Absolute principal component analysis of atmospheric aerosols in Mexico city

A receptor model is presented based on absolute principal component analysis (APCA) of elemental concentrations in atmospheric aerosols from Mexico City during the Summer of 1995. Elemental contents on samples collected with a Stacking Filter Unit of the Davis design was carried out using Particle Induced X-ray Emission (PIXE). The sampling device allowed the separation of particles with mean aerodynamic diameters ranging from 2.5 μm to 15 μm (coarse fraction) and smaller than 2.5 μm (fine fraction). Sampling was divided into morning, afternoon and night periods, with higher concentrations being found during the morning. Seasonal variation is observed when comparisons with other studies are carried out. The application of APCA allowed identification of four sources for each fraction, with a soil-derived dust predominance in the coarse one. The influence of meteorological parameters is studied using cluster analysis, showing that during the morning there is a transport of pollutants from the west towards the sampling site, while the night transport corresponds to soil-derived dust from the north.     

The influence of aerosols on photochemical smog in Mexico City

Aerosols in the Mexico City atmosphere can have a non-negligible effect on the ultraviolet radiation field and hence on the formation of photochemical smog. We used estimates of aerosol optical depths from sun photometer observations in a detailed radiative transfer model, to calculate photolysis rate coefficients (J_NO2) for the key reaction NO₂+hν→NO+O (λ<430 nm). The calculated values are in good agreement with previously published measurements of J_NO2at two sites in Mexico City: Palacio de Minerı́a (19°25′59″N, 99°07′58″W, 2233 masl), and IMP (19°28′48″N, 99°11′07″W, 2277 masl) and in Tres Marias, a town near Mexico City (19°03′N, 99°14′W, 2810 masl). In particular, the model reproduces very well the contrast between the two urban sites and the evidently much cleaner Tres Marias site. For the measurement days, reductions in surface J_NO2 by 10–30% could be attributed to the presence of aerosols, with considerable uncertainty due largely to lack of detailed data on aerosol optical properties at ultraviolet wavelengths (esp. the single scattering albedo). The potential impact of such large reductions in photolysis rates on surface ozone concentrations is illustrated with a simple zero-dimensional photochemical model.     

Trends of exhaust emissions from gasoline motor vehicles in the metropolitan area of Mexico city

Light duty gasoline vehicles account for most of CO, hydrocarbons and NO_x emissions to the urban environment in the metropolitan area of Mexico City. In order to ameliorate air pollution, several control measures have been imposed in the last decade, such as: up-grade of gasoline's quality, stringent environmental standards, and catalytic converters. On the other hand and from the beginning of 2001, Tier I emission standards became mandatory for all new model year sold in the country. Car manufacturers in Mexico do not guarantee the performance of their exhaust emissions systems for a given mileage. In this work, we present results on brand new vehicles that indicate that NO_x emission factors, though they are within the Tier I standard, deteriorate rapidly with the travelled distance (mileage).     

Measurement of hydroxymethanesulfonate in atmospheric aerosols

Hydroxymethanesulfonate (HMS), a product of the heterogeneous reaction between S(IV) and HCHO, was measured in atmospheric aerosol samples collected at two locations indicating it can exist outside of clouds. Sampling was performed through collection with filters and analysis using ion chromatography with a treatment step to distinguish between HMS and uncomplexed S(IV). Concentrations of HMS were found to range from below the detection limit to 6.5 ng m^-3 for samples collected in central New Mexico in the summer and around 30 ng m^-3 for two samples collected in Seattle, Washington in the spring. Higher concentrations were associated with greater occurrence of clouds, which is the presumed source of the HMS. In the samples collected in Seattle, about half of the HMS was found associated with fine particles. In the central New Mexico samples, molar ratios of HMS to sulfate in particulate matter were found to be less than 0.002 indicating that HMS was a minor contributor of aerosol sulfur under the conditions of the measurements.     

