Evaluation of SO2 dry deposition over short vegetation in Portugal

SO₂ dry deposition was studied over short vegetation, in Portugal, by means of the concentration gradient method. The experimental study involved one first phase of long-term measurements carried out in a grassland and, subsequently, a second period of several 1997 intensive field campaigns performed in three places representing different climate and surface conditions. Temporal and spatial patterns of dry deposition parameters show that downward fluxes of SO₂ are by some extent affected by surface processes. Median R_c varied from 140 s cm⁻¹ to values around 200 s cm⁻¹, in a wide range of environmental conditions. Stomatal uptake is an important sink when vegetation is biologically active, but its contribution is effectively low when compared with non-stomatal mechanisms, especially when the surface is wet. Under dry conditions R_c increases by a factor of two, but SO₂ deposition rates then still are significant. The parameterisation of the surface resistance for SO₂ proved to be difficult, but V_d derived with the Erisman parameterisation (Erisman et al., Atmos. Environ. 28 (16) (1994) 2595) compared best with measured values, at low time resolution scale and especially under moisture conditions.     

Identification,abundance and seasonal variation of anthropogenic organic aerosols from a mega-city in China

PM_2.5 aerosols were collected in Nanjing, a typical mega-city in China, during summer and winter 2004 and were characterized for aromatic and cyclic compounds using a GC/MS technique to understand the air pollution problem. They include polycyclic aromatic hydrocarbons (PAHs), hopanes, phthalates and hydroxy-PAHs (OH-PAHs). PAHs, hopanes and OH-PAHs presented higher concentrations in winter (26–178, 3.0–18, and 0.013–0.421 ng m⁻³, respectively) than in summer (12–96, 1.6–11, and 0.029–0.171 ng m⁻³, respectively) due to an enhanced coal burning for house heating and atmospheric inversion layers developed in the cold season. In contrast, phthalates are more abundant in summer (109–368 ng m⁻³, average 230 ng m⁻³) than in winter (33–390 ng m⁻³, average 170 ng m⁻³) due to an enhanced evaporation from plastics during the hot season and the subsequent deposition on the pre-existing particles. Generally, all the identified compounds showed higher concentrations in nighttime than in daytime due to inversion layers and increased emissions from heavy-duty trucks at night. PAHs, hopanes and phthalates in Nanjing aerosols are 5–100 times more abundant than those in Los Angeles, USA, indicating a serious air pollution problem in the city. Concentrations of OH-PAHs are 1–3 orders of magnitude less than their parent PAHs and comparable to those reported from other international cities. Source identification using diagnostic ratios of the organic tracers suggests that PAHs in Nanjing urban area are mainly derived from coal burning, whereas hopanes are more attributable to traffic emissions.     

Effect of winds on SO2 and SPM concentrations in Delhi

A study of concentrations of sulfur dioxide (SO₂) and suspended particulate matter (SPM) has been performed in Delhi. The monthly and seasonal variations of concentrations and winds are analyzed. The monthly mean SO₂ concentrations were in the range of 16.15–34.44 μg m⁻³ and showed regular seasonal variations with the highest concentrations in winter and lowest in monsoon season. On the other hand, the monthly mean SPM concentrations reached the highest (465.68 μg m⁻³) in November and the lowest (150.07 μg m⁻³) in August. It was observed that high SO₂ concentrations were generally associated with the wind blowing from WNW–NW directions, and the high SPM concentrations were usually related to the wind blowing from W–NW directions.     

