenced by agricultural activities. In the Corn Belt, recent changes in farm management practices are likely to affect water quality, yet there are few data on these linkages at the landscape scale. We report on trends in concentrations of N as ammonium (NH(4)) and nitrate (NO(3)), soluble reactive phosphorus (SRP), and suspended sediment (SS) in three Corn Belt streams with drainage areas of 12 to 129 km(2) for 1994 through 2006. During this period, there has been an increase in conservation tillage, a decline in fertilizer use, and consolidation of animal feeding operations in our study watersheds and throughout the Corn Belt. We use an autoregressive moving average model to include the effects of discharge and season on concentrations, LOWESS plots, and analyses of changes in the relation between discharge and concentration. We found significant declines in mean monthly concentrations of NH(4) at all three streams over the 13-yr period, declines in SRP and SS in two of the three streams, and a decline in NO(3) in one stream. When trend coefficients are converted to percent per year and weighted by drainage, area changes in concentration are -8.5% for NH(4), -5.9% for SRP, -6.8% for SS, and -0.8% for NO(3). Trends in total N and P are strongly tied to trends in NO(3), SRP, and SS and indicate that total P is declining, whereas total N is not.

Investigating the emissions and performance of high bypass ratio aero-engines

In the last decades the development of new technologies and methods of analysis has brought a continuous improvement of the performance of aeronautical jet engines among which are reductions of polluting substances and noise. In this vein, and in accordance with the general attention given environmental issues by the scientific community and governments, a first analysis has been conducted to assess the potential of new types of engines to meet the green environmental expectations. The objects of the study are two powerplants, one with bypass ratio (BPR) 5, used as baseline, and the other with BPR 12. A parametric analysis of the performance and the emissions of these engines was performed using software which directly coupled models of engine performance, emissions, and aircraft operations. Design-point (take-off) and off-design (whole of flight-mission) cases have been considered in order to show the different scenarios and information they could present. Lefebvre equations were used and calibrated to match reasonably the measured NO x and CO emissions of current combustors. Calibration method and errors are presented to clarify the approach. Trends of NO x and CO2 emissions relative to different operating parameters are presented for both cases. The principal differences in emissions for engines of high BPR are shown to be higher values of NO x associated with the off-design case.

Diesel combustion modelling using LES turbulence model with detailed chemistry

Diesel spray combustion and emissions are modelled in this study using large eddy simulation (LES) turbulence models coupled with spray breakup and detailed chemistry models. The objective of this study is to develop numerical models that can be used to predict the diesel spray combustion process. The LES filtering procedure yields unknown subgrid scale terms that need to be modelled. A one-equation, non-viscosity dynamic structure model is used to model the subgrid stress tensor and the gradient method is used to model the subgrid scalar flux. The model uses a tensor coefficient determined by a dynamic procedure and the sub-grid kinetic energy that is to enforce a budget on the energy flow between the resolved and unresolved scales. A skeletal n-heptane reaction mechanism is used to simulate the diesel fuel chemistry. A reduced NO x mechanism is incorporated into the n-heptane mechanism to simulate NO x formation, and the soot emission process is simulated by a phenomenological model that uses a competing formation and oxidation rate formulation. The present models were validated by comparing predicted results against experimental data of a diesel engine. The predicted and measured cylinder pressure history and heat release rate data are in good agreement. Trends of NO x and soot emissions are also captured with respect to different injection timings and exhaust gas recirculation (EGR) levels. Results indicated that the present models can capture the overall combustion process and can be further developed into a useful tool for engine combustion simulations.