The influence of ozone on atmospheric emissions of gaseous elemental mercury and reactive gaseous mercury from substrates

Experiments were performed to investigate the effect of ozone (O₃) on mercury (Hg) emission from a variety of Hg-bearing substrates. Substrates with Hg(II) as the dominant Hg phase exhibited a 1.7 to 51-fold increase in elemental Hg (Hg^o) flux and a 1.3 to 8.6-fold increase in reactive gaseous mercury (RGM) flux in the presence of O₃-enriched clean (50 ppb O₃; 8 substrates) and ambient air (up to ∼70 ppb O₃; 6 substrates), relative to clean air (oxidant and Hg free air). In contrast, Hg^o fluxes from two artificially Hg^o-amended substrates decreased by more than 75% during exposure to O₃-enriched clean air relative to clean air. Reactive gaseous mercury emissions from Hg^o-amended substrates increased immediately after exposure to O₃ but then decreased rapidly. These experimental results demonstrate that O₃ is very important in controlling Hg emissions from substrates. The chemical mechanisms that produced these trends are not known but potentially involve heterogenous reactions between O₃, the substrate, and Hg. Our experiments suggest they are not homogenous gas-phase reactions. Comparison of the influence of O₃ versus light on increasing Hg^o emissions from dry Hg(II)-bearing substrates demonstrated that they have a similar amount of influence although O₃ appeared to be slightly more dominant. Experiments using water-saturated substrates showed that the presence of high-substrate moisture content minimizes reactions between atmospheric O₃ and substrate-bound Hg. Using conservative calculations developed in this paper, we conclude that because O₃ concentrations have roughly doubled in the last 100 years, this could have increased Hg^o emissions from terrestrial substrates by 65–72%.     

Size distribution of atmospheric aerosols in seoul

Urban aerosol particle size distributions over the size range 0.01–1.0 μm have been determined by using an electrical aerosol analyzer at the Hanyang University in Seoul from May through to October 1984. The results indicate that the total average number distribution in the size range of 0.1–1.0 μm can be represented as the Junge distribution with dN/d(logD_p) = 28.9 D_p^−3.1. The total average surface distribution is characterized by a monomodal curve with its maximum in the vicinity of 0.13 μm, while the maximum of the total average volume distribution appears to be shifted to a larger size than that of the surface distribution showing an almost flat line for D_p 0.5 μm, which implies that a secondary maximum may exist for D_p 1.0 μm.The time development of the number size distributions for the total size range shows the minimum number in the early morning before 4 a.m. followed by a rapid increase by the morning rush hour. This development in the 0.1–1.0 μm range is characterized by the maximum numbers in the morning and evening rush hours when dense traffic and a low mixing depth coexist with high relative humidity. The photochemical production of small particles in the 0.01–0.1 μm range is verified from monthly variation of number concentration and duration of sunshine.In general the monthly development of the surface and volume distributions shows the highest values to be in spring with the lowest in summer.     

Quantitative analysis of mercury,arsenic, and bromine in atmospheric aerosols

The mercury, arsenic and bromine content of atmospheric aerosols sampled from air masses of widely varying origins was determined by neutron activation analysis. Concentrations ranged from 0.7 to 10ng Hg m⁻³, 0.05 to 20ng As m⁻³ and 1 to 90 ng Br m⁻³. Polluted air concentrations were at least an order of magnitude higher than those in unpolluted air. The average concentration of mercury and arsenic are similar (~ 2ng m⁻³) but a factor of 10 lower than that of bromine. The vertical distribution of the trace elements was measured on two days with different weather situations. The results are discussed with respect to the meteorological elements. A comparison of the particulate mercury concentrations from this study with gaseous mercury concentrations from the literature indicates that they are of the same order of magnitude. Thus it is necessary to take into account mercury in both phases when doing mercury budget studies.     

Modelling the formation and atmospheric transport of secondary inorganic aerosols with special attention to regions with high ammonia emissions

Regional simulations of sulfate, nitrate and ammonium aerosols were performed by a nested application of the online-coupled three-dimensional Eulerian model system COSMO-MUSCAT. This was done in a domain covering the northern part of Germany and surrounding regions for the full month of May and a 6-week period in August/September 2006 with the primary focus on secondary inorganic aerosol levels caused by ammonia emissions from domesticated animals and agricultural operations.The results show that in situations with westerly winds ammonium nitrate dominates with concentrations of about 5–10 μg m^?3 whereas the ammonium sulfate concentrations are about 5 μg m^?3. In situations with winds mainly from the East characterized by warmer and dryer air the ammonium sulfate concentrations have their maximum at about 10 μg m^?3 whereas at the same time no ammonium nitrate is present.A reduction of agricultural NH₃ emissions by 50% in a regional scale reduces the ammonium nitrate concentrations to a maximum of 30%, while the ammonium sulfate concentrations are unchanged. The reduction of NH₃ emissions in a more limited area (here in the Federal state of Germany Niedersachsen) does have no noticeable effect neither on ammonium sulfate nor on ammonium nitrate.     

