On the contribution of black carbon to the composite aerosol radiative forcing over an urban environment

This paper discusses the extent of Black Carbon (BC) radiative forcing in the total aerosol atmospheric radiative forcing over Pune, an urban site in India. Collocated measurements of aerosol optical properties, chemical composition and BC were carried out for a period of six months (during October 2004 to May 2005) over the site. Observed aerosol chemical composition in terms of water soluble, insoluble and BC components were used in Optical Properties of Aerosols and Clouds (OPAC) to derive aerosol optical properties of composite aerosols. The BC fraction alone was used in OPAC to derive optical properties of BC aerosols. The aerosol optical properties for composite and BC aerosols were separately used in SBDART model to derive direct aerosol radiative forcing due to composite and BC aerosols. The atmospheric radiative forcing for composite aerosols were found to be +35.5, +32.9 and +47.6 Wm^?2 during post-monsoon, winter and pre-monsoon seasons, respectively. The average BC mass fraction found to be 4.83, 6.33 and 4 μg m^?3 during the above seasons contributing around 2.2 to 5.8% to the total aerosol load. The atmospheric radiative forcing estimated due to BC aerosols was +18.8, +23.4 and +17.2 Wm^?2, respectively during the above seasons. The study suggests that even though BC contributes only 2.2–6% to the total aerosol load; it is contributing an average of around 55% to the total lower atmospheric aerosol forcing due to strong radiative absorption, and thus enhancing greenhouse warming.     

Light-absorbing aldol condensation products in acidic aerosols: Spectra,kinetics, and contribution to the absorption index

The radiative properties of aerosols that are transparent to light in the near-UV and visible, such as sulfate aerosols, can be dramatically modified when mixed with absorbing material such as soot. In a previous work we had shown that the aldol condensation of carbonyl compounds produces light-absorbing compounds in sulfuric acid solutions. In this work we report the spectroscopic and kinetic parameters necessary to estimate the effects of these reactions on the absorption index of sulfuric acid aerosols in the atmosphere. The absorption spectra obtained from the reactions of six different carbonyl compounds (acetaldehyde, acetone, propanal, butanal, 2-butanone, and trifluoroacetone) and their mixtures were compared over 190–1100 nm. The results indicated that most carbonyl compounds should be able to undergo aldol condensation. The products are oligomers absorbing light in the 300–500 nm region where few other compounds absorb, making them important for the radiative properties of aerosols. Kinetic experiments in 96–75 wt% H₂SO₄ solutions and between 273 and 314 K gave an activation energy for the rate constant of formation of the aldol products of acetaldehyde of −(70±15) kJ mol⁻¹ in 96 wt% solution and showed that the effect of acid concentration was exponential. A complete expression for this rate constant is proposed where the absolute value in 96 wt% H₂SO₄ and at 298 K is scaled to the Henry's law coefficient for acetaldehyde and the absorption cross-section for the aldol products assumed in this work. The absorption index of stratospheric sulfuric acid aerosols after a 2-year residence time was estimated to 2×10⁻⁴, optically equivalent to a content of 0.5% of soot and potentially significant for the radiative forcing of these aerosols and for satellite observations in channels where the aldol products absorb.     

Particulate contributions to light extinction and local forcing at a rural Illinois site

The light extinction and direct forcing properties of the atmospheric aerosol were investigated for a midwestern rural site (Bondville, IL) using field measurements, a semi-empirical light extinction model, and a radiative transfer code. Model inputs were based on the site measurements of the physical and chemical characteristics of atmospheric aerosol during the spring, summer, fall and winter of 1994. The light scattering and extinction coefficients were calculated and apportioned using the elastic light scattering interactive efficiency (ELSIE) model (Sloane and Wolff, 1985, Atmospheric Environment 19(4), 669–680). The average efficiencies calculated for organic carbon (OC, carbon measured as organic multiplied by 1.2) ranged from 3.81 m²/g OC at lower relative humidities (<63%) to 6.90 m²/g OC at higher relative humidities (>75%) while sulfate (assumed as ammonium sulfate) efficiencies ranged from 1.23 m²/g (NH₄)₂SO₄ to 5.78 m²/g (NH₄)₂SO₄ for the same range of relative humidities. Radiative transfer calculations showed that the rural aerosol at Bondville is most likely to have an overall negative (cooling) forcing effect on climate. Elemental carbon (EC), however, acts to counter sulfate forcing to a degree that has a significant seasonal variation, primarily due to the seasonal variation in the sulfate concentrations. Taking the loading to be the mean summer EC+ammonium sulfate loading and assuming [EC]/[(NH₄)₂SO₄] to be zero in one case (i.e. no soot present) and 0.025 (summer mean at Bondville) in another leads to a 37% difference in calculated forcing.     

