The Impact of Precipitation Scavenging on the Transport of Trace Gases: A 3-Dimensional Model Sensitivity Study

With the global Chemistry-Transport model MATCHsensitivity simulations were performed to determinethe degree to which especially upward transport ofgases from the earth's surface is limited byconvective and large-scale precipitation scavenging.When only dissolution of species in the liquid phaseis taken into account, mixing ratio reductions in themiddle and upper troposphere by 10% arecalculated for gases with a Henry's Law constant H of10³ mol/l/atm. The removal increases to 50% forH = 10⁴ mol/l/atm, and to 90% for H =10⁵ mol/l/atm. We also consider scavenging by theice phase, which is generally much less efficient thanby the aqueous phase. In fact, rejection of gases fromfreezing water droplets may be a source of trace gasat higher altitudes.H₂O₂ and the strong acids (H₂SO₄,HNO₃, HCl, HBr, HI) have such large solubilitiesthat they become largely removed by precipitation.When significant concentrations of these gases andsulfate aerosol exist above the liquid water domain ofthe atmosphere, they have likely been produced thereor at higher altitudes, although some could have comefrom trace gas rejection from ice particles or fromevaporating hydrometeors. Several other gases areaffected by precipitation, but not strongly enough toprevent fractional transfer to the middle and uppertroposphere: e.g., HNO₄, HNO₂ at pH 5,CH₂O, the organic acids at pH 6,CH₃SOCH₃, HOCl, HOBr, and HOI. NH₃ islargely removed by liquid phase scavenging at pH 7 and SO₂ atpH 7. At pH less thanabout 6, upward transport of SO₂ should largelydepend on the efficiency of oxidation processes in thewater droplets by O₃ and H₂O₂.Most gases have solubilities which are too low forsignificant precipitation scavenging and aqueous phaseoxidation to occur. This holds, e.g., for O₃, CO,the hydrocarbons, NO, NO₂, HCN, CH₃CN,CH₃SCH₃, CH₃O₂H, CH₃CHOandhigher aldehydes, CH₃OH and higher alcohols,peroxyacetylnitrate (PAN), CH₃COCH₃ andother ketones (note that some of these are not listedin Table I because their solubilities are below 10mol/l/atm). Especially for the short-lived gases,transfer from the boundary layer to the middle andupper troposphere is actually promoted by the enhancedupward transport that occurs in clouds.     

A note on smoke concentration in urban Athens,using data measured by two different methods

Summary The relationships between smoke concentrations, measured at two monitoring stations in Athens by the ASTM (American Society for Testing and Materials) and the OECD (Organization for Economic Co-Operation and Development) methods are studied by using a sample of 738 mean daily values of simultaneous measurements during the period 1984–1986. It is shown that an exponential model, different for each monitoring station, with smoke concentrations in COH (coefficient of haze) as the independent variable and smoke concentrations in gr/m³ as the dependent variable, was the most appropriate for transforming smoke data from COH to gr/m³. Also it is shown that the transformation models can be applied independent of the prevailing meteorological conditions. Finally some tentative conclusions are derived concerning the long term variations of smoke concentrations in gr/m³ for the period 1977–1990 and 1973–1990 at the two monitoring stations respectively. The trend analysis has shown that pollution abatement efforts resulted in an about 10% per year decreasing trend in smoke concentrations, during the period 1986–1990, at these two Athenian stations.With 4 Figures     

The budget of carbon dioxide variance in the surface layer over vegetated fields

The budget equation for carbon dioxide variance can be represented by production, dissipation and flux divergence terms. Each term is measured under near neutral to moderately unstable conditions over vegetated fields. The flux divergence term is about an order of magnitude smaller than production and dissipation terms, though it shows a loss for 0.006 < _v < 1 and a gain for 1 < - _v < 10. Here, _v is the Monin-Obukhov stability parameter including humidity effect. As expected from a closure of the budget, the nondimensional production and dissipation terms are basically identical and represented by the same functional form: (1–16 _v )^–1/2.     

