The relationship between dimethyl sulfide and particulate sulfate in the mid-atlantic ocean atmosphere

Dimethyl sulfide (DMS) and atmospheric aerosols were sampled simultaneously over the Atlantic Ocean in the vicinity of Bermuda using the NOAA King Air research aircraft. Total and fine (50% cutoff at 2 μm diameter) aerosol fractions were sampled using two independent systems. The average nonsea-salt (nss)SO₄²⁻ concentrations were 1.9 and 1.0 μg m⁻³ (as SO₄²⁻) for the total and the fine fractions in the boundary layer (BL) and 0.53 and 0.27 μg m⁻³ in the free troposphere (FT). Non-sea-salt SO₄²⁻ in the two aerosol fractions were highly correlated (r = 0.90), however a smaller percentage (55%) was found in the fine aerosol near Bermuda relative to that (90%) near the North American continent. The BL SO₄²⁻ concentrations measured in this study were higher than those measured by others at remote marine locations despite the fact that the 7-day air mass back trajectories indicated little or no continental contact at altitudes of 700 mb and below; the trajectories were over subtropical oceanic areas that are expected to be rich in DMS. DMS concentrations were higher near the ocean surface and decreased with increasing altitude within the BL; the average DMS concentration was 0.13 μg m⁻³. Trace levels of DMS were also measured in the FT (0.01 μg m⁻³). Computer simultation of the oxidation and removal of DMS in the marine atmosphere suggests that <50% of the SO₄²⁻ observed could be related to the natural S cycle.     

Background levels of air and precipitation quality for Europe

This paper is intended to be used by specialists engaged in air and precipitation quality management on regional and continental scales. Major goals are to establish definition, methodology and specific values of background air and precipitation quality for sulfur (S) and nitrogen (N) species to be used in practical applications of air resources management. Major findings are the following:

• 1.(a) 69% of SO₂ and 63 % of NO₂ concentration over Europe originate from continental scale anthropogenic sources,
• 2.(b) 15% of precipitation sulfate and 11% of precipitation nitrate over Europe are contributed by hemispheric background,
• 3.(c) hemispheric background pollution values for Europe were found as 1.25 μg (SO₂-S)m⁻³, 0.80 μg (SO₄²⁻-S)m⁻³, 0.157 mg (SO₄²⁻-S)l⁻¹ and 0.04 mg (NO₃⁻-N)ℓ⁻¹.

     

Continuous measurement of particulate and sulfur dioxide concentrations in Junk Bay,Hong Kong

As part of an environmental impact assessment for building a new town in Junk Bay, continuous measurement of SO₂ and particulate concentrations was carried out from October 1981 to June 1982 at three sites in Junk Bay to study the air quality in the area. Flame photometric SO₂ analyzers were used to measure ambient SO₂ level, whereas tape monitors were used to measure the ambient suspended particulate level (in terms of soiling index, or coefficient of haze per 1000 feet). It was found that the mean SO₂ concentrations at the three monitoring sites ranged from 5μg m⁻³ to 35μg m⁻³. Maximum daily values up to about 250 μg m⁻³ and hourly values up to 800 μg m ⁻³ had been recorded on occasion. Comparison of the hourly meteorological data and the hourly SO₂ concentrations in four high-SO₂-level days suggested that the ‘sulfur dioxide episodes’ were all associated with very light wind speeds and local sources. The mean coefficient of haze level in Junk Bay was less than one, which corresponded to very slight particulate pollution.     

Gaseous and particulate air pollution in the san gabriel mountains of southern california

