Absorption and fluorescence properties of rainwater during the cold season at a town in Western Portugal

This study aims at evaluating the variability of the optical properties of chromophoric dissolved organic matter (CDOM) of rainwater during the cold season, specifically between Autumn and Winter periods. The spectroscopic characteristics of rainwater samples collected at a town (Aveiro) in western Portugal were assessed by UV-Vis absorbance and three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopies. Rainwater samples showed similar characteristics to those of natural humic substances when analysed by UV-Vis absorbance spectroscopy, but a significant difference was observed in the volume weight average (VWA) of absorbances between Autumn and Winter. In general, the EEM fluorescence spectra of the Autumn and Winter samples disclosed the presence of six fluorophores with different VWA specific fluorescence intensities: three humic-like (λ _excitation/λ _emission ≈ 230/415 nm; 290/415 nm; and 340/415 nm) and three protein-like (λ _excitation/λ _emission ≈ 230/350 nm; 280/340 nm; and 225/300 nm), but one of the humic-like peaks (≈340/415 nm) does not always appear in the EEM fluorescence spectra of the Winter samples. During the cold season, chromophoric compounds are important constituents of rainwater dissolved organic matter and the presence of these highly absorbing and fluorescing compounds may exert a determining effect in atmospheric absorption of solar radiation.     

Manganese in coastal rainwater: speciation,photochemistry and deposition to seawater

Concentrations of manganese in 56 rain events in Wilmington, NC, USA rainwater from April 1, 2005 to March 31, 2006 were 11 ± 3 nM for dissolved Mn and 1.2 ± 0.4 nM for particulate Mn. Concentrations of both forms of Mn were higher in terrestrial storms relative to marine events. This observation along with the positive correlation of Mn with pollutant indicators suggests anthropogenic inputs to rain at this location, as has been observed at other locations. The ratio of Mn_part/Mn_diss was threefold larger in summer relative to winter rain, which matched the increase of particulate to dissolved Fe in rainwater suggesting influence of Saharan dust during the summer. Like Fe in rain, Mn undergoes photoreduction in rainwater, which has also been shown to be important in Mn cycling in seawater. The flux of Mn removed from the atmosphere via wet deposition is 1.5 × 10⁻⁵ moles m⁻² yr⁻¹ at this location, which is approximately twice the flux reported from two rainwater studies conducted in the early 1980s on Bermuda. Atmospheric input of Mn to the oceans is important because Mn like Fe is an essential and potentially limiting nutrient. Experiments mixing authentic rainwater and seawater demonstrate that rainwater dissolved Mn does not rapidly precipitate in seawater suggesting wet deposition is an important source of soluble, stable Mn to surface seawater.     

Atmospheric inorganic nitrogen in marine aerosol and precipitation and its deposition to the North and South Pacific Oceans

Aerosol and rain samples were collected between 48°N and 55°S during the KH-08-2 and MR08-06 cruises conducted over the North and South Pacific Ocean in 2008 and 2009, to estimate dry and wet deposition fluxes of atmospheric inorganic nitrogen (N). Inorganic N in aerosols was composed of ~68% NH₄⁺ and ~32% NO₃^– (median values for all data), with ~81% and ~45% of each species being present on fine mode aerosol, respectively. Concentrations of NH₄⁺ and NO₃⁻ in rainwater ranged from 1.7–55 μmol L⁻¹ and 0.16–18 μmol L⁻¹, respectively, accounting for ~87% by NH₄⁺ and ~13% by NO₃⁻ of total inorganic N (median values for all data). A significant correlation (r = 0.74, p < 0.05, n = 10) between NH₄⁺ and methanesulfonic acid (MSA) was found in rainwater samples collected over the South Pacific, whereas no significant correlations were found between NH₄⁺ and MSA in rainwater collected over the subarctic (r = 0.42, p > 0.1, n = 6) and subtropical (r = 0.33, p > 0.5, n = 6) western North Pacific, suggesting that emissions of ammonia (NH₃) by marine biological activity from the ocean could become a significant source of NH₄⁺ over the South Pacific. While NO₃⁻ was the dominant inorganic N species in dry deposition, inorganic N supplied to surface waters by wet deposition was predominantly by NH₄⁺ (42–99% of the wet deposition fluxes for total inorganic N). We estimated mean total (dry + wet) deposition fluxes of atmospheric total inorganic N in the Pacific Ocean to be 32–64 μmol m⁻² d⁻¹, with 66–99% of this by wet deposition, indicating that wet deposition plays a more important role in the supply of atmospheric inorganic N than dry deposition.     