Case studies of frontal passages in a mountain valley with direct access to the bavarian pre-Alpine region results from the German Front Experiment 1987

Summary The orographic impact on cold fronts is investigated in the Loisach River Valley area by means of two events obtained during the German Front Experiment 1987. The discussion is focussed on the frontal passages in the Garmisch area traced from continuous recordings of three meteorological/ air-chemical stations at different heights, viz: valley floor (735m), wank peak station (1780m) and Zugspitze station (2964m); the analysis also includes some aerological data (pibal ascents at Farchant near Garmisch) and sensitive pressure data from temporary stations in the Bavarian foreland, respectively.In the case of the west-east (or parallel to the mountains) moving front A (October 8, 1987), with prefrontal foehn, the following features were observed: i) formation of large (up to 7hPa/100 km up-valley directed pressure gradients after the frontal passage in the foreland, ii) intrusion of cold air from the foreland into the valley (lasting for several hours, filling the whole valley and even traceable at crest height), iii) destruction of upper level foehn current during the intrusion-phase, iv) retardation of the front (progressing rapidly in the foreland but being retarded by the mountains located to the west). In the case of the high-reaching, southward (or perpendicular to the mountains) moving D-front (December 18/19, 1987), such striking orographic modifications were not observed. In that case the mountains located to the west obviously did not act as noticeable barrier and, therefore, the flow around and over the mountains prevented the for ation of large pressure gradients between the foreland and the valley (and an intrusion into the valley).In both events it was confirmed that trace gas measurements provide a very promising tool to localize atmospheric flows indirectly.With 14 Figures     

The carbon isotopic ratio of atmospheric carbon dioxide at Tsukuba,Japan

To find out the secular and seasonal trends of the ¹³C value and CO₂ concentration in the surface air and the determination of the ¹³C in the atmospheric CO₂ collected at Tsukuba Science City was carried out during the period from July 1981 to October 1983. The monthly average of the ¹³C value of CO₂ in the surface air collected at 1400 LMT ranged from -7.52 to \s-8.45 with an average of -7.96±0.25 and the CO₂ concentration in the air varied from 334.5 l 1^-1 to 359 l 1^-1 with an average of 347.2±6.3 l 1^-1. The ¹³C value is high in summer and low in winter and is negatively correlated with the CO₂ concentration. In general, the relationship between the ¹³C and the CO₂ concentration is explainable by a simple mixing model of two different constant carbon isotopic species but the relationship does not always follow the model. The correlation between the ¹³C value and the CO₂ concentration is low during the plant growth season and high at other times. The observed negative deviation of the ¹³C value from the simple mixing model in the plant growth season is partly due to the isotopic fractionation process which takes place in the land biota.     

Winter storms in Switzerland North of the Alps 1864/1865–1993/1994

Summary In the framework of the Swiss National Research Program 31 Climate changes and natural disasters the question was brought up whether a global warming of the atmosphere would have an influence on the frequency and/or intensity of the extratropical storms of the Swiss winter season. In order to investigate a possible trend, time series of days with a minimum wind speed estimate or measurement equivalent to Beaufort 7, 8 and 9 were established. The longest being a record of the mesonet station Zürich from the period 1864 until 1993 (130 years). Slightly shorter time series for three additional stations in Northern Switzerland were compiled to control the behavior of the longest record and to verify the observed temporal trend. From the location of the four investigated wind records the observational domain was restricted to the part of Switzerland north of the Alps whereas for further meteorological considerations the whole North-Atlantic-European area was included.A negative regional trend in the number of storm days has been observed during the last century. In particular, the period before 1940 has to be interpreted as windier than the following decades. The duration of a storm event also decreased on average. In spite of an increase in cyclonic westwind situations since about 1960 over Europe, Switzerland was hit by fewer storms during the same period. One explanation could be that the whole westwind belt has moved slightly further north where a deepening of the cyclones was observed in recent times. Switzerland is usually situated at the southernmost edge of the particular storm fields and is therefore less influenced by strong gales. Rare exceptions are the cases when a secondary depression directly hits Central Europe, e.g. as happened in February 1990 (storm Vivian).With 11 Figures     

Modifications of the structure of cold fronts over the foreland and in a mountain valley

Summary The investigations during the German Frontal Experiment 1987 show the modification of structure and movement of cold fronts in the vicinity of the Alps and in an Alpine valley. The two cold fronts that moved nearly parallel to the mountains were modified at the mountain barrier by the channelling effect. In those cases the postfrontal air flowed into the Inn valley from the valley's exit up-valley. The upper and lower parts of the fronts became detached, and the structure of the lower part changes on its way from the foreland to the mountain barrier.In contrast the fourth cold front, which moved from north-west to south-east, crossed the mountains without appreciable deformation although it suffered some degree of blocking and retardation.With 7 Figures     

On the similarity in atmospheric fluctuations of carbon dioxide,water vapor and temperature over vegetated fields