Nitric oxide emissions from soils amended with municipal waste biosolids

Land spreading nitrogen-rich municipal waste biosolids (NO₃⁻-N<256 mg N kg⁻¹ dry weight, NH₃-N∼23,080 mg N kg⁻¹ dry weight, Total Kjeldahl N∼41,700 mg N kg⁻¹ dry weight) to human food and non-food chain land is a practice followed throughout the US. This practice may lead to the recovery and utilization of the nitrogen by vegetation, but it may also lead to emissions of biogenic nitric oxide (NO), which may enhance ozone pollution in the lower levels of the troposphere. Recent global estimates of biogenic NO emissions from soils are cited in the literature, which are based on field measurements of NO emissions from various agricultural and non-agricultural fields. However, biogenic emissions of NO from soils amended with biosolids are lacking. Utilizing a state-of-the-art mobile laboratory and a dynamic flow-through chamber system, in-situ concentrations of nitric oxide (NO) were measured during the spring/summer of 1999 and winter/spring of 2000 from an agricultural soil which is routinely amended with municipal waste biosolids. The average NO flux for the late spring/summer time period (10 June 1999–5 August 1999) was 69.4±34.9 ng N m⁻² s⁻¹. Biosolids were applied during September 1999 and the field site was sampled again during winter/spring 2000 (28 February 2000–9 March 2000), during which the average flux was 3.6±1.7 ng N m⁻² s⁻¹. The same field site was sampled again in late spring (2–9 June 2000) and the average flux was 64.8±41.0 ng N m⁻² s⁻¹. An observationally based model, developed as part of this study, found that summer accounted for 60% of the yearly emission while fall, winter and spring accounted for 20%, 4% and 16% respectively. Field experiments were conducted which indicated that the application of biosolids increases the emissions of NO and that techniques to estimate biogenic NO emissions would, on a yearly average, underestimate the NO flux from this field by a factor of 26. Soil temperature and % water filled pore space (%WFPS) were observed to be significant variables for predicting NO emissions, however %WFPS was found to be most significant during high soil temperature conditions. In the range of pH values found at this site (5.8±0.3), pH was not observed to be a significant parameter in predicting NO emissions.     

Estimates of air-sea exchange of mercury in the Baltic Sea

The concentrations of total gaseous mercury (TGM) in air over the southern Baltic Sea and dissolved gaseous mercury (DGM) in the surface seawater were measured during summer and winter. The summer expedition was performed on 02–15 July 1997, and the winter expedition on 02–15 March 1998. Average TGM and DGM values obtained were 1.70 and 17.6 ng m⁻³ in the summer and 1.39 and 17.4 ng m⁻³ in the winter, respectively. Based on the TGM and DGM data, surface water saturation and air-water fluxes were calculated. The results indicate that the seawater was supersaturated with gaseous mercury during both seasons, with the highest values occurring in the summer. Flux estimates were made using the thin film gas-exchange model. The average Hg fluxes obtained for the summer and winter measurements were 38 and 20 ng m⁻² d⁻¹, respectively. The annual mercury flux from this area was estimated by a combination of the TGM and DGM data with monthly average water temperatures and wind velocities, resulting in an annual flux of 9.5 μg m⁻² yr⁻¹. This flux is of the same order of magnitude as the average wet deposition input of mercury in this area. This indicates that reemissions from the water surface need to be considered when making mass-balance estimates of mercury in the Baltic Sea as well as modelling calculations of long-range transboundary transport of mercury in northern Europe.     

Influence of the surface microlayer on atmospheric deposition of aerosols and polycyclic aromatic hydrocarbons

Atmospheric dry deposition is an important process for the introduction of aerosols and pollutants to aquatic environments. The objective of this paper is to assess, for the first time, the influence that the aquatic surface microlayer plays as a modifying factor of the magnitude of dry aerosol deposition fluxes. The occurrence of a low surface tension (ST) or a hydrophobic surface microlayer has been generated by spiking milli-Q water or pre-filtered seawater with a surfactant or octanol, respectively. The results show that fine mode (<2.7 μm) aerosol phase PAHs deposit with fluxes 2–3 fold higher when there is a low ST aquatic surface due to enhanced sequestration of colliding particles at the surface. Conversely, for PAHs bound to coarse mode aerosols (>2.7 μm), even though there is an enhanced deposition due to the surface microlayer for some sampling periods, the effect is not observed consistently. This is due to the importance of gravitational settling for large aerosols, rendering a lower influence of the aquatic surface on dry deposition fluxes. ST (mN m⁻¹) is identified as one of the key factor driving the magnitude of PAH dry deposition fluxes (ng m⁻² d⁻¹) by its influence on PAH concentrations in deposited aerosols and deposition velocities (v_d, cm s⁻¹). Indeed, v_d values are a function of ST as obtained by least square fitting and given by Ln(v_d)=−1.77 Ln(ST)+5.74 (r²=0.95) under low wind speed (average 4 m s⁻¹) conditions.     

