A winter study of air,cloud and precipitation chemistry in Ontario,Canada

During January and February 1984, a field project was conducted near North Bay, Ontario, Canada. The principal objective was to characterize the chemical and microphysical properties of the air masses, clouds and precipitation in this region of NE North America during the winter season. Two extensively instrumented aircraft with some newly designed cloudwater and snow collectors were used, as well as a surface station continuously monitoring pollutant concentrations and a precipitation event sampling network. Pollutant concentrations at the surface were found to vary with the airmass back trajectory with the highest concentrations observed for trajectories from the S and SW and the lowest from the N. Vertical profiles of aerosol particle (0.2−2 μm diameter) and NOx concentrations show similar trends with maxima of 1200 cm⁻³ and 7 ppb, respectively near ground level with air mass trajectories from the S, in comparison to values of 250 cm⁻³ and 1 ppb obtained with trajectories from the N. Cloudwater, aircraft precipitation and ground precipitation samples had a daily median pH of 3.6,4.6 and 4.2, respectively with the cloudwater having the highest sulphate and nitrate concentrations. The nitrate/sulphate equivalent concentration ratios in the cloudwater, aircraft precipitation and ground precipitation samples were 0.7,0.6 and 1.4, respectively. The data suggest that precipitation scavenging of nitric acid below cloud base is an important process during the winter season.     

Importance of formaldehyde in cloud chemistry

A physical-chemical model which is an extension of that of Hong and Carmichael (1983) is used to investigate the role of formaldehyde in cloud chemistry. This model takes into account the mass transfer of SO₂, O₃, NH₃, HNO₃, H₂O₂, CO₂, HCl, HCHO, O₂, OH and HO₂ into cloud droplets and their subsequent chemical reactions. The model is used to assess the importance of S(IV)-HCHO adduct formation, the reduction of H₂O₂ by HCHO, HCHO-free radical interactions, and the formation of HCOOH in the presence of HCHO in cloud droplets.Illustrative calculations indicate that the presence of HCHO inhibits sulfate production rate in cloud droplets. The direct inhibition of sulfate production rate in cloud water due to nucleophilic addition of HSO⁻₃ to HCHO_(aq) to form hydroxymethanesulfonate (HMSA) is generally low for concentrations of HCHO typical of ambient air. However, inhibition of sulfate production due to formaldehyde-free radical interactions in solution can be important. These formaldehyde-free radical reactions can also generate appreciable quantities of formic acid.     

A Lagrangian approach for modelling turbulent transport and chemistry

A Lagrangian two-particle model for the relative diffusion and mixing of two reactive species is proposed. The model has been tested for consistency in simple geometrical configurations and has been compared to experimental data, Eulerian first- and second-order closure models as well as to other Lagrangian models. The model includes the covariance between the two species and therefore gives better predictions of both diffusion and chemistry than first-order closure models or the one-particle model proposed by Chock and Winkler (1994a,b, Journal of Geophysical Research, 99 D1, 1019–1031, 99 D1, 1033–1041). The model is a generalisation of the model presented by Komori et al. (1991, Journal of Fluid Mechanics 228, 629–659). The results of the calculations, although preliminary in character, indicate that the proposed algorithm is robust and efficient, and yields satisfactory results in turbulence with slow to moderate chemistry.     

Formation of particulate sulfur species (sulfate and methanesulfonate) during summer over the Eastern Mediterranean: A modelling approach

