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.     

Hydrogen Peroxide at the Bermuda Atlantic Time Series Station. Part 1: Temporal Variability of Atmospheric Hydrogen Peroxide and Its Influence on Seawater Concentrations

Diurnal and seasonal variations in atmospheric hydrogen peroxideconcentrations wereinvestigated during a summer and winter cruise aboard the R.V. `Endeavor' atthe BermudaAtlantic Time Series Station. Rainwater peroxide concentrations in Augustdisplayed dielvariability while no temporal H₂O₂ pattern was evidentin March rain. Averageconcentrations in March were also significantly lower than August whichindicates photochemicalprocesses are involved in controlling hydrogen peroxide concentrations inmarine rainwaterfalling over the open ocean. The range of gas phase hydrogen peroxideconcentrations wasbetween 1 and 6 ppbv and also exhibited a strong diurnal pattern during bothAugust and Marchwith concentration maxima in the early evening. The influence of atmosphericdeposition onsurface seawater hydrogen peroxide levels was also evaluated. Hydrogenperoxide depth profileswere measured on four separate occasions before and after rain events duringthe Augustsampling period. The input of rainfall hydrogen peroxide was observedthroughout the 25 metermixed layer with surface concentrations two fold larger in the morning aftera rain event. Theintegrated increase in hydrogen peroxide after the rain from 0 to 90 meterswas 1,720 molalmost all of which could be accounted for by the peroxide added from rain.The data presentedin this study represent the first detailed, simultaneous measurements ofhydrogen peroxide inmarine air, rain and surface seawater.     

Influence of dissolved organic carbon on photochemically mediated cycling of hydrogen peroxide in rainwater

The influence of sunlight and dissolved organic carbon (DOC) on the photochemically mediated cycling of hydrogen peroxide (H₂O₂) was investigated in rainwater samples collected in Wilmington, North Carolina USA. Upon exposure to simulated sunlight 14 of 19 authentic rainwater samples exhibited significant decreases in H₂O₂. The concentration of hydrogen peroxide did not change significantly in organic-free synthetic rainwater spiked with H₂O₂ in the light or in dark controls suggesting that the loss was not due to direct photolysis or dark mediated reactions. There was a significant correlation between pseudo-first order rate constants of H₂O₂ decay and initial H₂O₂ concentrations. There was also a significant correlation between the rate constant and the abundance of DOC suggesting that rainwater organic carbon plays an important role during photolytic decay either via direct reaction or indirectly through production of peroxide reactive species or scavenging of peroxide generating radicals. Several rain samples exhibited an initial increase in H₂O₂ during the first 2 h of irradiation. These increases were generally small and most likely do not represent a significant input of peroxide in precipitation. The photo-induced destruction of H₂O₂ is important because it may partly explain the late afternoon decrease of peroxide concentrations observed in earlier field studies and the substantial under saturation (<10%) of this oxidant in rainwater compared with gas phase concentrations.     

Temporal Variability of Iron Speciation in Coastal Rainwater

Iron occurs in rain as particulateand dissolved Fe and includes both Fe(II) and Fe(III)species. Model calculations and correlation analysisindicate Fe(II)(aq) occurs almost exclusively as thefree ion whereas Fe(III)(aq) occurs as both ironoxalate and Fe(OH)₂ ⁺(aq) with largevariations over the pH range from 4.0 to 5.0. Complexation with humic-like compounds may also beimportant for Fe(III)(aq); however, the concentrationand structural characteristics of these compounds haveyet to be determined. 112 rain samples were collectedfor iron analysis in Wilmington, North Carolina,between 1 July 1997, and 30 June 1999. Total iron,particulate iron and Fe(III)(aq) were higher inconcentration in summer and spring rain relative towinter and autumn rain. Fe(II)(aq) concentrations, incontrast, did not vary seasonally. Particulate iron,which was approximately half the total rainwater iron,was highest between noon and 6 p.m. (EST), probably dueto more intense regional convection including land-seabreezes during that time. The ratio ofFe(II)(aq)/Fe(III)(aq) was also highest in rainreceived between noon and 6 p.m., which most likelyreflects photochemical reduction of Fe(III)(aq)complexes to form Fe(II)(aq). A conceptual modeldepicting the interplay between iron species, lightintensity and organic ligands in rainwater ispresented.     