Ammonia fluxes and derived canopy compensation points over non-fertilized agricultural grassland in The Netherlands using the new gradient ammonia—high accuracy—monitor (GRAHAM)

During a measurement period from June till November 2004, ammonia fluxes above non-fertilized managed grassland in The Netherlands were measured with a Gradient Ammonia—High Accuracy—Monitor (GRAHAM). Compared with earlier ammonia measurement systems, the GRAHAM has higher accuracy and a quality control system.Flux measurements are presented for two different periods, i.e. a warm, dry summer period (from 18 July till 15 August) and a wet, cool autumn period (23 September till 23 October). From these measurements canopy compensation points were derived. The canopy compensation point is defined as the effective surface concentration of ammonia. In the summer period (negative) deposition fluxes are observed in the evening, night and early morning due to leaf surface wetness, while in the afternoon emission fluxes are observed due to high canopy compensation points. The mean NH₃-flux in this period was 4 ng m⁻² s⁻¹, which corresponds to a net emission of 0.10 kg N ha⁻¹ over the 28 day sampling period. The NH₃-flux in the autumn period mainly shows (negative) deposition fluxes due to small canopy compensation points caused by low temperatures and a generally wet surface. The mean NH₃-flux in this period is −24 ng m⁻² s⁻¹, which corresponds to a net deposition of 0.65 kg N ha⁻¹ over the 31 day sampling period.Frequency distributions of the NH₃-concentration and flux show that despite higher average ambient NH₃-concentrations (13.3 μg m⁻³ in the summer period vs. 6.4 μg m⁻³ in the autumn period) there are more emission events in the summer period than in the autumn period (about 50% of the time in summer vs. 20% in autumn). This is caused by the high canopy compensation points in summer due to high temperatures and a dry surface. In autumn, deposition dominates due to a generally wet surface that induces low canopy compensation points.For our non-fertilized agricultural grassland site, the derived canopy compensation points (at temperatures between 7 and 29 °C) varied from 0.5 to 29.7 μg m⁻³ and were on an average 7.0 μg m⁻³, which is quite high for non-fertilized conditions and probably caused by high nitrogen inputs in the past or high dry deposition amounts from local sources. The average value for the ratio between NH₄⁺ and H⁺ concentration in the canopy, Γ_c, that was derived from our data was 2200.     

Dry deposition of sulphur over eastern South Africa

Dry deposition of sulphur is estimated in three climatic regions of Mpumalanga, South Africa, using the inferential method. Data from June 1996 to May 1997 are used at Elandsfontein and Palmer on the industrialised highveld, as well as data from two-week monitoring campaigns in the late-winter and in summer at Blyde on the eastern escarpment and at Skukuza in the lowveld. Total dry deposition rates for sulphur range across the Mpumalanga highveld from 13.1 kg ha^-1 a^-1 at Elandsfontein to 3.1 kg ha^-1 a^-1 at Palmer, are associated with the strong SO₂ gradient between the two stations and are attributed mostly to dry deposition of sulphur from SO₂. The deposition flux varies less from Palmer eastward over the escarpment and the lowveld and ranges from 3.9 kg S ha^-1 a^-1 at Blyde to 3.3 kg S ha^-1 a^-1 at Skukuza. A weak seasonal variation in sulphur dry deposition flux occurs on the central highveld with the maximum in summer and the minimum in winter. Conversely, the maximum sulphur dry deposition on the periphery of the highveld, the escarpment and in the lowveld occurs in winter with the minimum in summer. More than 80% of the dry deposition of sulphur in Mpumalanga occurs during daytime in all seasons.     