On radiative effects of black carbon and sulphate aerosols

Most aerosol particles, such as sulphate and sea-salt particles, mainly scatter solar radiation, whilst soot (in the form of elemental carbon or “black” carbon, BC) in addition leads to considerable absorption. This scattering and absorption by the aerosol particles constitute the so-called direct aerosol effect. In this paper, we present results from a study of possible direct effects of tropospheric BC and sulphate aerosols, with an emphasis on BC aerosols, along a line from North Africa through Europe into the Arctic. Radiative budgets in a cloud-free atmosphere are estimated. Based on model-calculated distributions of BC and sulphate (provided by Seland and Iversen, 1998) and assumed size distributions of the background aerosol, new size distributions are obtained by adding natural, biomass burning and fossil fuel contributions to the background aerosol. Added nucleation mode particles are assumed externally mixed, whereas added accumulation mode BC and sulphate is internally mixed with the background according to condensational growth and Brownian coagulation theory. Humidity effects are taken into account by use of the Köhler equation. Mie calculations provide the resulting optical parameters, and the forcing is finally estimated by use of a radiative transfer model. A reference run and a series of eleven test-runs are performed to investigate the sensitivity of various assumptions on the contribution to upward TOA irradiance from BC and non-sea-salt sulphate. The tests suggest a high sensitivity to presence of BC and to particle swelling due to humidity. The sensitivity to assumed distribution of BC on particle size is more moderate. The same is true for the vertical resolution and the number concentration of the background aerosol. The effect of mixing organic carbon (OC) internally with biomass burning BC nucleation mode particles is characterized as moderate. The role of OC is, however, still uncertain. The same holds true for the optical thickness of the background atmosphere, for which we found a high sensitivity in this study. Other assumptions that were investigated had only small effects on the forcing. For the reference run we find a minimum in the aerosol forcing of approximately −5 W m^-2 near the most polluted areas in Europe, and a maximum of approximately 2 W m^-2 over North Africa. A warming effect is also found for the Arctic region, with forcing values up to 0.4 W m^-2.     

Inventory of aerosol and sulphur dioxide emissions from India. Part II—biomass combustion

A spatially resolved biomass burning data set, and related emissions of sulphur dioxide and aerosol chemical constituents was constructed for India, for 1996–1997 and extrapolated to the INDOEX period (1998–1999). Sources include biofuels (wood, crop waste and dung-cake) and forest fires (accidental, shifting cultivation and controlled burning). Particulate matter (PM) emission factors were compiled from studies of Indian cooking stoves and from literature for open burning. Black carbon (BC) and organic matter (OM) emissions were estimated from these, accounting for combustion temperatures in cooking stoves. Sulphur dioxide emission factors were based on fuel sulphur content and reported literature measurements. Biofuels accounted 93% of total biomass consumption (577 MT yr⁻¹), with forest fires contributing only 7%. The national average biofuel mix was 56 : 21 : 23% of fuelwood, crop waste and dung-cake, respectively. Compared to fossil fuels, biomass combustion was a minor source of SO₂ (7% of total), with higher emissions from dung-cake because of its higher sulphur content. PM_2.5 emissions of 2.04 Tg yr⁻¹ with an “inorganic fraction” of 0.86 Tg yr⁻¹ were estimated. Biomass combustion was the major source of carbonaceous aerosols, accounting 0.25 Tg yr⁻¹ of BC (72% of total) and 0.94 Tg yr⁻¹ of OM (76% of total). Among biomass, fuelwood and crop waste were primary contributors to BC emissions, while dung-cake and forest fires were primary contributors to OM emissions. Northern and the east-coast India had high densities of biomass consumption and related emissions. Measurements of emission factors of SO₂, size resolved aerosols and their chemical constituents for Indian cooking stoves are needed to refine the present estimates.     