In order to assess concentrations and daily patterns of air pollutants at a mountainous site in the South Coast Air Basin, a study was undertaken in the San Dimas Experimental Forest of the San Gabriel Mountains between April 1985 and October 1985. Continuous monitoring of O₃, NO, NO₂, SO₂, total S compounds and light scattering coefficient was conducted. Particulate aerosols were collected twice a week and concentrations of nitrate, ammonium and sulfate in fine (< 2.5 μm diameter) and coarse (> 2.5 μm diameter) modes were determined.For the June–August period, when the levels of photochemical smog were the highest, monthly 24-h average concentrations of the pollutants were: O₃, about 200 μg m⁻³; NO₂, 40–75 μg m⁻³; NO, 1–5 μg m ⁻³; and SO₂, 0.5–5 μgm⁻³. The concentrations of O₃ were about two times higher than in the neighboring stations of the South Coast Air Basin. O₃, SO₂ and total S concentrations peaked in the early afternoon, generally between 1500 and 1600 PST. Peak concentrations of NO occurred in the morning, generally between 1000 and 1100 PST. NO₂ concentrations typically peaked in the late afternoon between 1500 and 1800 PST, but occasionally (in 9 % of days) maximum NO₂ occurred in the morning, concurrently with the NO peaks. Daytime concentrations of the nitrate in fine aerosol fraction were generally between 100 and 600 nEq m ⁻³, those of ammonium between 50 and 300 nEq m ⁻³, and concentrations of sulfate between 60 and 250 nEq m⁻³. A 3-day denuder study showed that HNO₃can make up to 73 % of the total amount of total nitrate in the air. NO₂ was the most abundant N compound at Tan bark Flat (69–86% of the total amount of the monitored N compounds). Nitrate amounted to 9–15 %, HNO₃ to 4–11 %, ammonium to 3–9%, and NO to 1–2% of the total amount of the measured nitrogen compounds.     

Size distribution of large aerosol particles during AGASP-II: Absence of st. augustine eruptive particles in the Alaskan Arctic

Number distribution data for 0.1–45 μm diameter aerosol were obtained using optical counting and sizing probes flown over the Alaskan Arctic during the second Arctic Gas and Aerosol Sampling Program (AGASP-II), flights 201–203. Due to noise present in the lowest size channels of the optical probes, estimates of the H₂SO₄ component of Arctic haze were not attempted. Large particle (> 0.5 μm diameter) results are presented here. Large particle number and volume concentration were determined along with estimated mass, which was generally </ 0.1μg m⁻³. Lognormal fitting to > 0.3 μg m⁻³ mass loading sizedistributed aerosol data produced a means for comparing volume geometric median diameters (VGMD) for these higher-mass time intervals. These VGMDs showed that solid crustal particles previously observed during AGASP-II had VGMDs in the 1.2–1.6 μm range and that the shape of these fitted lognormal distributions was essentially constant. This result suggests very-long-range transport from a distant crustal source and, in conjunction with aerosol physical and chemical characterization data, argues against the presence of the Mt. Augustine eruptive particles during AGASP-II Alaskan Arctic sampling.     

An experimental study of the dry deposition of gaseous nitric acid to snow

A chamber placed in a constant temperature freezing room was used to study the surface resistance during deposition of HNO₃ to a snow surface. The resistance decreased with increasing temperature from larger than 5 s mm⁻¹ at − 18°C to about l s mm⁻¹ at −3°C. Measurements of gaseous and particulate nitrate concentrations during winter at a rural site in south central Sweden gave concentrations in the range of 0.4–5 μg HNO₃ m⁻¹ and 0.3–3 μg NO⁻₃ m⁻³ with a mean value of 1.3 μg HNO₃ m⁻³ and 0.7 μg NO⁻₃ m⁻³, respectively. The results indicate that for periods with temperatures below − 2°C estimated dry deposition of HNO₃ to snow is at most 4 % of measured wet deposition of nitrate in the area.     

Simultaneous sampling of NH3, HNO3, HC1, SO2 and H2O2 in ambient air by a wet annular denuder system

A new sampling device is described for the simultaneous collection of NH₃, HNO₃, HCl, SO₂ and H₂O₂ in ambient air. The apparatus is based on air sampling by two parallel annular denuder tubes. The gases are collected by absorption in solutions present in the annulus of the denuder tubes. After a sampling time of 30 min at flow rate of 32 ℓ min⁻¹ the solutions are extracted from the denuders and analyzed off-line. The detection limits of NH₃, HNO₃, HCL and SO₂ are in the order of 0.1–0.5 μm⁻³. For H₂O₂ the detection limit is 0.01 μm⁻³. The reproducibility is 5–10% at the level of ambient air concentrations. Comparison of this novel technique with existing methods gives satisfactory results. The compact set-up offers the possibility of field experiments without the need of extensive equipment.     

