Numerical studies of acidification processes within and below clouds with a flow-through chemical reactor model

A flow-through chemical reactor model has been exercised to assess the importance of various oxidation reactions and cloud processes on wet removal and redistribution of atmospheric pollutants and to investigate the effect of in-cloud acidification on precipitation chemistry at the surface. Preliminary results indicate that in-cloud acidification accounts for more than 60% of the wet deposited acids derived from acidification of initial SO₂, that 42–57% of water-soluble, non-reactive NH₃ and HNO₃ are removed by wet deposition. The pseudo-first-order conversion rate of SO₂ to SO₄²⁻ ranges from 3 to 25% h ⁻¹ depending on initial and boundary conditions.Sensitivity studies have been carried out to test the importance of time evolution of clouds on partitioning of pollutants in the atmosphere and to investigate the variability of precipitation chemistry due to changes in rate constants. The distributions of NH₃ and HNO₃ are found to be dependent largely on the cloud microphysical parameters, while the distributions of H₂O₂ and SO₂ depend largely on initial conditions of both species. Individual physical and chemical mechanisms can determine the overall rate of sulfate wet deposition at different stages of cloud evolution.     

Dissolution of trace metals from particles of industrial origin and its influence on the composition of rainwater

The dissolution of trace metals (Fe, Mn, Cu, Zn, Ca and Mg) from pulverized fuel ash and steel dust particles in aqueous suspensions was studied as a function of solution pH and aerosol mass loading. The rate of dissolution, the final concentration and the fraction of the metal leached increased with decreasing pH over the range of pH 2.5→ 5. The leaching of aerosol particles is an important source of trace metals in rainwater, and the results of the leaching experiments were found to be consistent with the observations of these metals in urban rainwater. Alkaline materials such as Ca and Mg leached from particles may be important in the electroneutrality balance of rainwater and they could affect the solution pH. The dissolution of metals such as Fe and Mn may provide catalysts which could be important in the aqueous oxidation of SO₂, and the leaching of Ca and Mg may have a ‘buffering’ effect in reacting cloud and aerosol droplets.     

Chemical and strontium isotope characterization of rainwater in Beijing,China

Major ion concentrations and Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) were measured in rainwater samples collected at an urban site in Beijing over a period of one year. The pH value and major ion concentrations of samples varied considerably, and about 50% of the rainwater studied here were acidic rain with pH values less than 5.0. Ca²⁺ and NH₄⁺ were the dominant cations in rainwaters and their volume weighted mean (VWM) values were 608 μeq l^?1 (14–1781 μeq l^?1) and 186 μeq l^?1 (48–672 μeq l^?1), respectively. SO₄^2? was the predominant anion with VWM value of 316 μeq l^?1 (65–987 μeq l^?1), next was NO₃^? with VWM value of 109 μeq l^?1 (30–382 μeq l^?1).Using Na as an indicator of marine origin, and Al for the terrestrial inputs, the proportions of sea salt and terrestrial elements were estimated from elemental ratios. More than 99% of Ca²⁺ and 98% of SO₄^2? in rainwater samples are non-sea-salt origin. The ⁸⁷Sr/⁸⁶Sr ratios were used to characterize the different sources based on the data sets of this study and those from literatures. Such sources include sea salts (⁸⁷Sr/⁸⁶Sr～0.90917), soluble soil dust minerals originating from either local or the desert and loess areas (～0.7111), and anthropogenic sources (fertilizers, coal combustion and automobile exhausts). The high concentrations of alkaline ions (mainly Ca²⁺) in Beijing atmosphere have played an important role to neutralize the acidity of rainwater. However, it is worth noting that there is a remarkable acidification trend of rainwater in Beijing recent years.     