Carbonaceous and inorganic composition in long-range transported aerosols over northern Japan: Implication for aging of water-soluble organic fraction

To better understand the influence of sources and atmospheric processing on aerosol chemical composition, we collected atmospheric particles in Sapporo, northern Japan during spring and early summer 2005 under the air mass transport conditions from Siberia, China and surrounding seas. The aerosols were analyzed for inorganic ions, organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and the major water-soluble organic compound classes (i.e., dicarboxylic acids and sugars). SO₄^2? is the most abundant inorganic constituent (average 44% of the identified inorganic ion mass) followed by NH₄⁺ (21%) and NO₃^? (13%). Concentrations of OC, EC, and WSOC ranged from 2.0–16, 0.24–2.9, and 0.80–7.9 μg m^?3 with a mean of 7.4, 1.0, and 3.1 μg m^?3, respectively. High OC/EC ratios (range: 3.6–19, mean: 8.7) were obtained, however WSOC/OC ratios (0.23–0.69, 0.44) do not show any significant diurnal changes. These results suggest that the Sapporo aerosols were already aged, but were not seriously affected by local photochemical processes. Identified water-soluble organic compounds (diacids + sugars) account for <10% of WSOC. Based on some marker species and air mass back trajectory analyses, and using stable carbon isotopic compositions of shorter-chain diacids (i.e., C₂–C₄) as photochemical aging factor of organic aerosols, the present study suggests that a fraction of WSOC in OC is most likely influenced by aerosol aging, although the OC loading in aerosols may be more influenced by their sources and source regions.     

Multi-year hourly PM2.5 carbon measurements in New York: Diurnal,day of week and seasonal patterns

Multi-year hourly measurements of PM_2.5 elemental carbon (EC) and organic carbon (OC) from a site in the South Bronx, New York were used to examine diurnal, day of week and seasonal patterns. The hourly carbon measurements also provided temporally resolved information on sporadic EC spikes observed predominantly in winter. Furthermore, hourly EC and OC data were used to provide information on secondary organic aerosol formation. Average monthly EC concentrations ranged from 0.5 to 1.4 μg m^?3 with peak hourly values of several μg m^?3 typically observed from November to March. Mean EC concentrations were lower on weekends (approximately 27% lower on Saturday and 38% lower on Sunday) than on weekdays (Monday to Friday). The weekday/weekend difference was more pronounced during summer months and less noticeable during winter. Throughout the year EC exhibited a similar diurnal pattern to NO_x showing a pronounced peak during the morning commute period (7–10 AM EST). These patterns suggest that EC was impacted by local mobile emissions and in addition by emissions from space heating sources during winter months. Although EC was highly correlated with black carbon (BC) there was a pronounced seasonal BC/EC gradient with summer BC concentrations approximately a factor of 2 higher than EC. Average monthly OC concentrations ranged from 1.0 to 4.1 μg m^?3 with maximum hourly concentrations of 7–11 μg m^?3 predominantly in summer or winter months. OC concentrations generally correlated with PM_2.5 total mass and aerosol sulfate and with NO_x during winter months. OC showed no particular day of week pattern. The OC diurnal pattern was typically different than EC except in winter when OC tracked EC and NO_x indicating local primary emissions contributed significantly to OC during winter at the urban location. On average secondary organic aerosol was estimated to account for 40–50% of OC during winter and up to 63–73% during summer months.     