Winter climate affects long-term trends in stream water nitrate in acid-sensitive catchments in southern Norway

Abstract. Controls of stream water NO3 in mountainous and forested catchments are not thoroughly understood. Long-term trends in stream water NO3 are positive, neutral and negative, often apparently independent of trends in N deposition. Here, time series of NO3 in four small acid-sensitive catchments in southern Norway were analysed in order to identify likely drivers of long-term changes in NO3. In two sites, stream water NO3 export declined ca 50% over a period of 25 years while in the other sites NO3 export increased with roughly 20%. Discharge and N deposition alone were poor predictors of these trends. The most distinct trends in NO3 were found in winter and spring. Empirical models explained between 45% and 61% of the variation in weekly concentrations of NO3, and described both upward and downward seasonal trends tolerably well. Key explaining variables were snow depth, discharge, temperature and N deposition. All catchments showed reductions in snow depth and increases in winter discharge. In two inland catchments, located in moderate N deposition areas, these climatic changes appeared to drive the distinct decreases in winter and spring concentrations and fluxes of NO3. In a coast-near mountainous catchment in a low N deposition area, these climatic changes appeared to have the opposite effect, i.e. lead to increases in especially winter NO3. This suggests that the effect of a reduced snow pack may result in both decreased and increased catchment N leaching depending on interactions with N deposition, soil temperature regime and winter discharge.

Speciation and Chemical Evolution of Nitrogen Oxides in Aircraft Exhaust near Airports

Measurements of nitrogen oxides from a variety of commercial aircraft engines as part of the JETS-APEX2 and APEX3 campaigns show that NOx (NOx [triple bond] NO + NO2) is emitted primarily in the form of NO2 at idle thrust and NO at high thrust. A chemical kinetics combustion model reproduces the observed NO2 and NOx trends with engine power and sheds light on the relevant chemical mechanisms. Experimental evidence is presented of rapid conversion of NO to NO2 in the exhaust plume from engines at low thrust. The rapid conversion and the high NO2/NOx emission ratios observed are unrelated to ozone chemistry. NO2 emissions from a CFM56-3B1 engine account for approximately 25% of the NOx emitted below 3000 feet (916 m) and 50% of NOx emitted below 500 feet (153 m) during a standard ICAO (International Civil Aviation Organization) landing-takeoff cycle. Nitrous acid (HONO) accounts for 0.5% to 7% of NOy emissions from aircraft exhaust depending on thrust and engine type. Implications for photochemistry near airports resulting from aircraft emissions are discussed.

The trend, seasonal cycle, and sources of tropospheric NO2 over China during 1997–2006 based on satellite measurement

The characteristics of spatial and temporal distribution of tropospheric NO2 column density concentration over China are presented, on the basis of measurements from the satellite instruments GOME and SCIAMACHY. From these observations, monthly averaged tropospheric NO2 variations are determined for the period of 1997 to 2006. The trend and seasonal cycle are also investigated. The possible source of tropospheric NO2 over megacity area is discussed in this paper. The results show a large growth of tropospheric NO2 over eastern China, especially above the industrial areas with a fast economical growth, such as, Yangtze Rive Delta region and Pearl River Delta region because of the prominent anthropogenic activity. There is a rapid increase of tropospheric NO2 over megacities in China. For instance, Shanghai had a linear significant increase in NO2 columns of ∼20% per year (reference year 1997) in the period of 1997–2006, which is the rapidest increase among all the selected cities. The seasonal pattern of the NO2 concentration shows a difference between the east and west in China. In the eastern part of China, an expected winter maximum in seasonal cycle is found because of the prominent anthropogenic activity and meteorological conditions. In the western part this cycle shows a NO2 maximum in summer time, which is attributed to natural emissions, especially soil emissions and lightning. A quickly increasing vehicle population may contribute to the increase of tropospheric NO2 over megacities in China for the remarkable correlation for vehicle population with tropospheric NO2.