Chromophoric Dissolved Organic Matter (CDOM) In Rainwater,Southeastern North Carolina,USA

The abundance and optical characteristics of dissolved organic matter (DOM) were determined in 120 rain samples collected in Wilmington, North Carolina, USA, between February 21, 2002 and August 11, 2003. All rainwater samples contained chromophoric dissolved organic matter (CDOM) as well as fluorescent compounds. The absorbance spectra of CDOM in the samples decreased exponentially with wavelength with little or no measurable absorbance past 550 nm. Fluorescence excitation emission spectra (EEMS) of the precipitation revealed the presence of four major peaks indicating both terrestrial and marine influences. There was a strong positive correlation between total integrated fluorescence and the absorbance coefficient at 300 nm in rainwater samples, suggesting that these optical properties are directly interrelated and that the compounds responsible for absorbance may be the same as those responsible for fluorescence. Air-mass back-trajectory analysis indicated elevated CDOM levels in continentally influenced rainwater relative to marine dominated events implying that anthropogenic and/or terrestrial sources are important contributors to CDOM levels in precipitation. The presence of highly absorbing and fluorescing CDOM in rainwater has significant ramifications in atmospheric chemistry and may play a previously unrecognized role in the wavelength dependent spectral attenuation of solar radiation by atmospheric waters.     

Rain-aerosol coupling in the tropical atmosphere of Southeast Asia: distribution and scavenging ratios of major ionic species

Both aerosol and rainwater samples were collected and analyzed for ionic species at a coastal site in Southeast Asia over a period of 9 months (January–September 2006) covering different monsoons. In general, the occurrence and distribution of ionic species showed a distinct seasonal variation in response to changes in air mass origins. Real-time physical characterization of aerosol particles during rain events showed changes in particle number distributions which were used to assess particle removal processes associated with precipitation, or scavenging. The mean scavenging coefficients for particles in the range 10–500 nm and 500–10 μm were 7.0 × 10⁻⁵ ± 2.8 × 10⁻⁵ s⁻¹ and 1.9 × 10⁻⁴ ± 1.6 × 10⁻⁵ s⁻¹, respectively. A critical analysis of the scavenging coefficients obtained from this study suggested that the wet removal of aerosol particles was greatly influenced by rain intensity, and was particle size-dependent as well. The scavenging ratios, another parameter used to characterize particle removal processes by precipitation, for NH₄ ⁺, Cl⁻, SO₄ ²⁻, and NO₃ ⁻ were found to be higher than those of Na⁺, K⁺, and Ca²⁺ of oceanic and crustal origins. This enrichment implied that gaseous species NH₃, HCl, and HNO₃ could also be washed out readily. These additional sources of ions in precipitation presumably counter-balanced the dilution effect caused by high total precipitation volume in the marine and tropical area.     

Atmospheric CO along the Trans-Siberian Railroad and River Ob: source identification using isotope analysis

The concentration, radiocarbon (¹⁴C) and stable isotope (¹³C and ¹⁸O) content of CO have been determined in air samples collected across Russia (about 8,500 km) and along the Ob river during the summer of 1999 to study the CO sources and sinks. An instrumented carriage on the Trans-Siberian railway and a boat on the river Ob were used as atmospheric measurement platforms. In general, CO mixing ratios, CO stable isotope ratios, as well as the abundances of ¹⁴CO over West Siberia were similar to those found at remote northern hemispheric baseline monitoring stations. Identified sources of CO along the Ob appear to be connected to methane oxidation based on an inferred δ¹³C_source = −36.8 ± 0.6‰, while the value for δ¹⁸O_source = 9.0 ± 1.6‰ identifies it as burning. Thus flaring in the oil and gas production can be supposed to be a source. The extreme ¹³C depletion and concomitant ¹⁸O enrichment for two of the boat samples unambiguously indicates contamination by CO from combustion of natural gas (inferred values δ¹³C_source = −40.3‰ and δ¹⁸O_source = 17.5‰). For these two samples, that have strongly elevated ¹⁴CO concentrations, the industrial area near Tomsk is identified as a source area using meteorological calculations. Along the Trans-Siberian Railroad background CO was to various degrees contaminated with CO from methane combustion (δ¹³C_source = −35.7 ± 6.2‰ and δ¹⁸O_source = 10.3 ± 1.8‰). The impact of industrial burning was discernable in the vicinity of Perm-Kungur.     