Modeling the spectral optical properties of ammonium sulfate and biomass burning aerosols: parameterization of relative humidity effects and model results

The importance of including the global and regional radiative effects of aerosols in climate models has increasingly been realized. Accurate modeling of solar radiative forcing due to aerosols from anthropogenic sulfate and biomass burning emissions requires adequate spectral resolution and treatment of spatial and temporal variability. The variation of aerosol spectral optical properties with local relative humidity and dry aerosol composition must be considered. Because the cost of directly including Mie calculations within a climate model is prohibitive, parameterizations from off-line calculations must be used. Starting from a log-normal size distribution of dry ammonium sulfate, we developed optical properties for tropospheric sulfate aerosol at 15 relative humidities up to 99%. The resulting aerosol size distributions were then used to calculate bulk optical properties at wavelengths between 0.175 and 4 μm. Finally, functional fits of optical properties were made for each of 12 wavelength bands as a function of relative humidity. Significant variations in optical properties occurred across the total solar spectrum. Relative increases in specific extinction and asymmetry factor with increasing relative humidity became larger at longer wavelengths. Significant variation in single-scattering albedo was found only in the longest near-IR band. This is also the band with the lowest single scattering albedo. A similar treatment was done for aerosols from biomass burning. In this case, two size distributions were considered. One was based on a distribution measured for Northern Hemisphere temperate forest fires while the second was based on a measured size distribution for tropical fires. Equilibrium size distributions and compositions were calculated for 15 relative humidities and five black carbon fractions. Mie calculations and band averages of optical properties were done for each of the resulting 75 cases. Finally, fits were made for each of 12 spectral bands as functions of relative humidity and black carbon fraction. These optical properties result in global average forcing from anthropogenic sulfate aerosols of −0.81 Wm^-2. The global average forcing for biomass aerosols ranged from −0.23 to −0.25 Wm^-2 depending on the assumed size distribution, while fossil fuel organic and black carbon are estimated to heat the atmosphere by about 0.16 Wm^-2.     

Aerosol optical,chemical and physical properties at Gosan,Korea during Asian dust and pollution episodes in 2001

In order to understand the influence of dust and anthropogenic pollution aerosols on regional climate in East Asia, we analyzed the aerosol optical, chemical and physical properties for two cases with high aerosol loading and assessed the radiative forcing of these cases. The 1st case study is a heavy dust episode (DE) in April (during ACE-Asia) 2001 and the 2nd case is a regional-scale pollution event in November 2001. The Ångström exponent (Å) for DE was 0.38 from sunphotometer measurements. The mean single scattering albedo (550 nm) at the surface reported during the pollution episode (PE, 0.88) was lower than that of DE (0.91). The concentrations of organic (OC) and elemental carbon (EC) measured during the PE were about 90% and 30% higher than DE. The aerosol mass scattering efficiency (α_s) of PE is a factor of about 2 higher than that of the DE. The difference in the mass absorption efficiency (α_a) of EC during DE and PE is small and within the measurement uncertainty. The diurnally averaged aerosol radiative forcing efficiency (ΔDFE, W m⁻² τ⁻¹) during DE is similar to results of other studies at Gosan.     

Primary emissions of fine carbonaceous particles in Europe

The European emissions of BC and OC in fine particles are calculated for the years 1990, 1995 and 2000 applying the RAINS model that, beyond fuel-sector distinction, explicitly includes various combustion technologies and the penetration of abatement options. The emission factors used are developed considering specific European conditions. The main sources of carbonaceous aerosols in Europe are emissions from traffic and residential combustion of solid fuels. Between 1990 and 2000, the BC and OC emissions are estimated to decline from 0.89 to 0.68 Tg and from 1.4 to 1.0 Tg, respectively. Most of the reduction occurred in the early 1990s in Eastern Europe owing to structural changes that resulted in energy efficiency improvements in industry and lower consumption of solid fuels in residential–commercial sector; the latter having strong impact on BC and OC emissions. Furthermore, the growth in transport volumes, and expected increase in emissions, was offset by introduction of stricter legislation for road transport from 1995. Focusing on the most important sectors, transport and residential combustion, the variation in measured carbonaceous emission shares and its impact on total emissions was evaluated. This analysis indicates a range of about −25% to +20% for BC and −7% and +15% for OC, compared to the central case.     