Chemical and strontium isotope characterization of rainwater in karst virgin forest,Southwest China

Strontium isotope ratios and concentrations of Ca²⁺, NH₄⁺, Na⁺, K⁺, Mg²⁺, Cl^?, SO₄^2?, NO₃^? and Al³⁺, Sr²⁺ were measured for 52 rainwater samples collected in virgin forest in a rural region between May 2007 and Dec. 2008. The rainwater pH values vary from 4.1 to 7.2 with a volume weight mean (VWM) value of 5.40. 40 of 52 samples have pH value above 5.0, indicating that the regional rainwater was not acidic. Among anions and cations, sulphate concentration (40.4 μeq l^?1, VWM) is the highest in the rainwater, followed by ammonium and calcium (30.2 and 20.8 μeq l^?1, VWM). Rainwater quality is characterized by low salinity and neutralized pH.The chemical compositions and ⁸⁷Sr/⁸⁶Sr ratios of the rainwater samples vary considerably. Using Na⁺ concentration as an indicator of marine origin, the proportions of sea salt and crustal elements were estimated from elemental ratios. The ⁸⁷Sr/⁸⁶Sr ratios were used to characterize different sources base on the data sets of this study and those from literatures. Such sources include weathering of limestone (⁸⁷Sr/⁸⁶Sr = 0.7075), remote soil dust (⁸⁷Sr/⁸⁶Sr > 0.7135) and anthropogenic source (fertilizers: ⁸⁷Sr/⁸⁶Sr = 0.7079). The results of the present study suggest that one likely source for high ammonium and calcium concentration is local soil. Due to a large contribution of these cations to the sulphate neutralization action, the rainwater in this region displays non-acidity, and thus has not significant environmental impact. The wet precipitation in the karst virgin forest in Guizhou province is strongly influenced by natural sources rather than anthropogenic sources.     

Two extreme types of mixing of dust with urban aerosols observed in Kosa particles: ‘After’ mixing and ‘on-the-way’ mixing

Besides well-known episodic Kosa during spring, high concentrations of Ca²⁺ in aerosols were observed early in summer as well as in the semi-continuous data of the aerosols at the summit of Mt. Fuji. We further analysed the data to study the chemical characteristics of the high calcium event during early summer. The back trajectory analyses of the event indicated that Ca was transported from arid and semi-arid regions (e.g. the Taklamakan desert) through the westerly-dominated troposphere higher than the height of the summit of Fuji. The amount of SO₄^2? was always equivalent to that of NH₄⁺ unlike the case of the normal Kosa period where SO₄^2? is in excess with respect to NH₄⁺. This shows the ‘after’ mixing of unreacted CaCO₃ of Kosa origin with (NH₄)₂SO₄, which was only realized by the downward injection of Kosa particles from higher altitudes to the air masses of different origin. In the case of normal Kosa, the air bearing Kosa particles passed through the polluted area to absorb unneutralized acids (‘on-the-way’ mixing), whereas in the case of the Kosa-like phenomena in summer, the acids from the polluted area have been neutralized by NH₄⁺ and become inactive before mixing with CaCO₃ (“after” mixing). We have simplified the chemistry of aerosols using their three major components, Ca²⁺, SO₄^2? and NH₄⁺, and introduced a new triangle diagram with the three assumed end-members of CaCO₃, CaSO₄ and (NH₄)₂SO₄ to quantify the contribution of the ‘after’ mixing to the aerosols (AMI; ‘after’ mixing index). Based on the back trajectories of some high AMI cases, CaCO₃ in Kosa particles was transported through the middle troposphere (5000–7000 m) and descended to meet another air mass where SO₄^2? had been already neutralized by NH₃.     

Time study of trace elements and major ions during two cloud events at the Mt. Brocken