Direct measurements and parameterisation of aerosol flux,concentration and emission velocity above a city

Articles have recently been published on aerosol size distributions and number concentrations in cities, however there have been no studies on transport of these particles. Eddy covariance measurements of vertical transport of aerosol in the size range 11 nm<D_p<3 μm are presented here. The analysis shows that typical average aerosol number fluxes in this size range vary between 9000 and 90,000 cm⁻² s⁻¹. With concentrations between 3000 and 20,000 cm⁻³ this leads to estimates of particle emission velocity between 20 and 75 mm s⁻¹. The relationships between number flux and traffic activity, along with emission velocity and boundary layer stability are demonstrated and parameterised. These are used to derive an empirical parameterisation for aerosol concentration in terms of traffic activity and stability. The main processes determining urban aerosol fluxes and concentrations are discussed and quantified where possible. The difficulties in parameterising urban activity are discussed.     

Characteristics of regional nucleation events in urban East St. Louis

Continuous measurements of aerosol size distributions (3 nm–2 μm) were carried out over a 26 month period (1 April 2001–31 May 2003; 650 days with valid data) in urban East St. Louis, IL, as a part of the US Environmental Protection Agency's Supersite program. This paper analyzes data for the 155 days on which “regional nucleation events” were observed during this study. Such events were observed during every month of the study except January 2003. We observed some differences, however, between events in the summer (defined here as April–September) and winter (December–February). Regional nucleation events were observed more frequently in summer months (36±13% of days) than in winter (8±7%), and nucleated particles grew faster in the summer (6.7±4.8 nm h⁻¹) than in winter (1.8±1.9 nm h⁻¹). The daily maximum in the number concentration of nanoparticles formed by nucleation (4.8±3.5×10⁴ cm⁻³) was highly variable and showed no clear seasonal dependence. Particle formation increased particle concentrations by an average factor of 3.1±2.8. Maximum daily rates of 3 nm particle production (17±20 cm⁻³ s⁻¹) were also highly variable and without a clear seasonal dependence. During these events, particle formation rates were typically near their maxima at 08:00–09:00 a.m., but particle production sometimes persisted at diminishing rates until late in the afternoon (15:00–16:00 p.m.).     

Particulate matter in the indoor air of classrooms—exploratory results from Munich and surrounding area

Numerous epidemiological studies have demonstrated the association between particle mass (PM) concentration in outside air and the occurrence of health related problems and/or diseases. However, much less is known about indoor PM concentrations and associated health risks. In particular, data are needed on air quality in schools, since children are assumed to be more vulnerable to health hazards and spend a large part of their time in classrooms.On this background, we evaluated indoor air quality in 64 schools in the city of Munich and a neighbouring district outside the city boundary. In winter 2004–2005 in 92 classrooms, and in summer 2005 in 75 classrooms, data on indoor air climate parameters (temperature, relative humidity), carbon dioxide (CO₂) and various dust particle fractions (PM₁₀, PM_2.5) were collected; for the latter both gravimetrical and continuous measurements by laser aerosol spectrometer (LAS) were implemented. In the summer period, the particle number concentration (PNC), was determined using a scanning mobility particle sizer (SMPS). Additionally, data on room and building characteristics were collected by use of a standardized form. Only data collected during teaching hours were considered in analysis. For continuously measured parameters the daily median was used to describe the exposure level in a classroom.The median indoor CO₂ concentration in a classroom was 1603 ppm in winter and 405 ppm in summer. With LAS in winter, median PM concentrations of 19.8 μg m⁻³ (PM_2.5) and 91.5 μg m⁻³ (PM₁₀) were observed, in summer PM concentrations were significantly reduced (median PM_2.5=12.7 μg m⁻³, median PM₁₀=64.9 μg m⁻³). PM_2.5 concentrations determined by the gravimetric method were in general higher (median in winter: 36.7 μg m⁻³, median in summer: 20.2 μg m⁻³) but correlated strongly with the LAS-measured results. In explorative analysis, we identified a significant increase of LAS-measured PM_2.5 by 1.7 μg m⁻³ per increase in humidity by 10%, by 0.5 μg m⁻³ per increase in CO₂ indoor concentration by 100 ppm, and a decrease by 2.8 μg m⁻³ in 5–7th grade classes and by 7.3 μg m⁻³ in class 8–11 compared to 1–4th class. During the winter period, the associations were stronger regarding class level, reverse regarding humidity (a decrease by 6.4 μg m⁻³ per increase in 10% humidity) and absent regarding CO₂ indoor concentration. The median PNC measured in 36 classrooms ranged between 2622 and 12,145 particles cm⁻³ (median: 5660 particles cm⁻³).The results clearly show that exposure to particulate matter in school is high. The increased PM concentrations in winter and their correlation with high CO₂ concentrations indicate that inadequate ventilation plays a major role in the establishment of poor indoor air quality. Additionally, the increased PM concentration in low level classes and in rooms with high number of pupils suggest that the physical activity of pupils, which is assumed to be more pronounced in younger children, contributes to a constant process of resuspension of sedimented particles. Further investigations are necessary to increase knowledge on predictors of PM concentration, to assess the toxic potential of indoor particles and to develop and test strategies how to ensure improved indoor air quality in schools.     