Quantification and radiocarbon source apportionment of black carbon in atmospheric aerosols using the CTO-375 method

To make progress towards linking the atmosphere and biogeosphere parts of the black carbon (BC) cycle, a chemothermal oxidation method (CTO-375), commonly applied for isolating BC from complex geomatrices such as soils, sediments and aquatic particles, was applied to investigate the BC also in atmospheric particles. Concentrations and ¹⁴C-based source apportionment of CTO-375 based BC was established for a reference aerosol (NIST RM-8785) and for wintertime aerosols collected in Stockholm and in a Swedish background area. The results were compared with thermal–optical (OC/EC) measurements. For NIST RM-8785, a good agreement was found between the BC_CTO-375 concentration and the reported elemental carbon (EC) concentration measured by the “Speciation Trends Network—National Institute of Occupational Safety and Health” method (EC_NIOSH) with BC_CTO-375 of 0.054±0.002 g g⁻¹ and EC_NIOSH of 0.067±0.008 g g⁻¹. In contrast, there was an average factor of ca. 20 difference between BC_CTO-375 and EC_NIOSH for the ambient Scandinavian wintertime aerosols, presumably reflecting a combination of BC_CTO-375 isolating only the recalcitrant soot-BC portion of the BC continuum and the EC_NIOSH metric inadvertently including some intrinsically non-pyrogenic organic matter. Isolation of BC_CTO-375 with subsequent off-line radiocarbon analysis yielded fraction modern values (fM) for total organic carbon (TOC) of 0.93 (aerosols from a Swedish background area), and 0.58 (aerosols collected in Stockholm); whereas the fM for BC_CTO-375 isolates were 1.08 (aerosols from a Swedish background area), and 0.87 (aerosols collected in Stockholm). This radiocarbon-based source apportionment suggests that contribution from biomass combustion to cold-season atmospheric BC_CTO-375 in Stockholm was 70% and in the background area 88%.     

Characterisation of fresh particulate vehicular exhausts near a Paris high flow road

Simultaneous continuous measurements of PM_2.5, PM₁₀, black carbon mass (BCae), Black smoke (BS) and particle number density (N) were conducted in the close vicinity of a high traffic road around Paris during a three-month period beginning in August 1997. In parallel some aerosol collection was performed on filters in order to assess the black carbon (BC), organic carbon (OC) and water soluble organic fractions (WSOC) of the freshly emitted traffic aerosols. The high hourly concentrations of PM_2.5 (39±20 μg m⁻³), BCae (14±7 μg m⁻³), and N (220,000±115,000 cm⁻³), were found to be well correlated with each other. On average PM_2.5 represented 66±13% of PM₁₀ and appears to be composed primarily of BC (43±20%). On the contrary no correlation was found between PM_2.5 and the coarse (PM₁₀–PM_2.5) mass fractions which was attributed to resuspension processes by vehicles. Black carbon mass concentrations obtained from both filter analyses (BC) and Aethalometre data (BCae) show a good agreement suggesting that the Aethalometre calibration based on a black carbon specific attenuation coefficient (σ) of 19 m² g⁻¹ is well adapted to nearby roadside measurements. Daily BC (used as a surrogate for fine particles) concentrations and wind speed were found to be anti-correlated. Average daily variations of BC could be related to traffic intensity and regime as well as to the boundary layer height. As expected for freshly emitted traffic aerosols, filter analyses indicated a high BC/TC ratio (29±5%) and a low mean WSOC/OC ratio (12.5±5%) for the bulk aerosol. For these two ratios no day/night differences were observed, the sampling station being probably too close to traffic to evidence photochemical modification of the aerosol phase. Finally, a linear relationship was found between BC and BS hourly concentrations (BC=0.10×BS+1.18; r²=0.93) which offers interesting perspectives to retrieve BC concentrations from existing BS archives.     

Estimation of direct radiative forcing due to non-methane hydrocarbons

The direct radiative forcing due to non-methane hydrocarbons (NMHCs) has not previously been quantified. We use new measurements of infrared absorption cross-sections and a narrow band radiative transfer model to estimate a forcing. An upper limit to the global mean anthropogenic forcing is likely to be in the region of 0.015 W m^-2, less than 1% of the forcing due to other greenhouse gases. However, taking account of the natural NMHC loading and the vertical profile of these gases the actual radiative forcing is likely to be somewhat less than this.     