To improve our understanding of the mechanisms of particulate sulfur formation (non sea-salt sulfate, nss-SO₄²⁻) and methanesulfonate (MS_x used here to represent the sum of gaseous methanesulfonic acid, MSA, and particulate methanesulfonate, MS⁻) in the eastern Mediterranean and to evaluate the relative contribution of biogenic and anthropogenic sources to the S budget, a chemical box model coupled offline with an aerosol–cloud model has been used.Based on the measurements of gaseous dimethyl sulfide (DMS) and methanesulfonic acid (MSA) and the MSA sticking coefficient determined during the Mediterranean Intensive Oxidant Study (MINOS) experiment, the yield of gaseous MSA from the OH-initiated oxidation of DMS was calculated to be about 0.3%. Consequently, MSA production from gas-phase oxidation of DMS is too small to explain the observed levels of MS⁻. On the other hand, heterogeneous reactions of dimethyl sulfoxide (DMSO) and its gas-phase oxidation product methanesulfinic acid (MSIA) can account for most of the observed MS⁻ levels. The modelling results indicate that about 80% of the production of MS⁻ can be attributed to heterogeneous reactions.Observed submicron nss-SO₄²⁻ levels can be fully explained by homogeneous (photochemical) gas-phase oxidation of sulfur dioxide (SO₂) to sulfuric acid (H₂SO₄), which is subsequently scavenged by (mainly submicron) aerosol particles. The predominant oxidant during daytime is hydroxyl radical (OH) showing very high peak levels in the area during summer mostly under cloudless conditions. Therefore, during summer in the east Mediterranean, heterogeneous sulfate production appears to be negligible. This result is of particular interest for sulfur abatement strategy. On the other hand only about 10% of the supermicron nss-SO₄²⁻ can be explained by condensation of gas-phase H₂SO₄, the rest must be formed via heterogeneous pathways.Marine biogenic sulfur emissions contribute up to 20% to the total oxidized sulfur production (SO₂ and H₂SO₄) in good agreement with earlier estimates for the area.     

Some fluid modeling studies of flow and dispersion over two-dimensional low hills

The results of wind tunnel experiments on flow and dispersion over two-dimensional low and ‘gentle’ hills of different shapes and aspect ratios are discussed. The hill-induced influences on velocity and concentration fields are studied as functions of the hills‘ aspect ratio (the ratio of the half-length to the height of the hill).The speed-up of flow on the hilltop is shown to have an inverse relationship with the aspect ratio ‘a’, as predicted by several theories. However, this does not increase indefinitely as a → 0, but has a maximum value at some optimum aspect ratio greater than unity. The velocity variances and Reynolds stress increase rapidly in the near wake region in the lee of the hills. In the far wake region, beyond about five hill heights, the hill-induced perturbations in mean velocity, Reynolds stress and variances of velocity decay in inverse proportion to the distance behind the hill.Dispersion from elevated sources located on the top of ‘gentle’ hills is not significantly affected by the hills. For low-level sources on the top of steeper hills, where flow separation may occur, the ground level concentrations are reduced (by as much as a factor of 3). On the other hand, the ground level concentrations from sources on the lee side of the hills are considerably enhanced (by as much as a factor of 15) near the source, but slightly reduced far downwind. The influence of‘gentle’ hills bears an inverse relationship with the hills' aspect ratio. For steep hills, however, the hill influence is intimately related to the dimensions of the cavity region.     

A wind tunnel study of the flow structure and dispersion from sources upwind of three-dimensional hills

The flow fields around moderately steep hills of triangular cross section and varying crosswind aspect ratio and around a bell-shaped hill were examined by using models immersed in a simulated neutral atmospheric boundary layer in a meteorological wind tunnel. The triangular hills ranged from an axisymmetric cone to a two-dimensional ridge. Concentration patterns resulting from sources of three heights placed upwind of each of these hills were examined to determine plume deformations and terrain amplification factors. The separated flow fields, increasing in size with increasing aspect ratio, appeared to have dominating influences on the entire flow structure; changes in several flow parameters were plausibly explained in terms of the notion that the effective hill shape was the hill-plus-recirculation region rather than the actual hill shape. The concentration measurements showed strong distortions of plume shapes effected by the hills, with convergence in vertical planes and divergence in horizontal planes. Plumes from elevated sources approached the hill surfaces much more closely the smaller the aspect ratio; streamline displacements were generally within the limits suggested by potential flow theory. The terrain amplification factor A, defined as the ratio of the maximum surface concentration in the presence of the hill to the maximum in flat terrain, was found to decrease with increasing aspect ratio. For the half-hill-height sources, values of A ranged from 4 for the bell-shaped hill down to 1.5 for the two-dimensional ridge; for the hill-height source, from 1.8 for the cone down to 0.6 for the two-dimensional ridge. The latter value is suspect, however, because larger concentrations are expected downwind of the reattachment point, a region not probed in the current study.     