Cloud water investigations have been performed at the highest elevation of Central Germany in 1997. Results of extensive trace element measurements are presented. Besides conductivity, pH, liquid water content and major ions the data set includes 49 minor and trace elements. Estimation of crustal enrichment factors (EFs) provides an indication of the anthropogenic contributions to the cloud water concentrations. The variation of cloud composition with time has been illustrated for two selected events with different air mass origins. The chemical composition of the cloud condensation nuclei on which the droplets grow mainly determines the cloud water chemistry. For a cloud event in June 1997 the concentrations of the crustally derived elements Si, Al, Fe, Ti, Ce, La and Nd follow each other closely. The fact that SO₄²⁻, NO₃⁻ and NH₄⁺ are only moderately correlated with the particular pollutants with high enrichment factors such as Cd, Sb, Pb, Zn, Cu, As, Bi, Sn, Mo, Ni, Tl and V indicates that their source regions are more widespread. During an event in October 1997 the time trends for most minor and trace elements follow rather closely those for the major ions NH₄⁺, SO₄²⁻ and NO₃⁻. Back trajectories show that the transport from continental and marine European sources was the likely cause of the sample concentrations. EFs of trace elements in cloud water samples during the June and October event show a strong correlation with those obtained for urban particulate matter. Although both events are influenced by air masses of different origin, there is a good agreement between the EF signatures.     

Observations and modelling of aerosol growth in marine stratocumulus—case study

Airborne measurements of the growth of the marine accumulation mode after multiple cycles through stratocumulus cloud are presented. The nss-sulphate cloud residual mode was log-normal in spectral shape and it’s mode radius was observed to progressively increase in size from 0.78 to 0.94 μm over 155 min of air parcel evolution through the cloudy marine boundary layer. The primary reason for this observed growth was thought to result from aqueous phase oxidation of SO₂ to aerosol sulphate in activated cloud drops. An aqueous phase aerosol–cloud-chemistry model was used to simulate this case study of aerosol growth and was able to closely reproduce the observed growth. The model simulations illustrate that aqueous phase oxidation of SO₂ in cloud droplets was able to provide enough additional sulphate mass to increase the size of activated aerosol. During a typical cloud cycle simulation, ≈4.6 nmoles kg^-1_air (0.44 μg m^-3) of sulphate mass was produced with ≈70% of sulphate production occurring in cloud droplets activated upon sea-salt nuclei and ≈30% occurring upon nss-sulphate nuclei, even though sea-salt nuclei contributed less than 15% to the activated droplet population. The high fraction of nss-sulphate mass internally mixed with sea-salt aerosol suggests that aqueous phase oxidation of SO₂ in cloud droplets activated upon sea-salt nuclei is the dominant nss-sulphate formation mechanism and that sea-salt aerosol provides the primary chemical sink for SO₂ in the cloudy marine boundary layer.     

Real-time wet scavenging of major chemical constituents of aerosols and role of rain intensity in Indian region

Real-time simultaneous studies on chemical characteristics of rainwater and PM₁₀ aerosols were carried out to understand the scavenging of major chemical components in Indian region. The concentrations of Ca²⁺, NH₄⁺, SO₄²⁻ and NO₃⁻ were observed to be lower in the aerosol samples collected during rain as compared to before and after rain events. The most significant reduction was noticed for Ca²⁺ (74%) during rain which showed highest scavenging ratio (SR) and indicated that below-cloud scavenging is an effective removal process for Ca²⁺ in Indian region. Among non-sea salt components, Ca²⁺ had highest SR at Hyderabad indicating typical characteristics of crustal influence as abundance of calcium carbonate in soil dust has been reported in India. However, the levels of these major chemical components gradually got build-up in due course of time. After rain events, the levels of SO₄²⁻ aerosols were noticed to be substantially higher (more than double) within 24 h. In general, scavenging ratios for all components (except Ca²⁺, NH₄⁺ and K⁺) were higher over BOB as compared to Hyderabad. The maximum fall in aerosol levels (BR minus AR) was observed during continuous and low intensity rain events that did not allow building up of aerosol concentrations.     