A theoretical estimate of O3 and H2O2 dry deposition over the northeast United States

Estimates of short-term, regional-scale spatial distributions of ozone (O₃) and hydrogen peroxide (H₂O₂) dry deposition over the northeast U.S. are presented. Dry deposition fluxes to surfaces are computed using a regional tropospheric chemistry model with deposition velocities which vary with local meteorology, land type, insolation, seasonal factors and surface wetness. A compilation of O₃ surface resistances is presented based on a survey of O₃ dry deposition measurements. The surface resistance for H₂O₂ is assumed to be small under most conditions, causing H₂O₂ to dry deposit at a rate which is frequently limited by surface-layer turbulence. Regional patterns of dry deposition velocities for these oxidants over the northeast U.S. are computed using landuse data and meteorological information predicted using a mesoscale meteorology model. Domain-averaged O₃ deposition velocities during a spring period reach a mid-day peak of 0.7–0.8 cm s⁻¹ and drop to 0.1–0.2 cm s⁻¹ at night. Domain-averaged H₂O₂ deposition velocities at a height of approximately 80 m are predicted to reach a mid-day peak of 1.6–2.0cm s⁻¹, and fall to 0.6–0.9 cm s⁻¹ at night. Time-averaged surface-layer H₂O₂ concentrations show a latitude dependence, with higher concentrations in the south. H₂O₂ concentrations are significantly reduced due to efficient wet removal and chemical destruction during the passage of a cyclonic frontal system. In contrast, O₃ concentrations are predicted to rise during the passage of a frontal system due to efficient vertical exchange of midtropospheric air into the boundary layer during convective conditions, followed by synoptic-scale subsidence occurring in the high pressure airmass following a cyclone. Maximum O₃ deposition during this 3-day springtime period occurs in polluted agricultural areas. In contrast, H₂O₂ dry deposition exhibits a latitude dependence with maximum 3-day accumulations occurring in the south. Domain-averaged mid-day deposition rates for O₃ and H₂O₂ were 45–50 μmol m⁻² h⁻¹ and 4–5 μmol m⁻² h⁻¹. At night, deposition rates were approximately 5–10 μmol m⁻² h⁻¹ and 1.5–2.5 μmol m⁻² h⁻¹ for O₃ and H₂O₂. These model results show that regional patterns of oxidant dry deposition are strongly influenced by oxidant concentrations, atmospheric stability, surface roughness and numerous other surface and meteorological factors. Each of these factors must be well-characterized before regional patterns of biological damage associated with oxidant dry deposition can be quantified.     

Controlled Exposures of Volunteers to Respirable Carbon and Sulf uric Acid Aerosols

Respirable carbon or fly ash particles are suspected to increase the respiratory toxicity of coexisting acidic air pollutants, by concentrating acid on their surfaces and so delivering it efficiently to the lower respiratory tract. To investigate this issue, we exposed 15 healthy and 15 asthmatic volunteers in a controlled- environment chamber (21°C, 50 percent relative humidity) to four test atmospheres: (i) clean air; (ii) 0.5-μm H2SO4 aerosol at =100 μg/m³, generated from water solution; (iii) 0,5-μm carbon aerosol at =250 μg/m³, generated from highly pure carbon black with specific surface area comparable to ambient pollution particles; and (iv) carbon as in (iii) plus =100 μg/m³ of ultrafine H2SO4 aerosol generated from fuming sulfuric acid. Electron microscopy showed that nearly all acid in (iv) became attached to carbon particle surfaces, and that most particles remained in the sub-μm size range. Exposures were performed double-blind, 1 week apart. They lasted 1 hr each, with alternate 10-min periods of heavy exercise (ventilation =50 L/min) and rest. Subjects gargled citrus juice before exposure to suppress airway ammonia. Lung function and symptoms were measured pre-exposure, after initial exercise, and at endexposure. Bronchial reactivity to methacholine was measured after exposure. Statistical analyses tested for effects of H₂SO₄ or carbon, separate or interactive, on health measures. Group data showed no more than small equivocal effects of any exposure on any health measure. One individual's responses were consistent with a clinically significant excess airway constriction from H₂SO₄ plus carbon, and 2-3 others showed slight excess responses to the combined pollutants, but all these observations might have reflected chance variations. We conclude that coexisting carbon aerosol did not increase respiratory irritancy of H₂SO₄, in most healthy and asthmatic subjects exposed for 1 hr under simulated "worst-case" ambient conditions.     