Dry deposition of ozone on building materials. Chamber measurements and modelling of the time-dependent deposition

Tests of the dry deposition of ozone to the surfaces of a concrete floor tile and an activated carbon cloth (ACC) sample were performed in a deposition chamber. The time-dependent deposition of ozone to the material surfaces was modelled with an adsorption, desorption, reaction model. This made it possible to find deposition velocities at equilibrium, at t=∞, from shorter time runs of 48 h. The total equilibrium deposition velocity on the concrete floor tile was found to decrease from 0.08(10) to 0.057(10) cm s⁻¹ in three consecutive runs on the same sample, and was found to be 0.137(8) cm s⁻¹ on an ACC. All at a linear airflow velocity of 0.092 cm s⁻¹, RH=50% and T=22°C. Varying the airflow in the deposition chamber, the surface deposition velocity was found to equal to the total deposition velocity for the concrete floor tile. A surface deposition velocity of 0.186(8) cm s⁻¹ was found for the ACC sample. The total real area and the reaction rate constant for the decomposition of ozone was found to be larger, and the adsorption rate constant, the desorption rate constant and the mass of ozone on the surface smaller, on the ACC sample than on the concrete floor tile.     

NOX fluxes from three kinds of agricultural lands in the Yangtze Delta,China

Static chamber method was adopted to measure the surface exchanges of NO and NO₂ between three kinds of agricultural lands and the atmosphere during spring–summer period in the Yangtze Delta, China. The average NO fluxes were 20.9, 27.4 and 21.4 ng N m⁻² s⁻¹, respectively, for cabbage (CA, cultivation of celery occurred along with cabbage), potato (PO) and soybean (SY) fields. The average NO₂ fluxes were −1.12, 0.93 and −0.68 ng N m⁻² s⁻¹, respectively, for the cabbage, potato and soybean fields. Apparently, negative linear correlation was found between the NO₂ fluxes from the CK plot (tilled conventionally but did not cultivate any seeds) and its ambient concentrations, and the compensation point was calculated to be 0.92 ppbv. The total NO emission from the vegetable lands and SY land in this region during spring–summer period was roughly estimated to be 15.9 Gg N, which accounted for about 11.2% of the estimated value of total NO emissions in the July of 1999 from Chinese agricultural fields.     

Transport of SO2 and aerosol over the Yellow sea

Aircraft measurements of air pollutants were made to investigate the characteristic features of long-range transport of sulfur compounds over the Yellow Sea for the periods of 26–27 April and 7–10 November in 1998, and 9–11 April and 19 June in 1999, together with aerosol measurements at the Taean background station in Korea. The overall mean concentrations of SO₂, O₃ and aerosol number in the boundary layer for the observation period ranged 0.1–7.4 ppb 32.1–64.1 ppb and 1.0–143.6 cm⁻³, respectively. It was found that the air mass over the Yellow Sea had a character of both the polluted continental air and clean background air, and the sulfur transport was mainly confined in the atmospheric boundary layer. The median of SO₂ concentration within the boundary layer was about 0.1–2.2 ppb. However, on 8 November, 1998, the mean concentrations of SO₂ and aerosol number increased up to 7.4 ppb and 109.5 cm⁻³, respectively, in the boundary layer, whereas O₃ concentration decreased remarkably. This enhanced SO₂ concentration occurred in low level westerly air stream from China to Korea. Aerosol analyses at the downstream site of Taean in Korea showed 2–3 times higher sulfate concentration than that of other sampling days, indicating a significant amount of SO₂ conversion to non sea-salt sulfate during the long-range transport.     