Role of ammonia chemistry and coarse mode aerosols in global climatological inorganic aerosol distributions

We use an inorganic aerosol thermodynamic equilibrium model in a three-dimensional chemical transport model to understand the roles of ammonia chemistry and natural aerosols on the global distribution of aerosols. The thermodynamic equilibrium model partitions gas-phase precursors among modeled aerosol species self-consistently with ambient relative humidity and natural and anthropogenic aerosol emissions during the 1990s.Model simulations show that accounting for aerosol inorganic thermodynamic equilibrium, ammonia chemistry and dust and sea-salt aerosols improve agreement with observed SO₄, NO₃, and NH₄ aerosols especially at North American sites. This study shows that the presence of sea salt, dust aerosol and ammonia chemistry significantly increases sulfate over polluted continental regions. In all regions and seasons, representation of ammonia chemistry is required to obtain reasonable agreement between modeled and observed sulfate and nitrate concentrations. Observed and modeled correlations of sulfate and nitrate with ammonium confirm that the sulfate and nitrate are strongly coupled with ammonium. SO₄ concentrations over East China peak in winter, while North American SO₄ peaks in summer. Seasonal variations of NO₃ and SO₄ are the same in East China. In North America, the seasonal variation is much stronger for NO₃ than SO₄ and peaks in winter.Natural sea salt and dust aerosol significantly alter the regional distributions of other aerosols in three main ways. First, they increase sulfate formation by 10–70% in polluted areas. Second, they increase modeled nitrate over oceans and reduce nitrate over Northern hemisphere continents. Third, they reduce ammonium formation over oceans and increase ammonium over Northern Hemisphere continents. Comparisons of SO₄, NO₃ and NH₄ deposition between pre-industrial, present, and year 2100 scenarios show that the present NO₃ and NH₄ deposition are twice pre-industrial deposition and present SO₄ deposition is almost five times pre-industrial deposition.     

Chemical composition of rainwater in the northeastern Romania, Iasi region (2003–2006)

Chemical composition of rainwater was studied in the northeastern Romania, Iasi region, and the concentrations of major inorganic and organic ions were measured in samples collected between April 2003 and December 2006. The pH of the rainwater is 5.92 (volume weighted mean average, VWM) suggesting a sufficient load of alkaline components neutralizing its acidity. On average, 97% of the acidity in the collected samples is neutralized by CaCO₃ and NH₃. Clear seasonal variations were observed for some of the identified ions (e.g., SO₄²⁻, NO₃⁻, Ca²⁺, NH₄⁺). The data obtained during this work revealed that both concentrations and fluxes of anthropogenic source-related ions (e.g., SO₄²⁻, NO₃⁻ and NH₄⁺) are among the highest reported for European sites. It is shown that meteorology and long-range transport processes may concur to their high levels.     

Coupling quasi-spectral microphysics with multiphase chemistry: a case study of a polluted air mass at the top of the Puy de Dôme mountain (France)

An explicit multiphase chemistry model (Atm. Environ. 34 (29/30) (2000) 5015) has been coupled with quasi-spectral microphysics, based upon Berry and Reinhardt's parameterizations (1974a, b). This coupled model has been initialized with polluted conditions as observed at the Puy de Dôme mountain in the center of France and for a maritime cloud.The presence of clouds results in two effects on multiphase chemistry: a direct effect through mass transfer, solubility and reactivity, and an indirect effect through microphysical transfer from cloud water into rainwater and redistribution of reactive soluble species among interstitial air, cloud droplets and raindrops.Results demonstrate that microphysical processes are necessary to sketch out the complex, nonlinear multiphase chemistry in a real cloud. In addition to the direct exchange through mass transfer, incorporation of reactive oxidants such as HO_x in droplets can arise and consequently make those species no longer available for reacting in the gas-phase. Moreover, microphysical coalescence conversions favor NO_x destruction and enhance the chemical nitric acid production. Coalescence of cloud drops to form rain transfers dissolved species into drops that are undersaturated compared to Henry's law equilibrium. The rain becomes a reservoir for these species, allowing aqueous chemistry to produce more nitric acid than would be possible without the presence of rain.Finally, for the different cloud types, the fate of those intermediate and reactive species is investigated, looking at their budget in clear sky situation versus cloudy and/or rainy situations.     