Temporal trends of black carbon concentrations and regional climate forcing in the southeastern United States

The effect of black carbon (BC) on climate forcing is potentially important, but its estimates have large uncertainties due to a lack of sufficient observational data. The BC mass concentration in the southeastern US was measured at a regionally representative site, Mount Gibbes (35.78°N, 82.29°W, 2006 m MSL). The air mass origin was determined using 48-h back trajectories obtained from the hybrid single-particle Lagrangian integrated trajectory model. The highest average concentration is seen in polluted continental air masses and the lowest in marine air masses. During the winter, the overall average BC value was 74.1 ng m⁻³, whereas the overall summer mean BC value is higher by a factor of 3. The main reason for the seasonal difference may be enhanced thermal convection during summer, which increases transport of air pollutants from the planetary boundary layer of the surrounding urban area to this rural site. In the spring of 1998, abnormally high BC concentrations from the continental sector were measured. These concentrations were originating from a biomass burning plume in Mexico. This was confirmed by the observations of the Earth probe total ozone mapping spectrometer. The BC average concentrations of air masses transported from the polluted continental sector during summer are low on Sunday to Tuesday with a minimum value of 256 ng m⁻³ occurring on Monday, and high on Wednesday to Friday with a maximum value of 379 ng m⁻³ occurring on Friday. The net aerosol radiative forcing (scattering effects plus absorption effects) per unit vertical depth at 2006 m MSL is calculated to be −1.38×10⁻³ W m⁻³ for the southeastern US. The magnitude of direct radiative forcing by aerosol scattering is reduced by 15±7% due to the BC absorption.     

Field validation of a semi-continuous method for aerosol black carbon (aethalometer) and temporal patterns of summertime hourly black carbon measurements in southwestern PA

Two methods for measuring aerosol elemental carbon (EC) are compared. Three-hour integrated carbon samples were collected on quartz filters during the summer of 1990 in Uniontown, PA, primarily during episodes of elevated particulate pollution levels. These samples were analyzed for EC and organic carbon (OC) using a thermo/optical reflectance (TOR) method. Aerosol black carbon (BC) was measured using an Aethalometer, a semi-continuous optical absorption method. The optical attenuation factor for ambient BC was supplied by the instrument manufacturer. Three-hour average concentrations were calculated from the semi-continuous BC measurements to temporally match the EC/OC integrated quartz filter samples. BC and EC concentrations are highly correlated over the study period (R²=0.925). The regression equation is BC (μg m^-3)=0.95 (±0.04) EC−0.2 (±0.4). The means of 3 h average measurements for EC and BC are 2.3 and 2.0 μg m^-3, respectively, average concentrations of EC and BC ranged from 0.6 to 9.4 and 0.5 to 9.0 μg m^-3 respectively. TOR OC and EC concentrations were not highly correlated (R²=0.22). The mean OC/EC ratio was 1.85.The 10-week Aethalometer hourly dataset was analyzed for daily and weekly temporal patterns. A strong diurnal BC pattern was observed, with peaks occurring between 7 a.m. and 9 a.m. local time. This is consistent with the increase in emissions from ground level combustion sources in the morning, coupled with poor dispersion before daytime vertical mixing is established. There was also some indication of a day-of-week effect on BC concentrations, attributed to activity of local ground level anthropogenic sources. Comparison of BC concentrations with co-located measurements of coefficient of haze in a separate field study in Philadelphia, PA, during the summer of 1992 showed good correlation between the two measurements (R²=0.82).     