Photobleaching of chromophoric dissolved organic matter (CDOM) in rainwater

Significant photodegradation of chromophoric dissolved organic matter (CDOM) in rainwater was observed after exposure to simulated sunlight. Fluorescence excitation emission spectra (EEMS) of precipitation revealed the presence of four major peaks all of which degraded upon photolysis with the greatest loss in the region characteristic of marine CDOM. Photobleaching of absorbance also occurred in the wavelength region between 250 and 375 nm with the greatest loss of absorbance in the upper end of the UV-A region near 275 nm. There was a strong positive correlation between absorbance loss and total integrated fluorescence loss suggesting these optical properties and the degree to which they are photobleached in rainwater are directly related. The quantum yield of CDOM photodegradation in rainwater decreased dramatically with increasing wavelength and decreasing energy of incoming radiation with the average quantum yield at 325 nm approximately an order of magnitude greater than at 460 nm. The similarity of photolytic response between rainwater and Cape Fear estuarine CDOM indicates that some fraction of the compounds that make up rainwater CDOM may be derived from surface sources and/or that the processes that produce or modify humic-like substances in the atmosphere result in similar types of compounds as non-atmospheric processes.     

Measurements of surface ozone at semi-arid site Anantapur (14.62°N, 77.65°E, 331 m asl) in India

In the present study, an attempt has been made to examine the governing photochemical processes of surface ozone (O₃) formation in rural site. For this purpose, measurements of surface ozone and selected meteorological parameters have been made at Anantapur (14.62°N, 77.65°E, 331 m asl), a semi-arid zone in India from January 2002 to December 2003. The annual average diurnal variation of O₃ shows maximum concentration 46 ppbv at noon and minimum 25 ppbv in the morning with 1σ standard deviation. The average seasonal variation of ozone mixing ratios are observed to be maximum (about 60 ppbv) during summer and minimum (about 22 ppbv) in the monsoon period. The monthly daytime and nighttime average surface ozone concentration shows a maximum (55 ± 7 ppbv; 37 ± 7.3 ppbv) in March and minimum (28 ± 3.4 ppbv; 22 ± 2.3 ppbv) in August during the study period. The monthly average high (low) O₃ 48.9 ± 7.7 ppbv (26.2 ± 3.5 ppbv) observed at noon in March (August) is due to the possible increase in precursor gas concentration by anthropogenic activity and the influence of meteorological parameters. The rate of increase of surface ozone is high (1.52 ppbv/h) in March and lower (0.40 ppbv/h) in July. The average rate of increase of O₃ from midnight to midday is 1 ppbv/h. Surface temperature is highest (43–44°C) during March and April months leading to higher photochemical production. On the other hand, relative humidity, which is higher during the rainy season, shows negative correlation with temperature and ozone mixing ratio. It can be seen that among the two parameters are measured, correlation of surface ozone with wind speed is better (R ²=0.84) in compare with relative humidity (R ²=0.66).     

Precipitation Chemistry and Wet Deposition in Kruger National Park, South Africa

The chemical composition, as well as the sources contributing to rainwater chemistry have been determined at Skukuza, in the Kruger National Park, South Africa. Major inorganic and organic ions were determined in 93 rainwater samples collected using an automated wet-only sampler from July 1999 to June 2002. The results indicate that the rain is acidic and the averaged precipitation pH was 4.72. This acidity results from a mixture of mineral acids (82%, of which 50% is H₂SO₄) and organic acids (18%). Most of the H₂SO₄ component can be attributed to the emissions of sulphur dioxide from the industrial region on the Highveld. The wet deposition of S and N is 5.9 kgS⋅ha⁻¹⋅yr⁻¹ and 2.8 kgN⋅ha⁻¹⋅yr⁻¹, respectively. The N deposition was mainly in the form of NH₄ ⁺. Terrigenous, sea salt component, nitrogenous and anthropogenic pollutants have been identified as potential sources of chemical components in rainwater. The results are compared to observations from other African regions.     