Chemical character of precipitation and related particles and trace gases in the North and South of China

Rainwater samples were collected at four sites, including Beijing and Mazhuang Town in the north of China, Shenzhen and Mangdang Mountain in the south of China. Character of atmospheric particles and gases were also measured at Mazhuang Town and Mangdang Mountain. Both of Beijing and Shenzhen are urban sites; Mazhuang Town and Mangdang Mountain are rural and remote sites respectively. The atmospheric pollution at rural plain site in the north of China was more serious than that at remote mountain site in the south of China. At Beijing, Mazhuang Town, Shenzhen and Mangdang Mountain the average pH values in rainwater were 6.02, 5.97, 4.72 and 4.81, respectively and the concentrations of total ions in rainwater were 1454, 1125, 187 and 191 μeq/l, respectively. While the acidity of the rain was higher in the south than that in the north, the rainwater in the north of China was more severely polluted than that in the south. The major acidic ion in the rainwater is SO₄^2-, and NH₄⁺ is the most important neutralizing ion in rainwater at the four sites, followed by Ca²⁺. The amounts of organic acid in precipitation were compared with other sites in the world. The ratios of organic acid to total free acid in rainwater at Mangdang Mountain was 13.8% and the influence of organic acid on acidity of rainwater at mountain site in the south of China is more important. The variation of atmospheric particles, gases and components in rainwater and cloud-fog water during special rain and cloud-fog events was discussed. The importance of washout process varied with atmospheric species. The impacts of rainfall, rain duration time and wind speed on wash-out process were estimated by regression analysis.     

Rainwater acidity and ion concentration correlations in a midwest storm system

The pH and the concentrations of sulfate, nitrate, ammonia, and calcium in rainwater were measured for two periods of a single midwest rainstorm which occurred over a mesometeorological network in central Illinois on 24–25 July 1979. Regression analysis was used to compare ion concentrations with rainfall amount, and ion balance was used to compare cation and anion concentrations at individual sites. Only the ions SO₄ ^2- and NO₃ ^- show any significant relationship to rainfall amount, decreasing as rainwater amounts increase (r=–0.57 and –0.60, respectively). During the first period of the rainstorm, a sequential sampler measurements allowed the calculation of detailed temporal variations in SO₄ ^2-, pH, and rain rate. SO₄ ^2- decreased, and pH increased as the rate increased and the opposite temporal pattern occurred as the rain decreased at the end of the period. Reasons for these variations are discussed.Research done while a visiting scientist at the Illinois State Water Survey, Champaign, Illinois, U.S.A.     

Rapid change in nitrate and sulfate concentrations observed in early stage of precipitation and their deposition processes

The concentrations of H⁺, nitrate (NO₃ ^-), and sulfate (SO₄ ^2-) in rainwater and their temporal changes were analyzed on the basis of continuous observation from 1 July 1991 to 30 June 1992 at a suburb of Nagoya, Japan. The yearly average for pH was 4.4. In general, an increasing pH with increase in precipitation amount was observed for rain events. Relatively high pH rainwater was sometimes observed at the beginning of rainfall, even though high concentrations of NO₃ ^- and SO₄ ^2- were involved. The high pH values were considered to be caused by the neutralization process with particulate matter containing cations. The yearly averaged ratio of equivalent concentration of nitrate to sulfate (N/S) in rainwater was 0.58. In the early stage of rain, the N/S value was usually more than 1.0 due to the difference of scavenging process between NO₃ ^- and SO₄ ^2-. High values of N/S ranging from 5 to 10 were found under the atmospheric conditions of calm winds and low humidity, during which it is possible that atmospheric particles float for a long time in the air before a rain event. The adsorption of NO₃ ^- in the early stage of rainfall by particulate matter was suggested from the difference in scavenging processes of NO₃ ^- and SO₄ ^2-. A possible scavenging process, called limb cloud scavenging, is presented to explain the interaction of particles and nitrate ions at the early stage of rain. In limb cloud scavenging, the repeated migration of cloud particles or raindrops between the inside and outside of clouds increases the absorption of ions to a highly condensed level, thus increasing the N/S value of rainwater. The influence of global scale seasonal phenomena with large amounts of particulates, such as typhoons or Asian dust storms, was also studied.     