Study of aerosol transport through precipitation chemistry over Arabian Sea during winter and summer monsoons

Precipitation samples over the Arabian Sea collected during Arabian Sea Monsoon Experiment (ARMEX) in 2002–2003 were examined for major water soluble components and acidity of aerosols during the period of winter and summer monsoon seasons. The pH of rain water was alkaline during summer monsoon and acidic during winter monsoon. Summer monsoon precipitation showed dominance of sea-salt components (∼90%) and significant amounts of non-sea salt (nss) Ca²⁺ and SO₄²⁻. Winter monsoon precipitation samples showed higher concentration of NO₃⁻ and NH₄⁺ compared to that of summer monsoon, indicating more influence of anthropogenic sources. The rain water data is interpreted in terms of long-range transport and background pollution. In summer monsoon, air masses passing over the north African and Gulf continents which may be carrying nss components are advected towards the observational location. Also, prevailing strong southwesterly winds at surface level produced sea-salt aerosols which led to high sea-salt contribution in precipitation. While in winter monsoon, it was observed that, air masses coming from Asian region towards observational location carry more pollutants like NO₃⁻and nss SO₄²⁻ that acidify the precipitation.     

Influence of dust composition on cloud droplet formation

Previous studies suggest that interactions between dust particles and clouds are significant; yet the conditions where dust particles can serve as cloud condensation nuclei (CCN) are uncertain. Since major dust components are insoluble, the CCN activity of dust strongly depends on the presence of minor components. However, many minor components measured in dust particles are overlooked in cloud modeling studies. Some of these compounds are believed to be products of heterogeneous reactions involving carbonates. In this study, we calculate Kohler curves (modified for slightly soluble substances) for dust particles containing small amounts of K⁺, Mg²⁺, or Ca²⁺ compounds to estimate the conditions where reacted and unreacted dust can activate. We also use an adiabatic parcel model to evaluate the influence of dust particles on cloud properties via water competition. Based on their bulk solubilities, K⁺ compounds, MgSO₄·7H₂O, Mg(NO₃)₂·6H₂O, and Ca(NO₃)₂·4H₂O are classified as highly soluble substances, which enable activation of fine dust. Slightly soluble gypsum and MgSO₃·6H₂O, which may form via heterogeneous reactions involving carbonates, enable activation of particles with diameters between about 0.6 and 2 μm under some conditions. Dust particles>2 μm often activate regardless of their composition. Only under very specialized conditions does the addition of a dust distribution into a rising parcel containing fine (NH₄)₂SO₄ particles significantly reduce the total number of activated particles via water competition. Effects of dust on cloud saturation and droplet number via water competition are generally smaller than those reported previously for sea salt. Large numbers of fine dust CCN can significantly enhance the number of activated particles under certain conditions. Improved representations of dust mineralogy and reactions in global aerosol models could improve predictions of the effects of aerosol on climate.     

Analysis of cloud and precipitation chemistry at Whiteface Mountain,NY

Federal and state programs over the past two decades have resulted in the reduction of emissions of precursors of acid rain. Concomitant with these changes, measured concentrations of acidity in precipitation and in watersheds have shown a downward trend or improvement. However, another pathway for these precursors is through cloud and fog events that often tend to occur at high-elevation regions affecting the fauna and flora as well. In this study we report on long-term measurements of cloud water and precipitation chemistry made from 1994 onwards at a high-elevation location, Whiteface Mountain NY, in the northeastern United States. Trends and inter-relationship between the ions were examined along with ambient SO₂ measurements and Adirondack lakes chemistry data.     

A comparison of model photochemical ozone formation potential with observed regional scale ozone formation during a photochemical episode over the United Kingdom in April 1987

A photochemical trajectory model has been employed to calculate the maximum potential for ozone generation in air parcels passing over the U.K. during a photochemical pollution episode in April 1987. In all, 11 trajectories have been studied and the model results compared against an objective analysis of the integrated ozone generation based on the observations reported from the U.K. Department of the Environment ground level O₃network. There is apparently good correlation between the observed O₃formation and the model O₃formation potential although the latter overestimates the former by a factor of about 2.8. The solar illumination conditions employed in the photochemical trajectory model may have caused this overestimation, since the model is formulated for O₃control strategy assessment and simulates the ‘worst case’ situation likely to give the maximum potential for secondary pollutant formation. In addition to the model results for O₃, a wide range of primary and secondary pollutant concentrations from the model were examined, together with the influence of precursor pollutant emissions.     