Contribution of carbonaceous material to cloud condensation nuclei concentrations in European background (Mt. Sonnblick) and urban (Vienna) aerosols

During four intensive measurement campaigns (two on Mt. Sonnblick, European background aerosol, and two in Vienna, urban aerosol), cloud condensation nuclei (CCN) were measured at supersaturations of 0.5%. Impactor measurements of the mass size distribution in the size range 0.1–10 μm were performed and later analyzed for Cl^-, NO^-₃, SO^2-₄, Na⁺, NH⁺₄, K⁺, Ca²⁺ and Mg²⁺ by ion chromatography, for total carbon (TC) using a combustion method, and for black carbon (BC) by an optical method (integrating sphere). Organic carbon (OC) was defined as the difference between TC (minus carbonate carbon) and BC. At all sites, the mass fraction of BC in the submicron aerosol was comparable (4–5%). CCN concentrations on Mt. Sonnblick were found to be 10–30% of those measured in Vienna, although high Mt. Sonnblick concentrations were comparable to low Vienna concentrations (around 800 cm^-3). The contribution of organic material was estimated from the mass concentrations of the chemical species sampled on the impactor stage with the lowest cut point (0.1–0.215 μm aerodynamic equivalent diameter). On Mt. Sonnblick, TC material contributed 11% to the total mass in fall 1995, and 67% in summer 1996, while the OC fraction was 6 and 61%. The combined electrolytes and mineral material contributed 18 and 16% in fall and summer. During the Vienna spring campaign, the contributions of OC and electrolytes to the total mass concentration in this size range were 48 and 36%, respectively.     

PM2.5 particles and size-segregated ionic species measured during fall season in three urban sites in Korea

Twelve hours integrated fine particles (PM_2.5) and 24-h average size-segregated particles were collected to investigate the chemical characteristics and to determine the size distribution of ionic species during October–December 1999 in three cities of different urban scale; Chongju, Kwangju, and Seoul, Korea. Concentrations of 5-min PM_2.5 black carbon (BC) and hourly criteria air pollutants (PM₁₀, CO, NO_x, SO₂, and O₃) were also measured using the Aethalometer and ambient air monitoring system, respectively.Highest PM_2.5 mass concentrations at Chongju, Kwangju, and Seoul sites were 63.0, 77.9, and 143.7 μg m⁻³, respectively. For the time period when highest PM_2.5 mass occurred, BC level out of PM_2.5 chemical species was highest at both Chongju and Kwangju, and highest NO₃⁻ (23.6 μg m⁻³) followed by BC (23.1 μg m⁻³) were observed at Seoul site, indicating that highest PM_2.5 pollution is closely associated with the traffic emissions. Strong relationships of Fe with BC and Zn at both Kwangju and Seoul sites support that the Fe and Zn measured there are originated partly from same source as BC, i.e. diesel traffics. However, it is suggested that the Fe measured at Chongju is most likely derived from dispersion of soil dust.The size distributions of SO₄²⁻, NO₃⁻, and NH₄⁺ ionic species indicated similar unimodal distributions at all sampling sites. However, different unimodal patterns in the accumulation mode size range with a peak in the smaller size (0.28–0.53 μm, condensation mode) in both Kwangju and Seoul, and in the relatively larger size (0.53–1.0 μm, droplet mode) in Chongju, were found. The potassium ion under the study sites dominates in the fine mode, and its size distribution showed unimodal character with a maximum in the size range 0.56–1.0 μm.     

A scheme for black carbon and sulphate aerosols tested in a hemispheric scale,Eulerian dispersion model