The effect of temperature on the gas–particle partitioning of reactive mercury in atmospheric aerosols

Measurements of gas–particle-partitioning coefficients for reactive mercury in dry urban and laboratory aerosol were found to strongly depend on ambient temperature. Samples of atmospheric and laboratory aerosols (defined as both the gas and particle phases) were collected using filter and absorbent methods and analyzed for reactive mercury using thermal desorption combined with cold vapor atomic fluorescence spectroscopy. Synthetic ambient aerosols were generated in the laboratory from ammonium sulfate and adipic acid mixed with mercuric chloride in a purpose-built aerosol reactor. The aerosol reactor was operated in a temperature-controlled laboratory. Linear relationships between the logarithm of inverse gas–particle partitioning and inverse temperature were observed and parameterized for use in the atmospheric modeling of reactive mercury. Reactive mercury was observed to partition from the particle to the gas phase as ambient temperature increased. Good agreement between measurements made using urban and laboratory aerosols was seen after gas–particle-partitioning coefficients were normalized for surface area instead of mass. Thermodynamic analyses of the urban and laboratory gas–particle-partitioning measurements revealed that the strength of interaction between reactive mercury and particle surfaces was suggestive of chemisorption. Gas–particle-partitioning coefficients made with the Tekran ambient mercury analyzer (AMA) also showed a dependence on temperature. However, the Tekran AMA partitioning coefficients did not agree well with partitioning coefficients measured using the filter-based methods. The disagreement is consistent with the 50 °C operational temperature of the Tekran AMA.     

The contribution of aircraft emissions to the atmospheric sulfur budget

An atmospheric general circulation model including the atmospheric sulfur cycle has been used to investigate the impact of aircraft sulfur emissions on the global sulfur budget of the atmosphere. The relative contribution from aircraft sulfur to the atmospheric sulfate burden is larger than the ratio between aircraft emissions and surface emissions due to the calculated long turn-over time of aircraft sulfate (about 12 days). However, in terms of the sulfate mass balance, aircraft emissions are small, contributing about 1% of the total sulfate mass north of 40°N where the aircraft emissions are largest. Despite this small contribution to sulfate mass, the aircraft emissions could potentially significantly enhance the background number concentration of aerosol particles. Based on the model calculations the increased stratospheric background aerosol mass observed during the last decades cannot be explained by increased aircraft sulfur emissions.     

Chemical characteristics of organic aerosols in Algiers city area: influence of a fat manufacture plant

Total concentrations and homologue distributions of organic fraction constituents have been determined in particulate matter emitted from different units of a fat manufacturer (i.e. oils refining and conditioning plants, and production and conditioning units of a soap industry) located in Algiers area, as well as in atmospheric aerosols. In particular n-alkanes, n-alkanoic and n-alkenoic acids, n-alkan-2-ones and polycyclic aromatic hydrocarbons (PAH) were investigated. Organic aerosol contents varied broadly among the plant units, depending upon nature of the manufactured products. The percent composition of all classes of compounds investigated in ambient atmosphere was similar to those observed indoor at industrial plant units. Organic acids, n-alkanoic as well as n-alkenoic, appeared by far the most abundant organic constituents of aerosols, both indoor and outdoor, ranging from 7.7 to 19.8 and from 12.7 to 17.1 μg m⁻³, respectively. The huge occurrence of acids and n-alkanes in ambient aerosols was consistent with their high levels present in oil and fat materials. Among minor components of aerosols, n-alkan-2-ones and PAH, seemed to be related to thermally induced ageing and direct combustion of raw organic material used for oil and soap production.     

The contribution of elemental carbon to the optical properties of rural atmospheric aerosols

Measurements of carbonaceous aerosol, aerosol light absorption and aerosol light scattering were made at two rural sites in southwestern Pennsylvania during August 1983. Aerosol light absorption ranged from 5.2 × 10⁻⁶ m⁻¹ to 6.4 × 10⁻⁵ m⁻¹ (average: 1.9 × 10⁻⁵ m⁻¹) and accounted for about 13% of the aerosol total light extinction. Elemental carbon, averaging 1.3 μg m⁻³ at the two sites (and comprising some 36 % of the aerosol carbon), accounted for effectively all (> 95 %) of the aerosol light absorption.     