A study of the ozone formation by ensemble back trajectory-process analysis using the Eta–CMAQ forecast model over the northeastern U.S. during the 2004 ICARTT period

The integrated process rates (IPRs) estimated by the Eta–CMAQ model at grid cells along the trajectory of the air mass transport path were analyzed to quantitatively investigate the relative importance of physical and chemical processes for O₃ formation and evolution over the northeastern U.S. during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) period. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model is used to determine the back trajectory of air masses reaching the northeast by linking a downwind receptor to upwind source areas. The process analysis is applied to a high O₃ episode occurring on July 22, 2004 at three selected sites in the northeastern U.S. The process analysis at the location of the site shows that during the daytime, the O₃ concentrations in the surface layer are mainly enhanced by the vertical diffusion of O₃-rich air from aloft, followed by horizontal advection (HADV) and chemical production (CHEM), whereas dry deposition (DDEP) and vertical advection (ZADV) mainly deplete O₃ concentrations at the sites of Valley Central (VC), NY and Castle Spring (CS), NH. By integrating the effects of each process over the depth of the daytime planetary boundary layer (PBL), it was found that at the VC site, CHEM and HADV contributed about 53% and 41%, respectively, to O₃ levels within the PBL. This confirms the significance of regional transport of O₃ from the industrialized areas into the Northeast. On the other hand, the process analysis results for O₃ formation in moving air masses indicate that on July 22, large chemical production of O₃ along the transport path over the polluted urban regions leads to significant increase in O₃ in the air mass reaching the VC site, whereas the low chemical production of O₃ along the transport path over the low emission regions leads to the low O₃ concentration at the site of Belleayre Mountain (BM), NY. The dramatic buildup of O₃ concentration from 50 ppb to 102 ppb in the air masses before reaching the VC site after 12:00 EST on 7/22 indicates the significant impact of pollution from the northeastern urban corridor at this site. On the basis of the results at the CS site, it was found that high NO_x emissions along the transport path led to large chemical production of O₃ in the air mass reaching the CS site on July 22. In contrast, the low chemical production of O₃ associated with low emission (relatively clean conditions) along the transport path over the northern portions of the domain is responsible for the low O₃ concentration at the CS site on July 26.     

A field study of the generation of nitrate in a hill cap cloud

A field experiment to investigate the formation of nitrate as an airstream passes through a hill cap cloud has been performed at the UMIST field station on Great Dun Fell. It has been shown that the aerosol nitrate concentration increased by about 0.5 microg m(-3) as the airstream passed through the cloud during the night. At sunrise the nitrate production disappeared. It is suggested that the most likely mechanism for this nitrate production was due to the solution of N2O5 and NO3 formed from the reaction of NO2 with O3. These higher oxides build up overnight in the absence of short wave radiation to photolyse them. Other possible mechanisms of nitrate production are also discussed.     

Process analysis of ozone formation under different weather conditions over the Kanto region of Japan using the MM5/CMAQ modelling system

We have assessed the contributions of individual physical and chemical atmospheric processes on ozone formation under different weather conditions during a typical summer month (August 2005) using the MM5/CMAQ modelling system. We found that the ozone episodes in the Kanto region are dominated by three major patterns, of which Patterns I and II are regular summertime pressure patterns with a 26% and 16% frequency of occurrence, respectively. A process analysis at two typical sites in the Kanto region – one located in the central region of Tokyo and the other located in the rural areas of Kanto – indicates that ozone formation is mainly controlled by advection, vertical diffusion, dry deposition, and chemical processes. The ground-level ozone concentrations are enhanced mainly by the vertical mixing of ozone-rich air from aloft, whereas the dry deposition and chemical processes mainly deplete ozone. By investigating the effects of each process under different weather conditions, we found that the significant decrease in ozone removal due to the chemical and advection processes under conditions of high stagnation is the most important cause of the enhanced levels of ozone in the central region of Tokyo. The results of this study can contribute to a better understanding of ozone formation in the Kanto region, and they may be valuable for local policy makers for further development planning.     

An experimental study of flow and dispersion in the wakes of three-dimensional low hills