Dry deposition fluxes and deposition velocities of PAHs at an urban site in Turkey

Even though dry deposition and air–water exchange of semivolatile organic compounds (SOCs) are important for surfaces in and around the urban areas, there is still no generally accepted direct measurement technique for dry deposition. In this study, a modified water surface sampler (WSS) configuration, including a filter holder and an XAD-2 resin column, was employed to investigate the polycyclic aromatic hydrocarbon (PAH) dry deposition in an urban area. The measured total (particle+dissolved) PAH fluxes to the WSS averaged to be 34 960±16 540 ng m⁻² d⁻¹. Average particulate PAH flux, determined by analyzing the filter in the WSS, was about 8% of the total PAH flux. Temporal flux variations indicated that colder months (October–April) had the highest PAH fluxes. This increase could be attributed to the residential heating as well as meteorological effects including lower mixing height. A high volume air sampler was concurrently employed to collect ambient air concentrations. The average total (gas+particle) atmospheric PAH concentration (456±524 ng m⁻³) was within the range of previously measured values at different urban locations. PAH concentrations in urban areas are more than two orders of magnitude higher than those measured in pristine areas and this result may indicate that urban areas have major source sectors and greater deposition rates are expected near to these areas. The average contribution of particle phase was about 10% in total concentration. Simultaneous particulate phase dry deposition and ambient air samples were collected in this study. Then, particulate phase apparent dry deposition velocities were calculated using the fluxes and concentrations for each PAH compound and they ranged from 0.1 to 1.2 cm s⁻¹. These values are in good agreement with previously reported values.     

Direct wet and dry deposition of ammonia,nitric acid,ammonium and nitrate to the Tampa Bay Estuary,FL, USA

The average total (wet plus dry) nitrogen deposition to the Tampa Bay Estuary was 7.3 (±1.3) kg-N ha⁻¹ yr⁻¹ or 760 (±140) metric tons-N yr⁻¹ for August 1996–July 1999, estimated as a direct deposition rate to the 104,000-ha water surface. This nitrogen flux estimate accounted for ammonia exchange at the air–sea interface. The uncertainty estimate was based on measurement error. Wet deposition was 56% of the total nitrogen deposition over this period, with an average 0.78 ratio of dry-to-wet deposition. Wet nitrogen deposition rates varied considerably, from near zero to 1.3 kg-N ha⁻¹ month⁻¹. About 40% of the total nitrogen flux occurred during the summer months of June, July and August when rainfall was the highest, except for 1997–1998 when the El Niño phenomenon brought unseasonal rainfall. Ammonia/ammonium contributed to 58%, and nitric acid/nitrate 42%, of the total nitrogen deposition over the 3-yr period. In one summer as waters of Tampa Bay warmed above 28°C and ammonium concentrations reached 0.03 mg l⁻¹, the estimated net flux of ammonia was from the Bay waters to the atmosphere.     

Measurements of JNO3− in snow by nitrate-based actinometry

Chemical actinometry was used to measure nitrate photolysis rate coefficients, J_NO3⁻, on and in snowpack at Summit, Greenland. Sealed glass tubes containing nitrate and a hydroxyl radical trapping system were buried in snow and exposed for between 2 and 24 h. Average J_NO3⁻ values for 2-h midday exposures in early June on surface snow were 10–14×10⁻⁷ s⁻¹. Averages over 24 h were 3.5–4.5×10⁻⁷ s⁻¹. These values reflect the integrated photon flux and also any variation of the nitrate photolysis rate with temperature. Attenuation of J_NO3⁻ within the firn was 0.03–0.04 cm⁻¹ for 24-h exposures and 0.08 cm⁻¹ for a 2-h exposure. Different attenuation coefficients may relate to differential light penetration due to changes in sun angle over the course of 24 h.     