Coastal rainwater hydrogen peroxide: Concentration and deposition

Correlation analysis between rainwater component concentrations (hydrogen peroxide, hydrogen ion, nitrate, nonseasalt sulfate and chloride ion) was used to investigate patterns of variation in hydrogen peroxide concentrations in rain collected in Wilmington, North Carolina, a coastal southeastern United States location, between October 1992, and October 1994. Rainwater hydrogen peroxide concentrations in general correlated positively with the pollutant components (hydrogen ion, nitrate and non-seasalt sulfate). This pattern suggests that destruction of hydrogen peroxide by sulfur dioxide is not the dominant factor controlling the concentration of hydrogen peroxide in this rainwater, with the possible exception of winter rain from coastal storms where an inverse correlation between hydrogen peroxide and nonseasalt sulfate was observed. Sequential sampling indicates rapid production of hydrogen peroxide and incorporation into rain within time periods of hours during summer daytime rains.Rain is an important transport mechanism for removal of atmospheric hydrogen peroxide, which may affect the oxidizing capacity of surface waters that receive the rain. During this study time, the annual deposition of hydrogen peroxide by rain was 12 mmole m^-2 yr^-1. An average rain event added approximately half of the resident amount of hydrogen peroxide to the shallow lakes typical of eastern North Carolina; extreme rain events can triple the amount normally present. The episodic nature of rain contributes to the variability in hydrogen peroxide concentration in surface waters. Higher hydrogen peroxide concentrations and greater rainfall amounts cause wet deposition of hydrogen peroxide to be approximately seven times greater during the warm season than the cold season.     

Thymol as a Biocide in Japanese Rainwater

Rainwater samples (wet-only; event samples) collected in Niigata in late autumn 1996 and springtime 1997 were used to assess the effectiveness of thymol as a biocide in Japanese rainwater. Upon collection each rainwater sample was divided into sub-samples, with thymol added to one sub-sample. Sub-samples with and without thymol were shipped to CSIRO, Australia, for chemical analysis. Comparison of analytical results for each pair of sub-samples proved the effectiveness of thymol in preventing biological action in this region where effects of rainwater microflaura and fauna on rainwater composition have not before been studied. Sub-samples without thymol exhibited lowered electrical conductivity, loss of the cations H⁺ and NH ₄ ^- , and loss of the anions HCOO^-, CH₃COO^-, C₂O ₄ ^2- , CH₃SO ₃ ^- and PO ₄ ^3- . Nitrate showed no change in all but one of the samples, indicating that ammonia was the preferred source of nitrogen for the biological processes that consumed the rainwater organic acids and phosphate. These results suggest that thymol is a suitable rainwater biocide for use under Japanese conditions.     

Covariations in oceanic dimethyl sulfide,its oxidation products and rain acidity at Amsterdam Island in the Southern Indian Ocean

Simultaneous measurements of rain acidity and dimethyl sulfide (DMS) at the ocean surface and in the atmosphere were performed at Amsterdam Island over a 4 year period. During the last 2 years, measurements of sulfur dioxide (SO₂) in the atmosphere and of methane sulfonic acid (MSA) and non-sea-salt-sulfate (nss-SO₄ ^2-) in rainwater were also performed. Covariations are observed between the oceanic and atmospheric DMS concentrations, atmospheric SO₂ concentrations, wet deposition of MSA, nss-SO₄ ^2-, and rain acidity. A comparable summer to winter ratio of DMS and SO₂ in the atmosphere and MSA in precipitation were also observed. From the chemical composition of precipitation we estimate that DMS oxidation products contribute approximately 40% of the rain acidity. If we consider the acidity in excess, then DMS oxidation products contribute about 55%.     

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.     

The effects of below-cloud aerosol on the acidification process of rain

Using a model of the acidification process of rain, we calculate and analyze the effects and contributions of a below-cloud aerosol in its different concentrations and acidities on the pH and ion components of rain (SO ₄ ^2– , H⁺, NO ₃ ^– , NH ₄ ⁺ , etc.) under the conditions of different concentrations of pollution gases. The results show that the aerosol has an acidification or alkalization effect on the rain which changes the pHs of rain and aerosol. As acidifying pollution gas concentrations (SO₂, HNO₃) are low, the acid aerosol has important effects on the pH and H⁺ of rain, but as the gas concentrations are high, the acid aerosol has very little effect. The alkalizing aerosol makes the pH of rain increase by between 0.3 and 0.5 and neutralizes about 60% of H⁺ in the rain. As alkalizing pollution gas NH₃ exists, the acid aerosol has important effects on the pH and H⁺ of rain. But the alkalizing aerosol has very little effect, especially as the NH₃ concentration is high. The percentage contribution of the aerosol to SO ₄ ^2– in rain is generally 7–15%, the contribution of the aerosol to NO ₃ ^– is nearly the same as that of HNO₃=1 ppb, and the contribution of the aerosol to NH ₄ ⁺ is nearly the same as that of NH₃, from 5 to 7 ppb, and is an important source of NH ₄ ⁺ in rain. Finally, according to the actual conditions of typical regions in the south and north of China (Chongqing and Beijing), we analyze the effects of aerosol and pollution gases on the ion components of rain.     