Rainfall chemistry: long range transport versus below cloud scavenging. A two-year study at an inland station (Opme, France)

The present study investigates the chemical composition of wet atmospheric precipitation samples on a daily and an intra-event timescales in Opme, an experimental meteorological station located near Clermont-Ferrand, France. The samples have been collected from November 2005 to October 2007. A total of 217 rainwater samples, integrated for 24 h, were collected and analyzed for pH, conductivity, Na⁺, K⁺, Mg²⁺, NH₄⁺, Ca²⁺, Cl⁻, NO₃⁻ , SO₄²⁻ , PO₄³⁻ and HCO₃⁻. The composition of the rainwater collected appeared to be controlled by the following potential sources: neutralisation process (association among calcium, ammonium with nitrate and sulphate), marine and terrestrial sources. In order to determine the role of long-range transport, the integrated events were classified according to four origins of air-masses: (1) West, (2) North and East, (3) South including Iberian and Italian Peninsulae and (4) local. This analysis allows identifying the source areas of the different association of elements defined. Although calcium is always dominant, total content of rainfall is variable and neutralisation process can be more or less efficient and specific. Rainout (long-range transport) and washout (below-cloud scavenging) were investigated through intra-event measurements of chemical species. Four rain-events have been selected according to the four classes of origins of air-masses. It appears that the first fractions are responsible for an important part of the chemical content of the whole event. Terrestrial species, locally emitted, induce the neutralisation process of acid species. Local meteorological conditions, such as wind’s speed and direction, play an important role as they could provoke recharges of the below cloud air column during the event.     

Acid rain and alkalization in southwestern China: chemical and strontium isotope evidence in rainwater from Guiyang

A comprehensive study on the chemical compositions of rainwater was carried out from June 2007 to December 2008 in Guiyang, a city located on the acid rain control zone of southwest China. All samples were analyzed for pH, major anions (F⁻, Cl⁻, NO₃⁻, SO₄²⁻), major cations (K⁺, Na⁺, Ca²⁺, Mg²⁺, NH₄⁺), Sr²⁺ and Sr isotope. The pH increase is due to the result of neutralization caused by the alkaline dust which contain large amount of CaCO₃. It was observed that Ca²⁺ was the most abundant cation with a volume-weighted mean (VWM) value of 217.6 μeq/L (52.7–1928 μeq/L), accounting for 66% (39%–88%) of the total cations. SO₄²⁻ was the most abundant anion with VWM value of 237.8 μeq/L (49.6-1643 μeq/L). SO₄²⁻ and NO₃⁻ were dominant among the anions, accounting for 66%–97% of the total measured anions. The Sr concentrations vary from 0.01 to 0.92 μmol/L, and strontium isotopic ratios vary in the range of 0.707684–0.710094, with an average of 0.708092. The elements ratios and the ⁸⁷Sr/⁸⁶Sr ratios showed that the solutes of rainwater mainly come from weathering of carbonate and secondary dust input. Moreover, urbanization results in the calcium-rich dust increased and the high concentrations of alkaline ions (mainly Ca²⁺) have played an important role to neutralize the acidity of rainwater, leading to the increase of arithmetic pH mean value by 0.5 units since 2002. It is worth noting that the emission of SO₂ and NO_x from the automobile exhaust is increasing and is becoming another important precursor of acid rain now.     