This paper describes the similarity between atmospheric fluctuations of carbon dioxide, water vapor and temperature using data which cover a wide range of instability (0.02 < < 10). The is the Monin-Obukhov stability parameter including the humidity effect.The spectral analysis shows that the coherency between fluctuations of carbon dioxide and water vapor or temperature is very close to unity, and the phase difference is basically out of phase for whole frequency ranges analyzed. The stability dependence of the normalized standard deviation of carbon dioxide is very similar to those of water vapor and temperature. The normalized standard deviation is about 2.5 under near neutral conditions, and it decreases with increasing instability following the -1/3; power law as (-)^-1/3. The skewness factors of carbon dioxide, water vapor and temperature show a systematic departure with increasing instabilities for 0.02 < s- < 1, and level off at high instabilities for 1 < -\s < 10. The stability dependence of the flatness factors is not so clear as that noted in the standrard deviation and skewness factors. Dissipation rates of carbon dioxide, water vapor and temperature variance are well related to the spectral peak wavelength. This seems to be real since the local production and local dissipation rates are the main terms, almost balancing one another in the variance budget equations for scalar entities.     

Simultaneous measurements of peroxy and nitrate radicals at Schauinsland

We present simultaneous field measurements of NO₃ and peroxy radicals made at night in a forested area (Schauinsland, Black Forest, 48° N, 8° N, 1150 ASL), together with measurements of CO, O₃, NO _x , NO _y , and hydrocarbons, as well as meteorological parameters. NO₂, NO₃, HO₂, and (RO₂) radicals are detected with matrix isolation/electron spin resonance (MIESR). NO₃ and HO₂ were found to be present in the range of 0–10 ppt, whilst organic peroxy radicals reached concentrations of 40 ppt. NO₃, RO₂, and HO₂ exhibited strong variations, in contrast to the almost constant values of the longer lived trace gases. The data suggest anticorrelation between NO₃ and RO₂ radical concentrations at night.The measured trace gas set allows the calculation of NO₃ and peroxy radical concentrations, using a chemical box model. From these simulations, it is concluded that the observed anthropogenic hydrocarbons are not sufficient to explain the observed RO₂ concentrations. The chemical budget of both NO₃ and RO₂ radicals can be understood if emissions of monoterpenes are included. The measured HO₂ can only be explained by the model, when NO concentrations at night of around 5 ppt are assumed to be present. The presence of HO₂ radicals implies the presence of hydroxyl radicals at night in concentrations of up to 10⁵ cm^–3.     

On the annual and monthly mean diurnal variations of diffuse solar radiation at a meteorological station in west Africa

Summary In addition to global solar radiationE ^g , the hourly diffuse componentE ^d incident on a horizontal surface has been measured from February 1993 to January 1995 at a meteorological station in tropical West Africa. The measured diffuse solar irradiance data was corrected for shadow band effects. The monthly mean diurnal variations of diffuse solar irradiance obtained for identical months in the two years have been compared and found to be generally consistent. The corresponding monthly mean hourly values ofE ^d for identical months in 1993 and 1994 agreed to within 9% while yielding correlation coefficients greater than 0.960. In addition, the monthly mean daily totals ofE ^d for identical months were found to agree mostly to within 6% and showed virtually the same annual variations in both years. The monthly mean daily total values of diffuse solar radiation for most months in the two years ranged between 7.94 MJm^–2d^–1 and 10.50 MJm^–2d^–1. The monthly mean of daily hourly maximum values ofE ^d obtained for identical months in the two years have been discussed in relation to the dominant atmospheric conditions during these months. The results been presented here have been compared with those of some investigators within and outside the Africa region.With 8 Figures     

Ab initio study on carbon Kinetic Isotope Effect (KIE) in the reaction of CH4+Cl•

Recent studies suggest that the destruction of methane by Cl in the marine boundary layer could be accounted for as another major sink besides the methane destruction by OH. High level ab initio molecular orbital calculations have been carried out to study the CH₄+Cl reaction, the carbon Kinetic Isotope Effect (KIE) is calculated using Conventional Transition-State Theory (CTST) plus Wigner and Eckart semiclassical tunneling corrections. The calculated KIE is around 1.026 at 300 K and has a small temperature variation. This is by far the largest KIE among different processes involving atmospheric methane destruction (e.g., OH, soil). A calculated mass balance of atmospheric methane including the KIE for the CH₄+Cl reaction is found to favor those methane budgets with enhanced biological methane sources, which have relatively lighter carbon isotope composition.     