The effect of three-dimensional, axisymmetric hills on the mean flow and turbulence structure of a boundary layer is investigated through experiments conducted in a large meteorological wind tunnel Two conical hills having slopes of 26.5° and 17.5° and height (H) about one-quarter of the simulated boundary layer thickness were considered. Measurements of mean and turbulent flow and diffusion on the lee sides of the hills are presented and compared with the observations made without the hills. Velocity measurements behind the steeper hill (26.5°) showed a well defined recirculating zone and the presence of weak trailing vortices in the lower region (z = 0.3H), whereas no such features could be detected behind the gentler hill.Perturbations in mean velocity, Reynolds stress and variances of velocity fluctuations in the far wake (beyond 4H) are found to decay with the downwind distance from the center of the hill according to some power laws. The decay rates of mean velocity deficit and turbulence pertubations in the far wake are compared with the theoretical predictions of Hunt (1971, Central Electricity Research Lab. Rep. 1974, ARC, Rep. 35601, Atmos. 229) and Lemberg (1973, PhD Dissertation, Rep. No. BLWT-3-73, Univ. of W Ontario) as well as with the results of other experiments.Comparisons of diffusion measurements at various distances from a point source with and without the hill bring out several interesting aspects of dispersion in the wake of the gentler hill. A 40–65% reduction in maximum ground-level concentration (glc) was observed when the source was placed at the top of the hill, whereas a marked increase (up to six times) in the glc was noticed when the source was moved to the downwind base of the hill. Enhanced vertical and lateral dispersion was also evident in the wake region.     

Night-time radical chemistry during the TORCH campaign

We present one of the most comprehensive studies of night-time radical chemistry to date, from the Tropospheric ORganic CHemistry experiment (TORCH) in the summer of 2003. TORCH provided a wealth of measurements with which to study the oxidizing capacity of the atmosphere. The measurements provided input to a zero-dimensional box model which has been used to study night-time radical chemistry during the campaign. Average night-time predicted concentrations of OH (2.6 × 10⁵ molecule cm^?3), HO₂ (2.9 × 10⁷ molecule cm^?3) and [HO₂+ΣRO₂] radicals (2.2 × 10⁸ molecule cm^?3) were an order of magnitude smaller than those predicted during the daytime. The model under-predicted the night-time measurements of OH, HO₂ and [HO₂+ΣRO₂] radicals, on average by 41%, 16% and 8% respectively. Whilst the model captured the broad features of night-time radical behaviour, some of the specific features that were observed are hard to explain. A rate of radical production assessment was carried out for the whole campaign between the hours of 00:00 and 04:00. Whilst radical production was limited owing to the absence of photolytic reactions, production routes via the reactions of alkenes with O₃ provided an effective night-time radical source. Nitrate radical concentrations were predicted to be 0.6 ppt on average with a peak of 18 ppt on August 9th during a polluted heat wave period. Overall, the nitrate radical contributes about a third of the total initiation via RO₂, mostly through reaction with alkenes.     

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.     

Alkyl nitrate photochemistry during the tropospheric organic chemistry experiment

Alkyl nitrates (C₁–C₅) were measured at two sites (near urban and rural) in southeast England during the Tropospheric Organic Chemistry Experiment (TORCH). Methyl nitrate was the dominant species during both campaigns accounting for on average about one third of the total measured alkyl nitrates. High mixing ratios (>50 pptv) and variability of methyl nitrate were observed at the near urban site (TORCH1) that were not seen at the rural site (TORCH2) and which could not be explained by local photochemical production or direct emissions. The diurnal variation of methyl nitrate during TORCH1 showed a morning maximum that would be consistent with nighttime chemistry followed by transport to the surface by boundary layer dynamics. Similarly, elevated morning mixing ratios were also observed during TORCH2 although the magnitudes were much smaller. As a result, methyl nitrate could represent a tracer for nighttime chemistry seen at the ground the following day. At both campaigns, the dominant source of short chain alkyl nitrates and carbonyl precursor radicals (≤C₄) were from decomposition of larger compounds. The magnitude of the source increased with decreasing carbon number consistent with increasing total precursor abundance. Non-photochemical emissions of acetaldehyde and acetone could not be accounted for by automobile exhaust emissions alone and indicated that other direct sources are likely important in this environment.     

A study of atmospheric acid formation in different environments

The mathematical model presented in this paper describes in detail the gas-phase chemistry (80 reactions), gas-phase/liquid-phase equilibrium (12 equilibria) and liquid-phase chemistry (27 reactions and equilibria) of acid formation. The model is used to simulate acid formation under various conditions:

• 1.(1) nonprecipitating clouds in the Adirondacks,
• 2.(2) raining clouds in the Ohio River valley,
• 3.(3) night-time fog in the Los Angeles basin and
• 4.(4) night-time nonprecipitating clouds in the Los Angeles basin. Model calculations are compared with observed values of sulfate and nitrate formation. The model is used to analyze the relative importance of sulfate and nitrate formation pathways under these various conditions.