Speciation and Photochemistry of Mercury in Rainwater

Mercury speciation was determined in rainwater from 76 storms in southeastern North Carolina between September 1, 2003 and September 30, 2005. Volume-weighted average concentrations of total Hg (THg), total dissolved Hg (TDHg), particulate Hg (Hg_part) and dissolved monomethyl Hg (MMHg) were 45.5 pM, 34.8 pM, 12.0 pM and 1.1 pM respectively. TDHg accounted for 77% of THg in precipitation which is similar to Cu but significantly higher than Cr or Fe. Concentrations of the various Hg species were very similar during summer and winter indicating that there was not a dominant seasonal influence on Hg speciation in rainwater at this location. THg, TDHg, and MMHg concentrations were also not significantly impacted by storm origin suggesting that they are relatively well mixed regionally and that air mass back trajectory is not the dominant factor controlling their concentration at this location. Concentrations of TDHg and Hg_part were inversely correlated in rainwater samples subjected to irradiation with simulated sunlight, suggesting the distribution between dissolved and particulate Hg may be controlled by photochemical transformations. Unlike TDHg and Hg_part, no significant changes in MMHg were observed upon photolysis of rainwater indicating that its distribution is not significantly driven by sunlight-mediated reactions, in contrast to what has been observed in surface waters. Results presented in this study indicate that the speciation of Hg in rainwater is dynamic and is driven by a complex combination of natural and anthropogenic processes as well as interactions with sunlight.     

Stratospheric final warmings in the Southern Hemisphere and their energetics

Summary.  Using 9 years (1985–1993) data, final stratospheric warmings in the Southern Hemisphere are studied. Interannual variations in the onset date and the temperatures are noted. In 1985 the stratosphere was colder by about 5 K and the wave activity was less. This year the final warming got delayed. In contrast in 1988 the final warming occurred earlier when compared with the mean picture and the wave activity was more. An examination of Eliassen-Palm fluxes showed the important role of planetary waves in the wave-mean flow interaction. In the energetics the most spectacular change is the reduction of zonal kinetic energy. Before the warming the energy exchanges were P_z → P_e → K_e → K_z ← P_z and after the warming they were P_z ← P_e ← K_e → K_z ← P_z. The dramatic reduction of zonal kinetic energy seems to be due to two effects: the reduction in K_e → K_z conversion and the weakening of direct meridional circulation. Received October 3, 2001; revised June 5, 2002     

Seasonal carbon dioxide balance and respiration of a high-arctic fen ecosystem in NE-Greenland

Summary  Turbulent fluxes of CO₂ were continuously measured by eddy correlation for three months in 1997 over a gramineous fen in a high-arctic environment at Zackenberg (74°28′12″N, 20°34′23″W) in NE-Greenland. The measurements started on 1 June, when there was still a 1–2 m cover of dry snow, and ended 26 August at a time that corresponds to late autumn at this high-arctic site. During the 20-day period with snow cover, fluxes of CO₂ to the atmosphere were small, typically 0.005 mg CO₂ m⁻² s⁻¹ (0.41 g CO₂ m⁻² d⁻¹), wheres during the thawed period, the fluxes displayed a clear diurnal variation. During the snow-free period, before the onset of vegetation growth, fluxes of CO₂ to the atmosphere were typically 0.1 mg CO₂ m⁻² s⁻¹ in the afternoon, and daily sums reached values up to almost 9 g CO₂ m⁻² d⁻¹. After 4 July, downward fluxes of CO₂ increased, and on sunny days in the middle of the growing season, the net ecosystem exchange rates attained typical values of about −0.23 mg m⁻² s⁻¹ at midday and max values of daily sums of −12 g CO₂ m⁻² d⁻¹. Throughout the measured period the fen ecosystem acted as a net-sink of 130 g CO₂ m⁻². Modelling the ecosystem respiration during the season corresponded well with eddy correlation and chamber measurements. On the basis of the eddy correlation data and the predicted respiration effluxes, an estimate of the annual CO₂ balance the calender year 1997 was calculated to be a net-sink of 20 g CO₂ m⁻² yr⁻¹. Received October 6, 1999 Revised May 2, 2000     

Regional effects of large-scale extreme wind events over orographically structured terrain