Local foehn effects in the upper Isar Valley, part 1: Observations

Summary An unusually strong nocturnal downvalley wind can be regularly observed in the upper Isar Valley close to Mittenwald (Bavarian Alps) when a high-pressure system is located over Central Europe or when ambient southerly winds are present. Due to the structure of the local topography, this downvalley wind has foehn-like properties in the sense that the breakthrough of the flow into the valley is characterized by a strong increase in temperature and a decrease in relative humidity. Therefore the author called this flow Minifoehn. In fact, wind speeds are low in comparison to deep foehn, but gusts may reach values up to 20ms^–1, even under the influence of high pressure systems with weak atmospheric pressure gradients. To investigate the Minifoehn, surface stations have been installed for collecting temperature, humidity, wind and pressure data. Measurements have shown that the Minifoehn represents the upper part of one of the drainage currents which flows over a mountain ridge into the valley at Mittenwald. Nocturnally cooled air drains from a plateau south of Mittenwald through different valleys which merge again near Mittenwald. It seems that the forcing of the nocturnal currents is dominated by the temperature difference between this plateau and the free atmosphere above Mittenwald at the same level. Strong temperature differences are found during clear winter nights and in case of subsidence inversions. Moreover, the appearance of the Minifoehn in autumn and winter is so frequent that we even may find a climatic effect: the upper Isar Valley is usually free of fog during these seasons and nocturnal temperatures are often considerably higher than in other Bavarian Alpine valleys at comparable altitude.     

On the temperature coefficient of PCO 2 in seawater

The commonly reported temperature coefficient of P. the equilibrium partial pressure of CO₂, is (P/T) _A,C ,which is about 15 ppm/°C, or 5% of the atmospheric partial pressure of CO₂. This coefficient, however, applies only to deep water, not to surface water which can exchange CO₂ with the atmosphere. The coefficient (P/T) _A,C ,, where designates constancy of the sum of atmospheric and surface-ocean CO₂, is the appropriate value for air-sea exchange. Numerical values are mass-dependent because the depth of the exchanging ocean layer must be specified. For a 100-m surface layer, the value is ca. 1.5 ppm/°C, or 0.5% of ambient CO₂. Editor's Note:In view of the interdisciplinary importance of the carbon dioxide-climate problem, this note on seawater chemistry should be of interest to specialists beyond the discipline of ocean chemistry.     

Observed variations of σθ and σΦ in the nocturnal drainage flow in a deep valley

The variations of and in the drainage flow in the Brush Creek valley of western Colorado are investigated using data from Doppler acoustic sodars and instrumented towers. The data were obtained on two experimental nights during the 1984 ASCOT field study. There is good agreement between the variations derived from low-level observations of the sodars and those derived from the towers located throughout the valley. The observed hourly average and in the nocturnal drainage flow are about 20 ° to 25 ° and 5 °, respectively; these values are much larger than those generally observed over flat terrain during nighttime stable conditions. After sunrise (about 0600 MST), as the valley warms and the flow direction changes to up-valley, these parameters increase sharply to their peak values at about 0800 MST and then decrease to their normal daytime values after about two hours.In the drainage flow, the hourly average varies inversely with wind speed according to the relation u 0.7ms^-1. The vertical standard deviation is much less enhanced by complex terrain than the horizontal standard deviation. The observed values are predicted fairly well by the local similarity theory.Oak Ridge Associated Universities (ORAU) Summer Research Participant at ATDD in 1987 andOak Ridge Associated Universities (ORAU) Summer Research Participant at ATDD in 1987 and     

The representation of foreseeable natural risks: A contribution to a new cartography of Avalanches

Summary Research carried out on a favourable site in the Pre-Alps of Savoy has allowed the development of risk cartography where avalanche phenomena are defined according to three criteria: maximum extension, type of avalanche, and frequency of avalanche. The maps, composed of simple symbols, allowing easy updating, could be a useful tool for mountain professionals.With 4 Figures     

A meteorological anomaly in Palestine 33 centuries ago: How did the sun stop?

Summary The meteorological conditions pertaining to the famous biblical episode during which Joshua stopped the sun when the Hebrew pursued the Amorites (Joshua 10, 10–13) are analyzed. The story, which cannot be interpreted as an anomaly in the astronomic motions, could be interpreted from the point of view of atmospheric optics. The event occurred at sundown, after an exceptional hailstorm, which left the ground of the valley where the Amorites stayed covered with ice. This resulted in a big drop in the air temperature, forming the conditions for a superior mirage. The Hebrew were in another nearby place, untouched by hail, on hot soil, in the usual daytime conditions of inferior mirage. When they reached the place of the Amorites, their optical path inverted curvature, displacing upwards the sun which was setting. Hail in Palestine is a quite rare meteorological event and the superior mirage would seem decidedly rare in the subtropical climate. Given that the event has been described in detail it could not have been invented, as the superior mirage was then unknown, in particular its connection with hailstorms. This would thus lead to the conclusion that the miracle described should be interpreted as a meteorological event which actually did happen.     