Size distribution of aerosols in Melbourne city air

A high volume sampler was operated in conjunction with an Anderson cascade impactor to determine the size distribution of particulates in city air. Measurements were carried out at several heights above street level ranging from 11.6 to 66.4 m at a commercial location in the city of Melbourne. Analysis of results showed no variation in size composition with height. The Melbourne aerosol was found to contain a higher percentage of larger particles than aerosols in any comparable study elsewhere. Although the assumption of log-normality was useful in interpreting the results, closer observation indicated the distribution was bimodal.     

A study of the formation of aerosols in filtered atmospheric air at Trombay,Bombay

A set of atmospheric measurements was carried out at Trombay, Bombay to study the correlation between the concentration variation of freshly formed atmospheric aerosols and the variations in the ambient concentrations of some of the atmospheric trace gases. Daily values of the aerosol concentration both in filtered and unfiltered air together with the concentrations of SO₂, NO, O₃ and α-activity were measured during a period of 11 months. Statistical analysis of the data shows that variation of particle concentration in the filtered air is a function of not only SO₂ variation but also O₃ and α-activity variations. The various parameters that are likely to be involved in the formation and the variations of small particle concentration in the atmosphere have been identified.     

On the oxidation of SO2 to sulfate in atmospheric aerosols

This paper presents the results of a study to investigate the atmospheric oxidation of sulfur dioxide (SO₂). A detailed model of gas-phase chemistry, aerosol thermodynamics and aerosol chemistry is employed to simulate atmospheric sulfate formation. The calculations indicate that, in addition to the gasphase oxidation by hydroxyl (OH) radicals, SO₂ oxidation in aqueous aerosols may also contribute significantly to sulfate formation. Reactions of SO₂ with hydrogen peroxide (H₂O₂) and O₂ (catalyzed by Fe³⁺ and Mn²⁺) are identified as principal aqueous-phase oxidation mechanisms. The results of this study confirm the conclusions drawn from the analysis of ambient aerosol data qualitatively. However, some discrepancies also exist between the results of our modeling study and field data. Such discrepancies emphasize the need for the collection of ambient data for a more rigorous and quantitative evaluation of atmospheric aerosol models.     

The effects of increasing atmospheric ozone on biogenic monoterpene profiles and the formation of secondary aerosols

Monoterpenes are biogenic volatile organic compounds (BVOCs) which play an important role in plant adaptation to stresses, atmospheric chemistry, plant–plant and plant–insect interactions. In this study, we determined whether ozonolysis can influence the monoterpenes in the headspace of cabbage. The monoterpenes were mixed with an air-flow enriched with 100, 200 or 400 ppbv of ozone (O₃) in a Teflon chamber. The changes in the monoterpene and O₃ concentrations, and the formation of secondary organic aerosols (SOA) were determined during ozonolysis. Furthermore, the monoterpene reactions with O₃ and OH were modelled using reaction kinetics equations. The results showed that all of the monoterpenes were unequally affected: α-thujene, sabinene and d-limonene were affected to the greatest extend, whereas the 1,8-cineole concentration did not change. In addition, plant monoterpene emissions reduced the O₃ concentration by 12–24%. The SOA formation was dependent on O₃ concentration. At 100 ppbv of O₃, virtually no new particles were formed but clear SOA formation was observed at the higher ozone concentrations. The modelled results showed rather good agreements for α-pinene and 1,8-cineole, whereas the measured concentrations were clearly lower compared to modelled values for sabinene and limonene. In summary, O₃-quenching by monoterpenes occurs beyond the boundary layer of leaves and results in a decreased O₃ concentration, altered monoterpene profiles and SOA formation.     

Size distributions of trace metals in atmospheric aerosols in the United Kingdom

The size distributions of Ba, Cd, Co, Cu, Hg, Mn, Ni, Pb, Sn, Se, Sr, Zn and Fe in atmospheric aerosols were measured using impactors at three background sites in central England and southern Scotland. Coarse aerosols (>10.0 μm) were found to be undercollected by a micro-orifice uniform deposit impactor (MOUDI) when compared to an isokinetic technique, to a degree dependent on the size distribution of individual metals. The size distributions obtained in Scotland, which were typically trimodal, differed from those in central England, where modes were more variable.Characteristic size distributions allowed identification of three main behavioural types: (i) metals whose mass resided mainly within the accumulation mode (Cd, Sn, Pb, Se), (ii) those which were distributed between fine, intermediate and coarse modes (Ni, Zn, Cu, Co, Mn, Hg), and (iii) those which were mainly found within coarse particles (Fe, Sr, Ba). The measured distributions are believed to result from a combination of processes including local anthropogenic and natural sources, long-range transport and resuspension.