Summary ?Above orographically structured terrain considerable differences of the regional wind field may be identified during large-scale extreme wind events. So far, these regional differences could not be resolved by climate models. To determine the relationships between large-scale atmospheric conditions, the influence of orography, and the regional wind field, data measured in the upper Rhine valley within the framework of the REKLIP Regional Climate Project were analyzed and calculations were made using the KAMM mesoscale model. In the area of the upper Rhine valley, ratios of the wind velocity in the Rhine valley at 10 m above ground level, ν_val, and the large-scale flow velocity, ν_lar, are between ν_val/ν_lar ≈ 0.1 and ν_val/ν_lar ≈ 1. The ν_val/ν_lar ratio exhibits a strong dependence on thermal stratification, δ, and decreases from ν_val/ν_lar ≈ 1 at δ = 0 K m⁻¹ to ν_val/ν_lar ≈ 0.2 at δ = 0.0075 K m⁻¹. In areas, where the lateral mountainous border of the Rhine valley is interrupted, the ν_val/ν_lar ratio increases again with increasing stability or decreasing Froude number. This is obviously due to flow around the Black Forest under stable stratification. It is demonstrated by model calculations that a complex wind field develops in the Rhine valley at small Froude numbers (Fr < 1) irrespective of the direction of large-scale flow. The ν_val/ν_lar ratio is characterized by small values in the direct lee side (ν_val/ν_lar ≈ 0.2) and high values on the windward side of the lateral mountainous border of the Rhine valley (ν_val/ν_lar ≈ 0.8). Received October 22, 2001; revised June 18, 2002; accepted June 23, 2002     

Effects of Monochromatic UV-Visible Light and Sunlight on Fe(III)-Catalyzed Oxidation of Dissolved Sulfur Dioxide

The effect of UV-visible light and natural sunlight on the Fe(III)-catalyzed oxidation of dissolved sulfur dioxide has been studied under the conditions representative for those of acidified atmospheric liquids. The experimental results have shown that both sunlight and UV-visible light enhance the rate of Fe(III)-catalyzed oxidation of aqueous sulfite with wavelength ranging from 300 to 575 nm. The light enhanced oxidation is mainly due to photochemical formation of OH radicals from Fe(OH)²⁺ complexes in the wavelength region below 420 nm and SO₃^•− free radicals from Fe(III) sulfite complexes above 420 nm in the absence of organic ligands. Like the Fe(III)-catalyzed thermal chemical oxidation, the Fe(III)-catalyzed photochemical oxidation is also first order with respect to sulfite ion concentration. The sunlight irradiation can increase the Fe(III)-catalyzed oxidation of S(IV) over 45%. The presence of organic complex ligands, such as oxalate, can completely inhibit the Fe-catalyzed oxidation of S(IV) in the dark. However, the photolysis of Fe(III)-oxalato complexes generates oxalate free radicals, leading to the formation of H₂O₂ and OH radicals and the oxidation of S(IV). The rate of Fe(III)-catalyzed oxidation of S(IV) species is found to increase with increasing light intensity. The effects of sunlight on the Fe(III)-catalyzed oxidation of S(IV) should be taken into account when predicting the daytime rates of sulfuric acid formation in atmospheric water droplets.     

Kinetics of the reactions of soot surface-bound polycyclic aromatic hydrocarbons with NO<Subscript>2</Subscript>

The kinetics of heterogeneous reactions of NO₂ with 17 polycyclic aromatic hydrocarbons (PAHs) adsorbed on laboratory generated kerosene soot surface was studied over the temperature range (255–330) K in a low pressure flow reactor combined with an electron-impact mass spectrometer. The kinetics of soot-bound PAH consumption due to their desorption and reaction with NO₂ were monitored using off-line HPLC measurements of their concentrations in soot samples as a function of reaction time, NO₂ concentrations in the gas phase being analyzed by mass spectrometer. No measurable decay of PAHs due to the reaction with NO₂ was observed under experimental conditions of the study (maximum NO₂ concentration of 5.5 × 10¹⁴ molecule cm⁻³ and reaction time of 45 min), which allowed to determine the upper limits of the first-order rate constants for the heterogeneous reactions of 17 soot-bound PAHs with NO₂: k < 5.0 × 10⁻⁵ s⁻¹ (for most PAHs studied). Comparison of these results to previous studies carried on different carbonaceous substrates, showed that heterogeneous reactivity of PAHs towards NO₂ is, probably, dependent on the substrate nature even for resembling, although different carbonaceous materials. Results show that particulate PAHs degradation by NO₂ alone is of minor importance in the atmosphere     