Dilution and aerosol dynamics within a diesel car exhaust plume—CFD simulations of on-road measurement conditions

Vehicle particle emissions are studied extensively because of their health effects, contribution to ambient PM levels and possible impact on climate. The aim of this work was to obtain a better understanding of secondary particle formation and growth in a diluting vehicle exhaust plume using 3-d information of simulations together with measurements. Detailed coupled computational fluid dynamics (CFD) and aerosol dynamics simulations have been conducted for H₂SO₄–H₂O and soot particles based on measurements within a vehicle exhaust plume under real conditions on public roads.Turbulent diffusion of soot and nucleation particles is responsible for the measured decrease of number concentrations within the diesel car exhaust plume and decreases coagulation rates. Particle size distribution measurements at 0.45 and 0.9 m distance to the tailpipe indicate a consistent soot mode (particle diameter D_p∼50 nm) at variable operating conditions. Soot mode number concentrations reached up to 10¹³ m⁻³ depending on operating conditions and mixing.For nucleation particles the simulations showed a strong sensitivity to the spatial dilution pattern, related cooling and exhaust H₂SO_4(g). The highest simulated nucleation rates were about 0.05–0.1 m from the axis of the plume. The simulated particle number concentration pattern is in approximate accordance with measured concentrations, along the jet centreline and 0.45 and 0.9 m from the tailpipe. Although the test car was run with ultralow sulphur fuel, high nucleation particle (D_p⩽15 nm) concentrations (>10¹³ m⁻³) were measured under driving conditions of strong acceleration or the combination of high vehicle speed (>140 km h⁻¹) and high engine rotational speed (>3800 revolutions per minute (rpm)).Strong mixing and cooling caused rapid nucleation immediately behind the tailpipe, so that the highest particle number concentrations were recorded at a distance, x=0.45 m behind the tailpipe. The simulated growth of H₂SO₄–H₂O nucleation particles was unrealistically low compared with measurements. The possible role of low and semi-volatile organic components on the growth processes is discussed. Simulations for simplified H₂SO₄–H₂O–octane–gasoil aerosol resulted in sufficient growth of nucleation particles.     

Measurement and Analysis of the Relationship between Ammonia,Acid Gases,and Fine Particles in Eastern North Carolina

Abstract

An annular denuder system, which consisted of a cyclone separator; two diffusion denuders coated with sodium carbonate and citric acid, respectively; and a filter pack consisting of Teflon and nylon filters in series, was used to measure acid gases, ammonia (NH₃), and fine particles in the atmosphere from April 1998 to March 1999 in eastern North Carolina (i.e., an NH₃?rich environment). The sodium carbonate denuders yielded average acid gas concentrations of 0.23 μg/m³ hydrochloric acid (standard deviation [SD] ± 0.2 μg/m³); 1.14 μg/m³ nitric acid (SD ± 0.81 μg/m³), and 1.61 μg/m³ sulfuric acid (SD ± 1.58 μg/m³). The citric acid denuders yielded an average concentration of 17.89 μg/m³ NH₃ (SD ± 15.03 μg/m³). The filters yielded average fine aerosol concentrations of 1.64 μg/m³ ammonium (NH₄ ⁺;SD ± 1.26 μg/m³); 0.26 μg/m³ chloride (SD ± 0.69 μg/m³), 1.92 μg/m³ nitrate (SD ± 1.09 μg/m³), and 3.18 μg/m³ sulfate (SO₄ ^2?; SD ± 3.12 μg/m³). From seasonal variation, the measured particulates (NH₄ ⁺,SO₄ ^2?, and nitrate) showed larger peak concentrations during summer, suggesting that the gas-to-particle conversion was efficient during summer. The aerosol fraction in this study area indicated the domination of ammonium sulfate particles because of the local abundance of NH₃, and the long-range transport of SO₄ ^2? based on back trajectory analysis. Relative humidity effects on gas-to-particle conversion processes were analyzed by particulate NH₄ ⁺ concentration originally formed from the neutralization processes with the secondary pollutants in the atmosphere.     