Continuous and filter-based measurements of PM2.5 nitrate and sulfate at the Fresno Supersite

PM(2.5) nitrate (NO-(3)) and sulfate (SO=(4)) were measured continuously with R&P8400N and R&P8400S instruments, respectively, and compared with filter-based measurements at the Fresno Supersite from October, 2000 through December, 2005. NO-(3) concentrations were higher in winter than summer with a long-term decreasing trend. Correlations between 24-h average continuous and filter-based NO-(3) were greater than 0.96 in 4 out of 5 years. Continuous NO-(3) was generally lower than filter-based NO-(3) although the difference decreased over time, from -52% in 2001 to +13% in 2005. These differences were similar in winter (-23%) and summer (-19%) while the corresponding differences between ambient and instrument temperature were -12 and 0.7 degrees C, respectively. Neither seasonal nor long-term trends in NO-(3) can be explained by variations in ambient temperature, the difference between ambient and instrument temperature, or changes in aerosol chemical composition. There were no seasonal or long-term trends in SO=(4) concentrations, partially due to low concentrations observed in Fresno. Long-term variability in the performance of R&P8400 NO-(3) and SO=(4) instruments suggest that collocation with filter measurements is needed for long-term measurements.

Multicomponent absorption of anions in commercial anion-exchange membranes

The equilibrium uptake of aqueous NaCl–NaBr and NaCl–NaNO 3 mixtures into Neosepta ® AMX and Selemion ® AMV anion-exchange membranes was measured experimentally and modeled theoretically, where the total external salt concentration was fixed at 0.1 M. The model, which considered the membrane microstructure as an array of cylindrical pores of identical radius and included electrostatic interactions and ion hydration effects, accurately predicted anion uptake in the membranes. During competitive uptake, the anion with the larger hard-sphere radius was preferentially absorbed, with an observed and computed selectivity trend of NO 3 − > Br − > Cl −. By matching experimental data to the model, the average radius of pores in water-equilibrated membranes was found to be 3.3 nm (AMX) and 2.4 nm (AMV), indicating that an AMV membrane (1.98 mmol/g IEC) contains a greater number of smaller pores as compared to AMX (1.35 mmol/g IEC). The calculated pore-wall charge density in the two membranes was nearly the same. The NO 3 −/Cl − uptake selectivity was greater than that for Br −/Cl − in both membranes, but the selectivity for a given anion pair was effectively the same for the two membranes at a given external solution composition. This result was explained by model calculations which showed that anion uptake selectivity was essentially independent of pore radius (for pores in the 1.5–4.0 nm range) and a strong function of the fixed-charge site concentration on the pore wall.

Interannual variation and trends in air pollution over Europe due to climate variability during 1958–2001 simulated with a regional CTM coupled to the ERA40 reanalysis

A three-dimensional Chemistry Transport Model was used to study the meteorologically induced interannual variability and trends in deposition of sulphur and nitrogen as well as concentrations of surface ozone (O3), nitrogen dioxide (NO2) and particulate matter (PM) and its constituents over Europe during 1958–2001. The model was coupled to the meteorological reanalysis ERA40, produced at the European Centre for Medium-range Weather Forecasts. Emissions and boundary conditions of chemical compounds and PM were kept constant at present levels.The average European interannual variation, due to meteorological variability, ranges from 3% for O3, 5% for NO2, 9% for PM, 6–9% for dry deposition, to about 20% for wet deposition of sulphur and nitrogen. For the period 1979–2001 the trend in ozone, due to climate variability is increasing in central and southwestern Europe and decreasing in northeastern Europe, the trend in NO2 is approximately opposite. The trend in PM is positive in eastern Europe. There are negative trends in wet deposition in southwestern and central Europe and positive trends in dry deposition overall. A bias in ERA40 precipitation could be partly responsible for the trends. The variation and trends need to be considered when interpreting measurements and designing measurement campaigns.