Chemical composition and seasonal variation of acid deposition in Guangzhou, South China: comparison with precipitation in other major Chinese cities

With the aim of understanding the origin of acid rains in South China, we analyzed rainwaters collected from Guangzhou, China, between March 2005 and February 2006. The pH of rainwater collected during the monitoring period varied from 4.22 to 5.87; acid rain represented about 94% of total precipitation during this period. The rainwater was characterized by high concentrations of SO₄²⁻, NO₃⁻, Ca²⁺, and NH₄⁺. SO₄²⁻ and NO₃⁻, the main precursors of acid rain, were related to the combustion of coal and fertilizer use/traffic emissions, respectively. Ca²⁺ and NH₄⁺ act as neutralizers of acid, accounting for the decoupling between high SO₄²⁻ concentrations and relatively high pH in the Guangzhou precipitation. The acid rain in Guangzhou is most pronounced during spring and summer. A comparison with acid precipitation in other Chinese cities reveals a decreasing neutralization capacity from north to south, probably related to the role and origin of alkaline bases in precipitation.     

Effect of high concentrations of inorganic seed aerosols on secondary organic aerosol formation in the m-xylene/NOx photooxidation system

High concentrations (>15 μm³ cm^?3) of CaSO₄, Ca(NO₃)₂ and (NH₄)₂SO₄ were selected as surrogates of dry neutral, aqueous neutral and dry acidic inorganic seed aerosols, respectively, to study the effects of inorganic seeds on secondary organic aerosol (SOA) formation in irradiated m-xylene/NO_x photooxidation systems. The results indicate that neither ozone formation nor SOA formation is significantly affected by the presence of neutral aerosols (both dry CaSO₄ and aqueous Ca(NO₃)₂), even at elevated concentrations. The presence of high concentrations of (NH₄)₂SO₄ aerosols (dry acidic) has no obvious effect on ozone formation, but it does enhance SOA generation and increase SOA yields. In addition, the effect of dry (NH₄)₂SO₄ on SOA yield is found to be positively correlated with the (NH₄)₂SO₄ surface concentration, and the effect is pronounced only when the surface concentration reaches a threshold value. Further, it is proposed that the SOA generation enhancement is achieved by particle-phase heterogeneous reactions induced and catalyzed by the acidity of dry (NH₄)₂SO₄ seed aerosols.     

Drop size-dependent chemical composition in clouds and fogs. Part I. Observations

The first observations of size-dependent cloud and fog drop inorganic ion and trace metal concentrations obtained using the Colorado State University 5-Stage cloud water collector (CSU 5-Stage) during field studies of orographic clouds (Whiteface Mountain, NY, July 1998) and radiation fogs (Davis, CA, January 1999) are reported. Although some mixing between drop sizes occurs, the CSU 5-Stage effectively separates the largest drops (>≈30 μm in diameter) from the smallest ones (<≈10 μm in diameter) permitting the discernment of size-dependent drop composition not possible with previous two- or three-stage collectors. At Whiteface, pH and the concentrations of the “major” ions −NH₄⁺, NO₃⁻, and SO₄²⁻—appeared largely independent of drop size as measured by a two-stage collector. The same major ion concentrations differed in Davis fogs by up to a factor of approximately 10 in the two-stage collector with consistently higher small drop concentrations. In both locations, CSU 5-Stage data generally indicate a greater range of concentrations is present across the drop size spectrum. CSU 5-Stage data show “U”- shaped profiles of major ion concentration vs. drop size at Whiteface and “L”- shaped profiles at Davis and the maximum/minimum concentration differences between fractions increased up to a factor of 2 (Whiteface) and 30 (Davis). Lower concentration species at both locations showed multiple concentration vs. drop size profiles with CSU 5-Stage data again exhibiting more variability than observed with the two-stage collector. While rarely reported, significant nitrite concentrations—relatively higher in the larger drops—were observed, and copper concentrations merit further investigation in the Davis fogs. The findings presented here are consistent with other studies. The implications and benefits of the increased resolution of size-dependent drop composition provided by the CSU 5-Stage are explored for the Davis fogs in a companion paper (Moore et al., Atmos. Environ. (2004), this issue).     