Arctic air pollution: An overview of current knowledge

From December to April, the Arctic air mass is polluted by man-made mid-latitudinal emissions from fossil fuel combustion, smelting and industrial processes. In the rest of the year, pollution levels are much lower. This is the outcome of less efficient pollutant removal processes and better south (S) to north (N) transport during winter. In winter, the Arctic air mass covers much of Eurasia and N. America. Meteorological flow fields and the distribution of anthropogenic SO₂ emissions in the northern hemisphere favor northern Eurasia as the main source of visibility reducing haze. Observations of SO₄²⁻ concentrations in the atmosphere throughout the Arctic yield, depending on location and year, a January–April mean of 1.5–3.9 μg m⁻³ in the Norwegian Arctic to 1.2–2.2 μg m⁻³ in the N. American Arctic. An estimate of the mean vertical profile of fine particle aerosol mass during March and April shows that, on average, pollution is concentrated in the lower 5 km of the atmosphere. Not only are anthropogenic particles present in the Arctic atmosphere but also gases such as SO₂, perfluorocarbons and pesticides. The acidic nature and seasonal variation of Arctic pollution is reflected in precipitation, the snowpack and glacier snow in the Arctic. A pH of 4.9–5.2 in winter and ~ 5.6 in summer is expected in the absence of calcareous wind blown soil. Glacial records indicate that Arctic air pollution has undergone a marked increase since the mid 1950s paralleling a marked increase in SO₂ and NO_x emissions in Europe. Effects of Arctic pollution include a reduction in visibility and perturbation of the solar radiation budget in April–June. Potential effects are the acidification and toxification of sensitive ecosystems.     

An evaluation of artifact SO42− formation on nylon filters under field conditions

The extent of SO₂ conversion on Membrana (Ghia) Nylasorb nylon filters under field conditions has been evaluated and found to be quite variable. The S-SO₄²⁻ loading on the nylon filters is higher at higher SO₂ concentrations, and on a long term basis approaches a saturatio limit of 2.5 μg S-SO₄²⁻ on a 47mm disc, at a dosage of 230 μg SO₂ approximately. The % conversion decreases as the SO₂ concentration increases. On a long term basis, at an SO₂ concentration range of 1.0–7.7 μg m⁻³, the conversion ranges from 8.2% to 2.1%. The dependence of SO₂ conversion on nylon filters on relative humidity displays a diurnal pattern. An expression has been derived to explain the observed % SO₂ conversion on nylon filters as a combined effect of the ambient SO₂ concentration and relative humidity.     

Roadside Particulate Air Pollution in Bangkok

Abstract

Airborne fine particles of PM_2.5-10 and PM_2.5 in Bangkok, Nonthaburi, and Ayutthaya were measured from December 22, 1998, to March 26, 1999, and from November 30, 1999, to December 2, 1999. Almost all the PM₁₀ values in the high-polluted (H) area exceeded the Thailand National Ambient Air Quality Standards (NAAQS) of 120 μg/m³. The low-polluted (L) area showed low PM₁₀ (34–74 μg/m³ in the daytime and 54–89 μg/m³ at night). PM_2.5 in the H area varied between 82 and 143 μg/m³ in the daytime and between 45 and 146 μg/m³ at night. In the L area, PM_2.5 was quite low both day and night and varied between 24 and 54 μg/m³, lower than the U.S. Environmental Protection Agency (EPA) standard (65 μg/m³). The personal exposure results showed a significantly higher proportion of PM_2.5 to PM₁₀ in the H area than in the L area (H = 0.80 ± 0.08 and L = 0.65 ± 0.04).