Trends in Summer Chemistry Linked to Productivity in Lakes Recovering from Acid Deposition in the Adirondack Region of New York

The US Environmental Protection Agency established the Adirondack Effects Assessment Program (AEAP) to evaluate and monitor the status of biological communities in lakes in the Adirondack region of New York that have been adversely affected by acid deposition. This program includes chemical analysis of 30 lakes, sampled two to three times each summer. Results of trends analysis for lake chemistry and chlorophyll a (chlor a) are presented for 1994 to 2003, and a general comparison is made with recent results of the Adirondack Long-Term Monitoring (ALTM) Program, which included chemical analysis of all but two of these lakes (plus an additional 24 lakes) monthly, year-round for 1992–2004. Increases in pH were found in 25 of the 30 AEAP lakes (P < 0.05) and increases in acid-neutralizing capacity (ANC) were found in 12 of the 30 lakes (P < 0.05). Concentrations of both SO42− and Mg2+ decreased in 11 lakes (P < 0.05), whereas concentrations of NO3− decreased in 20 lakes (P < 0.05). Concentrations of NH4+ decreased in 10 lakes at a significance level of P < 0.05 and in three other lakes based on P < 0.1. Concentrations of inorganic and organic monomeric aluminum generally were below the reporting limit of 1.5 μmol L−1, but decreases were detected in four and five lakes, respectively (P < 0.1). Concentrations of chlor a increased in seven lakes at a significance level of P < 0.05 and two lakes at a significance level of P < 0.1. A significant inverse correlation was also found between chlor a and NO3− concentrations in nine lakes at a significance level of P < 0.05 and two lakes at a significance level of P < 0.1. Results of AEAP analysis of lake chemistry were similar to those of the ALTM Program, although decreases in SO42− concentrations were more evident in the year-round ALTM record. Overall, the results suggest (a) a degree of chemical recovery from acidification during the summer, (b) an increase in phytoplankton productivity, and (c) a decreasing trend in NO3− concentrations resulting from the increased productivity.

Evolution of the urban aerosol during winter temperature inversion episodes

Winter temperature inversions are for Nordic urban sites a major cause for exceeding air-quality legislation thresholds for most primary pollutants. In this study, number particle size distributions have been measured and compared to other tracers for traffic emissions. Concentrations during winter days with and without morning temperature inversion were compared. Morning temperature inversion resulted in high concentrations of traffic-related pollutants, including CO, NO and NO 2 together with ultrafine particles, while the pollution levels where considerably lower during mornings without temperature inversion. The specific time trends of NO x species could be well understood when considering the reaction with O 3. The two different particle measures used in this study, i.e. the number concentration of ultrafine particles (10–100 nm) and the mass of particles below 10 μm (PM 10), both increased during morning rush hours. When the morning inversion broke up and ground-level air mixed with air aloft, the number of particles decreased more rapidly than PM 10 concentrations. LIDAR measurements were used to follow the vertical distribution of particles, and they clearly showed how the mixing processes started after the morning inversion and resulted in rising of the inversion followed by a relatively well-mixed boundary layer with a height of 1 km around 14:00.

Long-term observation of real-world road traffic emission factors on a motorway in Switzerland

Long-term air quality measurements from a roadside monitoring site of the Swiss national air pollution-monitoring network (NABEL) are analysed for average real-world road traffic emission factors (EFs) by evaluation of concentration differences under upwind and downwind conditions. Time series of relative EFs for NO x , NO, NO 2, CO, SO 2, number of fine particles, and elemental carbon (EC) are estimated for the total vehicle fleet and converted into absolute EFs by scaling them with emission factors from an emission model [HBEFA, 2004. Handbuch für Emissionsfaktoren des Strassenverkehrs (handbook of emission factors for road traffic). Umweltbundesamt Berlin, Bundesamt für Umwelt, Wald und Landschaft Bern, Infras AG, Bern (published on CD-ROM, see also www.hbefa.net] for a specific reference year. For NO x , NO, NO 2, CO, and SO 2 the EFs of the total vehicle fleet decreased during the time period from 1992 to 2004 by 44%, 51%, 5%, 71%, and 55%, respectively. The much smaller negative trend of NO 2 compared to NO x indicates that the NO 2/NO x road traffic emissions ratio increased during this period (from 14% in 1992 to 23% in 2004). It is shown that this trend is caused by increasing primary NO 2 road traffic emissions rather than by increasing secondary NO 2 formation via reaction of NO with O 3. Data for total particle number concentrations and EC are only available since the year 2004. For 2004, the estimated average emission of total particles with diameter larger than 7 nm is 7.9×10 14 km −1 veh −1, for EC an average emission factor of 21 mg km −1 veh −1 is obtained.