The influence of diesel—truck exhaust particles on the kinetics of the atmospheric oxidation of dissolved sulfur dioxide by oxygen

The automobile exhausts are one of the major sources of particulate matter in urban areas and these particles are known to influence the atmospheric chemistry in a variety of ways. Because of this, the oxidation of dissolved sulfur dioxide by oxygen was studied in aqueous suspensions of particulates, obtained by scraping the particles deposited inside a diesel truck exhaust pipe (DEP). A variation in pH showed the rate to increase with increase in pH from 5.22 to about ～6.3 and to decrease thereafter becoming very slow at pH?=?8.2. In acetate-buffered medium, the reaction rate was higher than the rate in unbuffered medium at the same pH. Further, the rate was found to be higher in suspension than in the leachate under otherwise identical conditions. And, the reaction rate in the blank reaction was the slowest. This appears to be due to catalysis by leached metal ions in leachate and due to catalysis by leached metal ions and particulate surface both in suspensions. The kinetics of dissolved SO₂ oxidation in acetate-buffered medium as well as in unbuffered medium at pH?=?5.22 were defined by rate law: k _obs ?=?k ₀?+?k _cat [DEP], where k _obs and k ₀ are observed rate constants in the presence and the absence of DEP and k _cat is the rate constant for DEP-catalyzed pathway. At pH?=?8.2, the reaction rate was strongly inhibited by DEP in buffered and unbuffered media. Results suggest that the DEP would have an inhibiting effect in those areas where rainwater pH is 7 or more. These results at high pH are of particular significance to the Indian subcontinent, because of high rainwater pH. Conversely, it indicates the DEP to retard the oxidation of dissolved SO₂ and control rainwater acidification.

     

Acidification of a solonetzic soil by nitrogenous fertilizers

Abstract

Annual applications of (NH₄)₂SO₄ NH₄NO₃ and urea on a Solonetzic soil at 112 kg N/ha for 10 consecutive years reduced pH levels from 5.6 for the check to 4.4, 4.9 and 5.3, respectively for (NH₄)₂SO₄, NH₄NO₃ and urea. (NH₄)₂SO₄ generated twice as much exchange acidity as NH₄NO₃ and four times as much as urea. Net extractable cations leached from the Ap horizon closely approximated the amount of exchange acidity generated by (NH₄)₂SO₄ and NH₄NO₃ fertilizers. The levels of soil extractable Al and Mn were greatly enhanced by (NH₄)₂SO₄ as were plant contents. Similar acidifying effects to that produced by the (NH₄)₂SO₄ occurred when NH₄NO₃ was applied at 300 kg N/ha annually for 12 consecutive years in another field experiment on the same soil. Liming samples of the field (NH₄)₂SO₄ acidified soils in the greenhouse, significantly increased yields and lowered the Al and Mn contents of the plants to normal levels.     

Drop size-dependent chemical composition of clouds and fogs. Part II: Relevance to interpreting the aerosol/trace gas/fog system

Size-resolved fog drop chemical composition measurements were obtained during a radiation fog campaign near Davis, California in December 1998/January 1999 (reported in Reilly et al., Atmos. Environ. 35(33) (2001) 5717; Moore et al., Atmos. Environ. this issue). Here we explore how knowledge of this size-dependent drop composition—particularly from the newly developed Colorado State University 5-Stage cloud water collector—helps to explain additional observations in the fog environment. Size-resolved aerosol measurements before and after fog events indicate relative depletion of large (>2 μm in diameter) particles during fog accompanied by a relative increase in smaller aerosol particle concentrations. Fog equivalent air concentrations suggest that entrainment of additional particles and in-fog sedimentation contributed to observed changes in the aerosol size distribution. Calculated deposition velocities indicate that sedimentation was an important atmospheric removal mechanism for some species. For example, nitrite typically has a larger net deposition velocity than water and its mass is found preferentially in the largest drops most likely to sediment rapidly. Gas–liquid equilibria in fog for NO₃⁻/HNO₃, NH₄⁺/NH₃, and NO₂⁻/HONO were examined. While these systems appear to be close to equilibrium or relative equilibrium during many time periods, divergences are observed, particularly for low liquid water content (<0.1 g m⁻³) fogs and in different drop sizes. Knowledge of the drop size-dependent composition provided additional data useful to the interpretation of these deviations. The results suggest that data from multi-stage cloud water collectors are useful to understanding fog processes as many depend upon drop size.     