Roadside PM₁₀ was measured simultaneously with the Thailand Pollution Control Department (PCD) monitoring station at the same site and at the intersections where police work. The result from dual simultaneous measurements of PM₁₀ showed a good correlation (correlation coefficient: r = 0.93); however, PM levels near the roadside at the intersections were higher than the concentrations at the monitoring station. The relationship between ambient PM level and actual personal exposures was examined. Correlation coefficients between the general ambient outdoors and personal exposure levels were 0.92 for both PM_2.5 and PM₁₀.

Bangkok air quality data for 1997–2000, including 24-hr average PM₁₀, NO₂, SO₂, and O₃ from eight PCD monitoring stations, were analyzed and validated. The annual arithmetic mean PM₁₀ of the PCD data at the roadside monitoring stations for the last 3 years decreased from 130 to 73 μg/m³, whereas the corresponding levels at the general monitoring stations decreased from 90 to 49 μg/m³. The proportion of days when the level of the 24-hr average PM₁₀ exceeded the NAAQS was between 13 and 26% at roadside stations. PCD data showed PM₁₀ was well correlated with NO₂ but not with SO₂, suggesting that automobile exhaust is the main source of the particulate air pollution. The results obtained from the simultaneous measurement of PM_2.5 and PM₁₀ indicate the potential environmental health hazard of fine particles. In conclusion, Bangkok traffic police were exposed to high levels of automobile-derived particulate air pollution.     

Unambiguous identification and measurement of sulfuric acid cluster chemiions in aircraft jet engine exhaust

Measurements of negative chemiions (CI) emitted by a jet engine at the ground were made with an ion trap mass spectrometer. The new instrument offered a high-mass resolution, which led to a first unambiguous identification of negative CI formed by a jet engine. The observed ions are HSO₄⁻(H₂SO₄)_a clusters proved by an isotope study. From the mass spectra an efficiency ε for fuel sulfur conversion to S^VI of 2%±0.8 could be inferred. In addition thermodynamic properties of the observed cluster ions were inferred from measured ion abundance ratios. An effective free energy ΔG_a−1,a⁰=−14 kcal/mol was calculated (for a=3) and an enthalpy of ΔH_a−1,a⁰=−24 (for a=3) kcal/mol was estimated. This indicates a low stability of HSO₄⁻(H₂SO₄)_a (a⩾3) cluster ions against thermal detachment of H₂SO₄ at the high temperatures of our experiment. However the low temperatures at cruise altitudes around 10–12 km lead to high H₂SO₄/H₂O supersaturation and therefore a rapid growth of HSO₄⁻(H₂SO₄)_a cluster ions seems to be possible which is not hindered by thermal H₂SO₄ detachment.     

Total SO2 emission control strategies for the management of air pollution in ulsan industrial complex

Since emission regulations in Korea concentrate mainly on the limitation of pollutant concentration in the stack gas, it is difficult to achieve a desirable air quality in a heavily industralized city like Ulsan. To ensure a suitable air quality in the future, a total emission control method is proposed with a stack height formula of H = 10.6 q^0.5, where H is the stack height (m) and q is the SO₂ emission rate (m³ h⁻¹ reduced to 0°C). The total emission permitted can be allocated to industries

• 1.(1) at an uniform reduction rate,
• 2.(2) by the formula Q = aQ_o^0.925, where Q is the emission allowed (g s⁻¹), a is a constant, and Q_o is the emission before control (g s⁻¹), or
• 3.(3) by using a linear programming technique.

The above three approaches were evaluated using the TCM 2 air quality model. In order to achieve the air quality goal set for the area, the first approach requires 38.7 % reduction of SO₂ emission from industries, the second 53.3 %, and the third 4.3 %. The linear programming method is found to be very economical, but there are some administrative difficulties in enforcement.     