Representativeness of Urban Highest Polluted Zones for Sitting Traffic-Oriented Air Monitoring Stations in a Chinese City

Passive sampling technique was used to preliminary assess the spatial and temporal distribution of air pollution in Suzhou (P.R. China) in 2003, with the aim to determine the representativeness of zones where to monitor air quality. 100 sites were selected to measure 15-days average concentrations of SO2, NOx, NO2, O3, NH3, Benzene, Toluene and Xylene in winter and summer. The distribution trend of NO and NO2, as well as the ratio of NO2/NOx was used to identify the heavy traffic polluted zones and design a network of monitoring stations (macrositing). NO2/NOx ratios varied throughout the two selected seasons and from site to site clearly demonstrating the different extent and completeness of NO → NO2 oxidation processes in time and space. The exceedance frequencies of pollutants was used to design the type of monitoring equipment to be installed at each station (micrositing). An air quality monitoring network composed by 9 monitoring stations has been finally established in 2004 according to the result of this survey and is now currently running.

Response of surface water chemistry to reduced levels of acid precipitation: comparison of trends in two regions of New York, USA

AbstractIn light of recent reductions in sulphur (S) and nitrogen (N) emissions mandated by Title IV of the Clean Air Act Amendments of 1990, temporal trends and trend coherence in precipitation (1984–2001 and 1992–2001) and surface water chemistry (1992–2001) were determined in two of the most acid‐sensitive regions of North America, i.e. the Catskill and Adirondack Mountains of New York. Precipitation chemistry data from six sites located near these regions showed decreasing sulphate (SO42−), nitrate (NO3−), and base cation (CB) concentrations and increasing pH during 1984–2001, but few significant trends during 1992–2001. Data from five Catskill streams and 12 Adirondack lakes showed decreasing trends in SO42− concentrations at all sites, and decreasing trends in NO3−, CB, and H+ concentrations and increasing trends in dissolved organic carbon at most sites. In contrast, acid‐neutralizing capacity (ANC) increased significantly at only about half the Adirondack lakes and in one of the Catskill streams. Flow correction prior to trend analysis did not change any trend directions and had little effect on SO42− trends, but it caused several significant non‐flow‐corrected trends in NO3− and ANC to become non‐significant, suggesting that trend results for flow‐sensitive constituents are affected by flow‐related climate variation. SO42− concentrations showed high temporal coherence in precipitation, surface waters, and in precipitation–surface water comparisons, reflecting a strong link between S emissions, precipitation SO42− concentrations, and the processes that affect S cycling within these regions. NO3− and H+ concentrations and ANC generally showed weak coherence, especially in surface waters and in precipitation–surface water comparisons, indicating that variation in local‐scale processes driven by factors such as climate are affecting trends in acid–base chemistry in these two regions. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Evidence of an increasing NO/NO emissions ratio from road traffic emissions