Net CO2 exchange of a subarctic mountain birch ecosystem

Summary  Net ecosystem CO₂ exchange was measured over a mountain birch forest in northern Finland throughout the growing season. The maximal net CO₂ uptake rate of about − 0.5 mg(CO₂) m⁻² s⁻¹ was observed at the end of July. The highest nocturnal respiration rates in early August were 0.2 mg(CO₂) m⁻² s⁻¹. The daily CO₂ balances during the time of maximal photosynthesis were about −15 g(CO₂) m⁻² d⁻¹. The mountain birch forest acted as a net sink of CO₂ from 30 June to 28 August. During that period the net CO₂ balance was −448 g(CO₂)m⁻². The interannual representativeness of the observed balances was studied using a simplified daily balance model, with daily mean global radiation and air temperature as the input parameters. The year-to-year variation in the phenological development was parameterised as a function of the cumulative effective temperature sum. The daily balance model was used for estimating the variability in the seasonal CO₂ balances due to the timing of spring and meteorological factors. The sink term of CO₂ in 1996 was lower than the 15-year mean, mainly due to the relatively late emergence of the leaves. Received October 11, 1999 Revised April 25, 2000     

Annual methane emission from Finnish mires estimated from eddy covariance campaign measurements

Summary  Measurements of landscape-scale methane emission were made over an aapa mire near Kaamanen in Finnish Lapland (69° 8′ N, 27° 16′ E, 155 m ASL). Emissions were measured during the spring thaw, in summer and in autumn. No effect of water table position on CH₄ emission was found as the water table remained at or above the surface of the peat. Methane emission fluxes increased with surface temperature from which an activation energy of −99 kJ mol⁻¹ was obtained. Annual emission from the site, modelled from temperature regression and short-term flux measurements made in three separate years, was calculated to be 5.5 ± 0.4 g CH₄ m⁻² y⁻¹ of which 0.6 ± 0.1 g CH₄ m⁻² y⁻¹ (11%) was released during the spring thaw which lasted 20 to 30 days. The effect of global warming on the CH₄ budget of the site was estimated using the central scenario of the SILMU (Finnish Research Programme on Climate Change) model which predicts annual mean temperature increases of 1.2, 2.4 and 4.4 °C in 2020, 2050 and 2100, respectively. Maximum enhancements in CH₄ emission due to warming were calculated to be 18, 40 and 84% for 2020, 2050 and 2100, respectively. Actual increases may be smaller because prediction of changes in water table are highly uncertain. Received September 17, 1999 Revised October 16, 2000     

Rainwater chemistry and bulk atmospheric deposition in a tropical semiarid ecosystem: the Brazilian Caatinga

We assessed the rainwater chemistry, the potential sources of its main inorganic components and bulk atmospheric deposition in a rural tropical semiarid region in the Brazilian Caatinga. Rainfall samples were collected during two wet seasons, one during an extremely dry year (2012) and one during a year with normal rainfall (2013). According to measurements of the main inorganic ions in the rainwater (H⁺, Na⁺, NH₄ ⁺, K⁺, Ca²⁺, Mg²⁺, Cl^?, NO₃ ^?, and SO₄ ^2?), no differences were observed in the total ionic charge between the two investigated wet seasons. However, Ca²⁺, K⁺, NH₄ ⁺ and NO₃ ^? were significant higher in the wetter year (p < 0.05) which was attributed to anthropogenic activities, such as organic fertilizer applications. The total ionic contents of the rainwater suggested a dominant marine contribution, accounting for 76 % and 58 % of the rainwater in 2012 and 2013, respectively. The sum of the non-sea-salt fractions of Cl^?, SO₄ ^2?, Mg²⁺, Ca²⁺ and K⁺ were 19 % and 33 % in 2012 and 2013, and the nitrogenous compounds accounted for 2.8 % and 6.0 % of the total ionic contents in 2012 and 2013, respectively. The ionic ratios suggested that Mg²⁺ was probably the main neutralizing constituent of rainwater acidity, followed by Ca²⁺. We observed a low bulk atmospheric deposition of all major rainwater ions during both wet seasons. Regarding nitrogen deposition, we estimated slightly lower annual inputs than previous global estimates. Our findings contribute to the understanding of rainfall chemistry in northeastern Brazil by providing baseline information for a previously unstudied tropical semiarid ecosystem.