Measurement of the vapor phase deposition of polychlorinated bipheyls (PCBs) using a water surface sampler

A water surface sampler (WSS) was employed in combination with greased knife-edge surface deposition plates (KSSs) to measure the vapor phase deposition rates of PCBs to the sampler at an urban site, Chicago, IL. This sampler employed a water circulation system that continuously removed deposited PCBs. Total (gas+particle) and particulate PCB fluxes were collected with the WSS and KSSs, respectively. Gas phase PCB fluxes were then calculated by subtracting the KSS fluxes (particulate) from the WSS fluxes (gas+particle). The calculated gas phase PCB fluxes averaged 830±910 ng m⁻²d⁻¹. This flux value is, in general, higher than the fluxes determined using simultaneously measured air–water concentrations in natural waters and is in the absorption direction. This difference is primarily because the PCBs were continuously removed from the WSS water keeping the water PCB concentration near zero.Concurrently, ambient air samples were collected using a modified high volume air sampler. The gas phase PCB concentrations ranged between 1.10 and 4.46 ng m⁻³ (average±SD, 2.29±1.28 ng m⁻³). The gas phase fluxes were divided by the simultaneously measured gas phase ambient concentrations to determine the overall gas phase mass transfer coefficients (MTCs) for PCBs. The average gas phase overall MTCs (K_g) for each homolog group ranged between 0.22 and 1.32 cm s⁻¹ (0.54±0.47 cm s⁻¹). The average MTC was in good agreement with those determined using similar techniques.     

Dry deposition of ammonia,nitric acid,ammonium, and nitrate to alpine tundra at Niwot Ridge,Colorado

Micrometeorological measurements and ambient air samples, analyzed for concentrations of NH₃, HNO₃, NH₄⁺, and NO₃⁻, were collected at an alpine tundra site on Niwot Ridge, Colorado. The measured concentrations were extremely low and ranged between 5 and 70 ng N m⁻³. Dry deposition fluxes of these atmospheric species were calculated using the micrometeorological gradient method. The calculated mean flux for NH₃ indicates a net deposition to the surface and indicates that NH₃ contributed significantly to the total N deposition to the tundra during the August–September measurement period. Our pre-measurement estimate of the compensation point for NH₃ in air above the tundra was 100–200 ng N m⁻³; thus, a net emission of NH₃ was expected given the low ambient concentrations of NH₃ observed. Based on our results, however, the NH₃ compensation point at this alpine tundra site appears to have been at or below about 20 ng N m⁻³. Large deposition velocities (>2 cm s⁻¹) were determined for nitrate and ammonium and may result from reactions with surface-derived aerosols.     

Atmospheric distribution of polycyclic aromatic hydrocarbons and deposition to Galveston Bay,Texas, USA

Estimates of the atmospheric deposition to Galveston Bay of polycyclic aromatic hydrocarbons (PAHs) are made using precipitation and meteorological data that were collected continuously from 2 February 1995 to 6 August 1996 at Seabrook, TX, USA. Particulate and vapor phase PAHs in ambient air and particulate and dissolved phases in rain samples were collected and analyzed. More than 95% of atmospheric PAHs were in the vapor phase and about 73% of PAHs in the rain were in the dissolved phase. Phenanthrene and napthalene were the dominant compounds in air vapor and rain dissolved phases, respectively, while 5 and 6 ring PAH were predominant in the particulate phase of both air and rain samples. Total PAH concentrations ranged from 4 to 161 ng m⁻³ in air samples and from 50 to 312 ng l⁻¹ in rain samples. Temporal variability in total PAH air concentrations were observed, with lower concentrations in the spring and fall (4–34 ng m ⁻³) compared to the summer and winter (37–161 ng m⁻³). PAHs in the air near Galveston Bay are derived from both combustion and petroleum vaporization. Gas exchange from the atmosphere to the surface water is estimated to be the major deposition process for PAHs (1211 μg m^{− 2} yr^{− 1}), relative to wet deposition (130 μg m⁻² yr^{− 1}) and dry deposition (99 μg m⁻² yr^{− 1}). Annual deposition of PAHs directly to Galveston Bay from the atmosphere is estimated as 2  t yr⁻¹.