Abstract A statistical analysis of roadside concentrations of nitrogen oxides (NOX) and nitrogen dioxide (NO2) in London shows that from 1997 to 2003 there has been a statistically significant downward trend (at the p = 0.004 level) in NOX averaged across a network of 36 sites. Conversely, there has been no statistically significant trend in the concentrations of NO2 over the same period. Hourly modelling using a simple constrained chemical model shows that the NO2/NOX emissions ratio from road traffic has increased markedly from a mean of about 5–6 vol% in 1997 to about 17 vol% in 2003. Calculations show that if the NO2/NOX emissions ratio had remained the same as that towards the beginning of each time series, 14 out of the 36 sites would have shown a statistically significant downward trend in NO2 at the p = 0.10 level compared with only five that did. The increase in the NO2/NOX emissions ratio from road traffic in recent years has therefore had a significant effect on recent trends in roadside NO2 concentrations. It is shown that the increased use of certain types of diesel particulate filters fitted to buses is likely to have made an important contribution to the increasing trends in the NO2/NOX emissions ratio. However, it is unlikely that these filters account for all of the observed increase and other effects could be important, such as the increased penetration of diesel cars in the passenger car fleet and new light- and heavy-duty engine technologies and management approaches.

Indicators of nitrate leaching loss under different land use of clayey and sandy soils in southeastern Oklahoma

Abstract Evidence of increasing nitrate (NO 3 − ) leaching losses from soils under various land-use systems has elevated the interest and need to find better land-management practices. An essential step is to find and understand the common parameters that closely relate to leaching losses across a wide range of land-use systems. The overall objective of this study was to relate NO 3 − leaching losses to various properties across soil types and land-use systems. We chose two locations in southeastern Oklahoma with different soil types. The land-use systems were old-field (abandoned grassland), forest and cultivated arable crop. Parameters determined were soil biomass C, biomass N, total C, total N, extractable mineral N, pH, moisture, CO 2 emission and lysimeter-drained water NO 3 − concentration. Firstly, stepwise multiple linear-regression analyses were performed between biannual NO 3 − leaching losses and monthly measured independent variables. Secondly, annual and biannual timescale-independent variables and respective NO 3 − leaching losses were used to perform a regression model. Monthly measured parameters explained over 75% of the biannual NO 3 − leaching losses, but often with different explanatory variables. Land use was the dominant parameter on monthly models, while soil total C, total C/N ratio, extractable NH 4 + and NO 3 − concentrations and water flux were the most frequently included parameters. When annual- and biannual-mean parameter values were used in regression, respective NO 3 − leaching losses were explained by somewhat similar parameters to monthly models. Together, these parameters explained only 63, 53 and 44% of the total variation in NO 3 − leaching loss for the first, second and biannual period, respectively. These models explained a consistently lower amount of variation than the monthly models. Additionally, more than 70% of the total variation could be explained by NO 3 − concentration and the tested soil-type related parameters. Results from this study revealed that annual and biannual trends of NO 3 − leaching losses across land-use systems can be explained by a number of parameters collectively rather than a single parameter. Thus, there is no single parameter that can be used to fully predict NO 3 − leaching losses across different land-use systems on a monthly, annual and biannual basis. Additionally, there is no specific month or season for sampling to better predict NO 3 − leaching losses across different soil and land-use systems.

Evaluation of long‐term tropospheric NO2 data obtained by GOME over East Asia in 1996–2002

Long‐term tropospheric nitrogen dioxide (NO2) column data obtained by the Global Ozone Monitoring Experiment (GOME) (G‐NO2) are evaluated to confirm the trends found in tropospheric NO2 abundances over East Asia between 1996 and 2002. For three locations in Central and East Asia, the G‐NO2 values are compared with tropospheric columns estimated from coincident observations of total NO2 by ground‐based UV/visible spectrometers and stratospheric NO2 by satellite solar occultation sensors (E‐NO2). The comparisons show a slight linear drift in G‐NO2 data from 1996 to 2002. However, it is much smaller than the standard deviation of the differences between G‐NO2 and E‐NO2 and much smaller than the increasing trends in NO2 seen by GOME over the industrial areas of China, demonstrating the validity of the trends estimated using the GOME data.