Chemical composition of precipitation at Marion Island (sub-Antarctic)

Over a 1-year period 16.40g Cl⁻m⁻², 10.35 g Na m⁻², 2.11 g SO₄-S m⁻², 1.24g Mg m⁻², 0.39 g K m⁻², 0.37 g Ca m⁻² and 0.21 g inorganic N m⁻² were deposited in precipitation 450 m inland on the eastern coastal plain of sub-Antarctic Marion Island (46°54′S, 37°45′E). Dissolved PO₄-P and organic forms of N were not detected in the precipitation samples. Concentrations of Cl⁻, Na, Mg, Ca and K, as well as the total ionic concentration in the precipitation samples were significantly negatively correlated with the amount of precipitation. The ionic concentration order (Cl > Na > SO₄-S > Mg > K ≈ Ca) in the precipitation was very similar to that in the surrounding ocean. It is likely that most of the inorganic N found in the precipitation originated in penguin rookeries on the nearby shore zone. A comparison is made between precipitation inputs of nutrients at the island and those at other subpolar sites in the S and N Hemispheres.     

Analysis of a Summertime PM2.5 and Haze Episode in the Mid-Atlantic Region

Abstract

Observations of the mass and chemical composition of particles less than 2.5 μm in aerodynamic diameter (PM_2.5), light extinction, and meteorology in the urban Baltimore-Washington corridor during July 1999 and July 2000 are presented and analyzed to study summertime haze formation in the mid-Atlantic region. The mass fraction of ammoniated sulfate (SO₄ ^2-) and carbonaceous material in PM_2.5 were each ～50% for cleaner air (PM_2.5 < 10 μg/m³) but changed to ～60% and ～20%, respectively, for more polluted air (PM_2.5 > 30 μg/m³). This signifies the role of SO₄ ^2- in haze formation. Comparisons of data from this study with the Interagency Monitoring of Protected Visual Environments network suggest that SO₄ ^2? is more regional than carbonaceous material and originates in part from upwind source regions. The light extinction coefficient is well correlated to PM_2.5 mass plus water associated with inorganic salt, leading to a mass extinction efficiency of 7.6 ± 1.7 m²/g for hydrated aerosol. The most serious haze episode occurring between July 15 and 19, 1999, was characterized by westerly transport and recirculation slowing removal of pollutants. At the peak of this episode, 1-hr PM_2.5 concentration reached ～45 μg/m³, visual range dropped to ～5 km, and aerosol water likely contributed to ～40% of the light extinction coefficient.     

NCAQ Report to Congress: Review and Response

Airborne measurements of gaseous and particulate sulfur and nitrogen pollutants were made in southwestern Kentucky on the afternoon of October 21, 1979. Back-trajectory analysis indicates that the sampled air parcel moved over northern Florida, Alabama, and western Tennessee during the two days prior to sampling. Before moving over Florida, the air parcel was over the Atlantic Ocean for at least five days. Analytical long-range transport (LRT) model predictions based on anthropogenic emissions account for only about 75% of the airborne measured concentrations of 14.7 μg m^?3 for SO₂ and 4.8 μg m^?3 for SO₄ ^2?. The remaining 25 % is thought to be due to biogenic sulfur emissions from the extensive wetland areas along the Gulf Coast.

Forward-trajectory analysis indicates that the air parcel moved to the Adirondack Mountains of New York State 24 hours after sampling. Model predictions indicate that SO₂ and SO₄ ^2? mean layer concentrations at the Adirondacks were 24 and 16 μg^?3, respectively. Almost half of this sulfur was estimated to come from emissions in the heavily industrialized region along the Ohio River Valley.

Further comparisons used a measurement data base obtained in southeastern Canada and the state of Arkansas during August 1976. An air parcel was tracked for seven days as it entered the north central United States, stagnated over the lower midwest, and then moved to eastern Canada. Model predictions were in substantial agreement with regional SO₄ ^2? concentrations measured at a number of ground-level sites. Average SO₄ ^2? concentrations measured in central Arkansas on August 10, 1976 were 20 μ m^?3 vs. a modeled value of 19 μ m^?3. Average SO₄ ^2? concentrations measured in Nova Scotia four days later were 22 μg^?3 vs. a modeled estimate of 24 μg^?3.