Observations of flow structure in a small forested valley system

Summary Features of the mean flow structure in a small valley system in the Rosalian mountain range are discussed using data from a wind measurement network. Tethered balloon measurements during periods of clear sky form the basic dataset for the analysis of drainage winds and temperature inversions. During periods of weak ambient winds the existence of a pure thermally driven nocturnal valley wind system is shown. With strong ambient winds opposing the drainage flow, a reduced drainage height but the same jet maximum as with weak ambient winds is found. On the other hand with aiding flow the drainage winds are suppressed and flow reversal can occur. This strong valley flow interaction with the ambient wind indicates considerable dynamic influence on the evolution of drainage winds and on the breakup of temperature inversion structure for small valleys.With 15 Figures     

An indication of climatic change as seen from the rainfall data of a mauritanian station

Summary Daily precipitation amounts from the meteorological station at Kaédi (Mauritania) are available since 1904, the country's longest record. The series constitutes an excellent tool for studying variations in the precipitation climate. Analysis of the annual precipitation amounts reveals the existence of a statistically highly significant downward trend which started in the sixties and is still continuing at present.It was further found that the entire reference period 1904–1988 needs, for reasons of data stationarity, to be split into two parts, corresponding respectively to the periods 1904–1967 and 1968–1988, the year 1967 being estimated as the change-point. The latter period corresponds to the drought which has prevailed over the Sahel region since 1968. The jump disclosed in the hydroclimatic system is indicative of the existence of multiple stable climate regimes.With 7 Figures     

Precipitation Chemistry in the Sahelian Savanna of Niger, Africa

Within the framework of the IDAF (IGAC DEBITS AFRICA) network, we present in this paper data on precipitation and aerosol chemistry in the semiarid savanna of the Sahelian region of Niger. An automatic wet-only precipitation collector was operated at the Banizoumbou station during the entire 1996 rainy season (June to September 1996). Inorganic (Na⁺, NH ₄ ⁺ , K⁺, Mg²⁺, Ca²⁺, Cl^-, NO ₃ ^- , SO ₄ ^2- ) and organic contents of the precipitation (HCOOH, CH₃COOH, C₂H₅COOH) were determined by Ion Chromatography (IC) in 29 rainfall events. Once per week, bulk particle samples were collected on the same site, and soluble water material was determined by IC. We examined the influence of atmospheric gas and particle sources on the precipitation and aerosol chemical contents. We established the influence of marine, terrigenous, and biogenic sources in the Sahelian region. The terrigenous signature is dominant and related to Sahelian soil erosion, with a high calcium content in precipitation (31.2 eq L^-1) and in aerosols (1.8 g m^-3). Two other signatures of atmospheric sources are highlighted by the relatively high nitrogenous (ammonium and nitrate) and organic contents (formate, acetate) in the precipitation. Ammonium (12.9 eq L^-1) and nitrate (12.3 eq L^-1) contents confirm respectively the biogenic source of ammonia released by domestic animal excreta in Niger and the natural emissions from semiarid savannas soils, perturbed by wild or domestic animal grazing. In spite of a high potential acidity given by nitrate, formate and acetate; a weak acidity (H⁺ (2.1 eq L^-1) is calculated from the mean pH of 5.67 measured. A statistical analysis of the aerosol chemical composition clearly indicates that nitrates are strongly correlated at the 1% level with terrigenous ions, i.e., Ca²⁺ and Mg²⁺ (0.95 < r < 1). We observed a similar relationship between all the terrigenous ions and nitrate in the precipitation. In the Sahelian region, alkaline soil dust representative of the terrigenous contribution interact, with gaseous nitrogenous and carbonaceous compounds, leading to the neutralization of acid gases and subsequent weak acidity in precipitation. Finally, taking into account the main chemical characteristics of Banizoumbou precipitations and aerosols, which demonstrate the importance of heterogeneous and multiphase chemical processes, we propose a conceptual model of the atmospheric chemistry in the Sahelian region.     

The Kolmogorov constant for carbon dioxide in the atmospheric surface layer over a paddy field

From measurements in the atmospheric surface layer over a paddy field, the Kolmogorov constants for CO₂ and longitudinal wind velocity were obtained. In this study, the nondimensional dissipation rate _nc = (1–16 _v )^-1/2 for CO₂ variance and = (1–16 _v )^-1/4 – _v for turbulent energy were used, assuming the equality of the local production term and the local dissipation term, and neglecting the divergence flux term in the budget equation. The value of the constant for CO₂ was consistent with recent determinations for temperature and humidity. The constant for longitudinal wind velocity showed good agreement with other recent observations.