Past and future simulations of NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; from a coupled chemistry-climate model in comparison with observations

Abstract. Trends in derived from a 45 year integration of a chemistry-climate model (CCM) run have been compared with ground-based measurements at Lauder (45° S) and Arrival Heights (78° S). Observed trends in at both sites exceed the modelled trends in N2O, the primary source gas for stratospheric NO2. This suggests that the processes driving the trend are not solely dictated by changes in but are coupled to global atmospheric change, either chemically or dynamically or both. If CCMs are to accurately estimate future changes in ozone, it is important that they comprehensively include all processes affecting NOx (NO+NO2) because NOx concentrations are an important factor affecting ozone concentrations. Comparison of measured and modelled NO2 trends is a sensitive test of the degree to which these processes are incorporated in the CCM used here. At Lauder the 1980-2000 CCM NO2 trends (4.2% per decade at sunrise, 3.8% per decade at sunset) are lower than the observed trends (6.5% per decade at sunrise, 6.0% per decade at sunset) but not significantly different at the 2σ level. Large variability in both the model and measurement data from Arrival Heights makes trend analysis of the data difficult. CCM predictions (2001-2019) of NO2 at Lauder and Arrival Heights show significant reductions in the rate of increase of NO2 compared with the previous 20 years (1980-2000). The model results indicate that the partitioning of oxides of nitrogen changes with time and is influenced by both chemical forcing and circulation changes.

Long-term trend of chemical constituents in precipitation in Tokyo metropolitan area, Japan, from 1990 to 2002

In order to understand the actual status and mechanism of acid rain, it is important to know the pH of precipitation and its chemical constituents on a continuous and regular basis over a wide area. This study examines acid rain over a wide area using an observational network in the Tokyo metropolitan area of Japan, and analyzes the major chemical constituents of every precipitation sample. Precipitation was collected continuously for a period of 12 years from June 1990 to May 2002 at several sampling sites in the Tokyo metropolitan area, and its pH and chemical constituent concentrations were measured. The average pH ranged from 4.23 to 4.62, clearly indicating acidification of precipitation over the entire Tokyo metropolitan area. A time-trend model was applied to describe temporal variations of chemical constituent concentrations, including annual change rate, seasonal variation, and precipitation effects. Seasonal and annual trends for the past 12 years were examined with the model, using the least squares method. Nonsea salt (nss)-Ca 2+ shows a maximum value in early spring, a seasonality probably caused by calcium-rich particles in airborne yellow dust from Asia. Slightly decreasing annual trends of nss-SO 4 2− may correspond to the recent decreasing trend of atmospheric SO 2 gas concentrations in the Tokyo metropolitan area. The annual trends of NO 3 −, NH 4 +, and nss-Ca 2+ show a large site-to-site difference. The increasing NO 3 −, NH 4 +, and nss-Ca 2+ concentrations at inland suburban sites may be caused by increases in their local sources such as vehicle traffic and municipal waste incineration. The annual change rate of H + is slightly negative or almost zero at every site, so the acidification of precipitation has not become worse since 1990 over the Tokyo metropolitan area.

A new software suite for NO 2 vertical profile retrieval from ground-based zenith-sky spectrometers

Here we present an operational method to improve accuracy and information content of ground-based measurements of stratospheric NO 2. The motive is to improve the investigation of trends in NO 2, and is important because the current trend in NO 2 appears to contradict the trend in its source, suggesting that the stratospheric circulation has changed. To do so, a new software package for retrieving NO 2 vertical profiles from slant columns measured by zenith-sky spectrometers has been created. It uses a Rodgers optimal linear inverse method coupled with a radiative transfer model for calculations of transfer functions between profiles and columns, and a chemical box model for taking into account the NO 2 variations during twilight and during the day. Each model has parameters that vary according to season and location. Forerunners of each model have been previously validated. The scheme maps random errors in the measurements and systematic errors in the models and their parameters on to the retrieved profiles. Initialisation for models is derived from well-established climatologies. The software has been tested by comparing retrieved profiles to simultaneous balloon-borne profiles at mid-latitudes in spring.