SO2 Oxidation in a rainband: Effects of in‐cloud H2O2 Production

Abstract

Aqueous‐phase H₂O₂ production in a rainband and its possible effect on sulphate production are studied by means of a two‐dimensional numerical model. In‐cloud peroxide production is incorporated into this chemistry model and its simulation results are compared with those in which aqueous‐phase H₂O₂ came only from the dissolution of gaseous H₂O₂ from the cloud interstitial air.

Results are presented for two different polluted situations ‐ Case 1 having initial SO₂ and sulphate aerosol profiles representative of a moderately polluted air mass, and Case 2 having chemical profiles expected to increase the relative importance of oxidation to nucleation as a means of contributing sulphate to cloud and rain. Sulphate production increased in both cases, although in Case 1 the effect of this increase on the concentration of sulphate in rain is negligible because nucleation and scavenging of aerosol are the major processes by which sulphate enters cloud and rain. In Case 2, sulphate concentrations in rain increase by 5–10%. Under environmental conditions of low sulphate aerosol, where oxidation reactions are the dominant means for sulphate to enter cloud and rain, the neglect of sulphate produced by the additional H₂O₂ may lead to error. The usual uncertainties in the initial SO₂ and sulphate aerosol vertical profiles, however, could be a more significant source of error in simulations of the chemistry of cloud and precipitation than the neglect of aqueous‐phase peroxide production during the lifetime of even a long‐lived system.     

Measurements of atmospheric hydroperoxides at a rural site in central Japan

Measurements of hydroperoxides (H₂O₂ and MHP) at ground level were made from 2012 to 2015 in Imizu City, Toyama Prefecture in central Japan. H₂O₂ and MHP concentrations ranged from 0.01 to 3.5 ppb and from below the level of detection (< 0.01 ppb) to 1.4 ppb, respectively. The concentrations of H₂O₂ and MHP were high in the summer and low in the winter. The H₂O₂ concentration was at its maximum in July and August, whereas the concentration of O₃ in the daytime was highest in May and June. The ratio of [H₂O₂]/[SO₂] presented clear seasonal variations. Many cases showed the condition of [H₂O₂] < [SO₂], called oxidant limitation especially in the cold months. Hydroperoxide concentrations in the rainwater were also high in the summer. The concentrations of MHP were much lower than those of H₂O₂ in the rain water. High concentrations of H₂O₂ (> 2.5 ppb) were detected in the summer during the inflow of air pollution. The concentrations of H₂O₂ were significantly high in July and August of 2013. The H₂O₂ was well correlated with the O₃ in July and August whereas there was no correlation between O₃ and H₂O₂ in May and June. There was a negative correlation between NO_X and H₂O₂.     

Influence of local sources on rainwater chemistry over Pune region,India

Rainwater samples were collected at five locations in the Pune region, an urban area in the south-west part of India, during 2006–2009. These locations; viz., Swargate (Traffic), Bhosari (Industrial), Pashan, Sangvi (Urban) and Sinhagad (Rural and High Altitude), represent different environments in this region. The study based on chemical analyses of these samples reveals that, on average, rainwater was alkaline at all the locations with pH values of 6.7, 6.16, 5.94, 6.04 and 5.92, respectively. Higher pH value of rainwater at the traffic location than those at the other locations is due mainly to the abundance of Ca²⁺ caused by vehicle-driven road-side dust. The maximum SO₄^2? and NO₃^? concentrations were found at Bhosari and Swargate respectively caused by local industrial and vehicular emissions. The average Fractional acidity over Pune area is 0.024, indicating about 98% acidity is neutralized by alkaline constituents. Factor analysis of the results indicated the influence of various sources, such as anthropogenic, soil dust, sea salt and biomass burning.     