A computer modelling study of the chemistry occurring during cloud formation over hills

A model has been developed which describes the chemical processes occurring during the formation of hill cloud. This model has been applied to the study of four cases which should be typical of the chemical conditions which can exist during the formation of such cloud over hills in the N of England.The results show that a wide range of chemical behaviour is possible in cloud droplets, depending on the history of the airmass. The composition of the water-soluble aerosol is of importance in determining the chemical composition of cloud droplets, with most of the sulphate and large proportions of the other ions being derived from this source. Gas-phase chemistry is also important, not only because it dictates the initial concentrations of oxidants such as H₂O₂ and O₃ and of acidic gases such as HNO₃ at the onset of condensation, but also because of the chemical processes which occur concurrently with those in clouds. These are important sources of radicals, as well as of nitric acid, which continue to dissolve in solution. Gasphase HCl, HNO₃ and NH₃ are all of primary importance in describing the acidity of cloud droplets. The various oxidation mechanisms for SO₂ can all be important, with their relative contributions varying from case to case. H₂O₂ does not invariably dominate this process, especially in winter when its concentration may be low.The chemistry of hill clouds is far from trivial and oversimplification can result in a misunderstanding of the behaviour which occurs. There is a need, rather, to consider a wide range of reactions if the nature of the overall processes and the impact of manmade emissions on the environment via this route are to be understood.     

Analysis of Hong Kong daily bulk and wet deposition data from 1994 to 1995

Results from a 1-year daily rainwater sampling program, employing both wet and bulk deposition samplers with replicate samples, from 1994 to 1995 in Hong Kong are presented and analysed. Analyte concentrations were found to vary over a wide range of several orders of magnitude, with [H⁺] for example, from 0.16 to 208.9 μeq dm^-3. Diurnal pH values less than 3.83 were measured on five occasions. A significant correlation between pH and lognormal windspeed has been found. This is taken to indicate the minor importance of long-range transport in determining rainwater acidity, since local pollutant emissions accumulate and react under conditions of atmospheric stability in the sub-tropical climate. The H⁺ wet deposition flux onto a polythene surface was 90 meq m^-2 yr^-1 during 1994–1995 at City University. Dry deposition exerts a neutralizing influence upon the acidity from this wet deposition. Although paired t-tests indicated significant differences between the bulk versus wet deposition datasets for cations, but not anions, the dataset means consequently showed such large standard deviations that t-tests indicated no significant differences. In rainwater, the charges from SO^2-₄ and NO^-₃ anions seldom balance the proton charges, implying that they are also derived from solubilization of primary and secondary airborne Ca²⁺, Mg²⁺ and NH⁺₄ particulate matter in rainwater. Use of the [SO^2-₄]/[NO^-₃] ratios in rainwater in fingerprinting pollutant origins has drawbacks, but is generally indicative of a predominantly regional contribution of these secondary pollutants to rainwater. Bulk deposition pH in Hong Kong would be in the region of 4.1 if basic Ca²⁺ compounds alone did not neutralize acidity. The regional rainout pH, inferred after exhaustive below-cloud scavenging, is about 5. The temporal trends in Hong Kong rainwater acidity are blurred.     

Transformation of atmospheric ammonia and acid gases into components of PM<Subscript>2.5</Subscript>: an environmental chamber study

Introduction  

The kinetics of the transformation of ammonia and acid gases into components of PM_2.5 has been examined. The interactions of existing aerosols and meteorology with the transformation mechanism have also been investigated. The specific objective was to discern the kinetics for the gas-to-particle conversion processes where the reactions of NH₃ with H₂SO₄, HNO₃, and HCl take place to form (NH₄)₂SO₄, NH₄NO₃, and NH₄Cl, respectively, in PM_2.5.