Production, transport and deposition of sulphate and black carbon (BC) are simulated separating aerosol modes by production mechanisms, thus facilitating calculation of aerosol physical properties. The scheme uses explicit sulphur chemistry with prescribed oxidants and aerosol transformation based on Brownian coagulation rates, and is implemented in a 3-D, hemispheric-scale transport model with off-line meteorology from ECMWF. The components are DMS, SO₂, sulphate and BC. Simulations are made for the year 1988. Predicted results for sulphate agree well with the measurements, except for considerable underestimations in the Arctic. In Europe SO₂ is slightly overestimated and sulphate is slightly underestimated in cold seasons, whilst trends are less clear in North America. Compared to many other models, we estimate a smaller effective oxidation rate for SO₂ due to reduced rates in cold clouds; a shorter turnover time for sulphate (3.7 d) due to a probably too large below-cloud scavenging ratio; and slightly smaller sulphate column burdens. Our BC results are similar to Liousse et al.’s (1996, J. Geophys. Res. 101, 19.411–19.432) except in USA, whilst the concentrations in remote areas and the turnover time (3.7 d) are considerably smaller than Cooke and Wilson’s (1996, J. Geophys. Res. 101, 190.395–19.409). Agreements with available measurements are quite close to large anthropogenic emissions (including USA), but they are considerably underestimated in Arctic winter. Transition from hydrophobic to hydrophilic BC due to coagulation is swift (6% h^-1). Sensitivity tests emphasize that sub-cloud scavenging coefficients rely on careful assumptions about size distributions, and that more research is needed on sulphate production in ice-clouds. Emphasis should also be put on production of sulphate and hydrophilic BC boundary-layer clouds; inclusion of hydrophobic accumulation mode BC and sub-grid transition to hydrophilic BC in emitting grid squares.     

Application of a Lagrangian particle model to assess the impact of harbour,industrial and urban activities on air quality in the Taranto area,Italy

This paper evaluates the relative impact on air quality of harbour emissions, with respect to other emission sources located in the same area. The impact assessment study was conducted in the city of Taranto, Italy. This area was considered as representative of a typical Mediterranean harbour region, where shipping, industries and urban activities co-exist at a short distance, producing an ideal case to study the interaction among these different sources. Chemical and meteorological field campaigns were carried out to provide data to this study. An emission inventory has been developed taking into account industrial sources, traffic, domestic heating, fugitive and harbour emissions. A 3D Lagrangian particle dispersion model (SPRAY) has then been applied to the study area using reconstructed meteorological fields calculated by the diagnostic meteorological model MINERVE. 3D short term hourly concentrations have been computed for both all and specific sources. Industrial activities are found to be the main contributor to SO₂. Industry and traffic emissions are mainly responsible for NO_x simulated concentrations. CO concentrations are found to be mainly related to traffic emissions, while primary PM₁₀ simulated concentrations tend to be linked to industrial and fugitive emissions. Contributions of harbour activities to the seasonal average concentrations of SO₂ and NO_x are predicted to be up to 5 and 30 μg m⁻³, respectively to be compared to a overall peak values of 60 μg m⁻³ for SO₂ and 70 μg m⁻³ for NO_x. At selected urban monitoring stations, SO₂ and NO_x average source contributions are predicted to be both of about 9% from harbour activities, while 87% and 41% respectively of total concentrations are predicted to be of industrial origin.     

Radiative forcing from household fuel burning in Asia

Household fuel use in developing countries, particularly as biomass and coal, is a major source of carbonaceous aerosols and other air pollutants affecting health and climate. Using state-of-the-art emission inventories, a global three-dimensional photochemical tracer/transport model of the troposphere, and a global radiative transfer model based on methods presented in the latest IPCC Assessment Report (2007-AR4), we estimate the radiative forcing (RF) attributable to household fuel combustion in Asia in terms of current global annual-mean RF and future global integrated RF for a one-year pulse of emissions (2000) over two time horizons (100 and 20 years). Despite the significant emissions of black carbon (BC) aerosols, these estimates indicate that shorter-lived (non-Kyoto) air pollutants from household fuel use in the region overall seem to exert a small net negative RF because of the strong influence of reflective aerosols. There are, however, major uncertainties in emission estimates for solid fuel burning, and about the sustainability of household fuel wood harvesting in Asia (the carbon neutrality of harvesting). In addition, there is still substantial uncertainty associated with the BC radiative forcing. As a result we find that the sign of the RF from household biomass burning in the region cannot be established. While recognizing the value of integrating climate change and air pollution policies, we are concerned that for a ‘Kyoto style’ post-Kyoto treaty (with global cap-and-trade and the Global Warming Potential as the metric) expanding the basket of components with a selection of short-lived species without also including the wider range of co-emitted species may lead to unintended consequences for global-scale climate. Additional measurement, modelling, and policy research is urgently needed to reduce the uncertainties so that the net impact on climate of emissions and mitigation measures in this sector can be accurately assessed.