Factors Influencing Nitrogen Speciation in Coastal Rainwater

Rainwater was collected from 129 rain events between February 2002 and August 2003 and analyzed for ammonium (NH₄⁺), nitrate (NO₃⁻), organic nitrogen (ON) and free amino acids (AA). Inorganic nitrogen (NO₃⁻ + NH₄⁺) was the dominant form of N representing 85% of total nitrogen based on volume-weighted averages. The remainder of the N occurred as organic nitrogen species of which free amino acids contribute approximately 17%. A significant, and in some cases the majority (> 75%), of the remaining ON could be accounted for by macromolecular uncharacterized humic like substances. This has important ramifications with respect to the long range transport of atmospheric ON because humic materials are recalcitrant and therefore may travel long distances from their source. There was a distinct seasonality to the N speciation data with maximum concentrations of NH₄⁺, ON and AA occurring in the spring. Air-mass back trajectory analysis indicates there is a strong anthropogenic component to the NO₃⁻, NH₄⁺ and AA signal but not ON. There was a strong positive correlation between amino acid concentrations and ammonium which suggests they have similar sources and sinks in rainwater. Finally, large episodic additions of NH₄⁺ and AA during tropical events could significantly impact short term bioavailable N budgets in estuaries impacted by these storms. Approximately three times as much NH₄⁺ and AA were deposited during Hurricane Isabel (317 μ moles ⋅ m⁻² and 84 μ moles ⋅ m⁻² respectively) compared to the mean impact of average summertime rain events at this location.     

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.     

Chemical composition of rainwater and anthropogenic influences in Chengdu,Southwest China

A comprehensive study on the chemical compositions of rainwater was carried out from Jan. to Dec. in 2008 in Chengdu, a city located on the acid rain control zone of southwest China. All samples were analyzed for pH and major ions (F^–, Cl^?, NO₃^?, SO₄^2?, K⁺, Na⁺, Ca²⁺, Mg²⁺, and NH₄⁺). The pH increased due to the result of neutralization caused by the base ions. It was observed that Ca²⁺ was the most abundant cation with a VWM value of 196.6 μeq/L (17.3–1568.7 μeq/L), accounting for 49.7% (9.4%–79.2%) of the total cations. SO₄^2? was the most abundant anion with VWM value of 212.8 μeq/L (41.8–1227.6 μeq/L). SO₄^2? and NO₃^? were dominant among the anions, accounting for 90.4%–99.1% of the total measured anions.The concentrations of NO₃^? were higher than the most polluted cities abroad, which indicated Chengdu has been a severe polluted city over the world. The high fuel consumption from urbanization and the rapid increase of vehicles resulted in the high emission of SO₂ and NO_x, which were the precursor of the high concentration of acidic ions NO₃^? and SO₄^2?. It was the main reason of the severe acid rain in Chengdu.The high concentrations of alkaline ions (mainly Ca²⁺, NH₄⁺) in Chengdu city atmosphere have played an important role to neutralize the acidity of rainwater and the pH value has increased by 0.7 units since 1989. It is worth noting that the emission of NO_x from the automobile exhaust is increased and is becoming the important precursor of acid rain now.     

Trend in chemical composition of precipitation during 2005–2009 at a rural station of Bhubaneswar, eastern India

Precipitation samples collected during 2005–2009 from a rural forest station of Bhubaneswar were analyzed for their chemical composition. The samples were collected through a wet-only (WO) collector and two bulk (B1 and B2) collectors. The ions were evenly balanced indicating good data quality. The overall pH of rainwater was slightly acidic and ~47% of all rain events during the period were acidic (pH?<?5.6). Multilinear regression analysis showed relation between the free acidity (H⁺) and other components in rainwater. Enrichment factors (EF) of the major components with respect to their sources such as marine and crustal were calculated. Maximum EF was observed for NO ₃ ^? for both marine and crustal sources for all the three collectors. Source apportionments were also carried for the ions. Trend analysis showed continuous increase in most of the ions over years during the study period driven by anthropogenic emissions. Statistical/factorial analysis established correlation among different ions.     

A Long Term Study on Chemical Composition of Rainwater at Dayalbagh, a Suburban Site of Semiarid Region

Rainwater samples were collected for the monsoon period of 1988 and 1991–1996 at Dayalbagh (Agra), a suburban site situated in semiaridregion. The mean pH was 7.01 ±1.03 well above 5.6, which is the reference pH. Concentration of Ca²⁺ was observed to be highest followed by Mg²⁺, NH₄ ⁺,SO₄ ^2–, Cl^–,NO₃ ^–, Na⁺, F^– and K⁺. The ratios of SO₄ ^2– + NO₃ ^– andCa²⁺ + Mg²⁺ (TA/TC) have been considered as indicatorfor acidity. In the Agra region ratio of TA/TC is quite below 1.0 indicating alkaline nature of rainwater. The lowest value of 0.24 was observed in 1991 likely due to the lowest rain depth of the decade. The highest value of 0.54 was observed in 1996, a year with a large rain depth and increase in line (vehicular traffic) and area sources (population growth). Good correlation between Ca²⁺ and NO₃ ^–,Ca²⁺ and SO₄ ^2– andSO₄ ^2– and NO₃ ^–,indicates that wind carried dust and soil play a significant role in neutralization of precipitation acidity.