Aerosol Condensation Research Articles

Mid-latitude North-Atlantic aerosol characteristics in clean and polluted air

Aerosol number concentrations ( r>1.5 nm) and size distributions (5 nm–150 μm radius) are reported for air masses typical of clean and polluted air masses over a North–East Atlantic coastal site (Mace Head). In clean marine air, total particle concentration is typically 400–600 cm −3, rising to between 600 and 1500 cm −3 for modified maritime air, and in the most polluted air, concentrations increase to ≈7000 cm −3. In all air masses, regular bursts of ultra-fine (1.5–5 nm) particles (UFP) were observed over several hours duration and coincided with low tide occurrence in the presence of solar radiation. During these bursts, measured UFP concentrations exceeded 180,000 cm −3. Typically, the sub-micron marine size distribution possessed a bi-modal shape, typical of cloud-induced aerosol growth, with an accumulation mode centred at 0.1–0.2 μm (radius) and a fine mode ≈0.02 μm, while super-micron sea-spray particles possessed a mode at ≈1–5 μm, extending to ≈10 μm under moderately low wind speeds of 5–6 m s −1. Under higher wind speed conditions (10–11 m s −1), a spume drop mode, extending to sizes of 150 μm, was observed. In anthropogenically influenced air masses, the sub-micron size distribution remained bi-modal, although to a lesser extent. Number concentration is dominated by sub-micron aerosol in all air masses, in marine air masses, surface area is generally dominated by super-micron particles, and volume is overwhelmingly dominated by the mode centred on 10 μm under moderate conditions, and 100 μm under higher winds. By comparison, in polluted air, surface area is dominated by sub-micron sizes and volume dominated by super-micron sizes. After nucleation bursts, growth of new, ultra-fine, particles into sizes in the accumulation mode can be observed under some conditions with growth rates of the order of 10 nm/h. Calculating the pre-existing aerosol condensation sink and the growth rate of newly formed particles allowed estimation of the condensing vapour concentration producing the observed growth along with the source rate of the condensing vapour. The concentration of condensable vapour and its source strength were 10–14×10 7 molecules cm −3 and 3–14×10 5 molecules cm −3 s −1, respectively.

On the formation, growth and composition of nucleation mode particles

Taking advantage of only the measured aerosol particles spectral evolution as a function of time, a new analytical tool is developed to derive formation and growth properties of nucleation mode aerosols. This method, when used with hygroscopic growth-factors, can also estimate basic composition properties of these recently-formed particles. From size spectra the diameter growth-rate can be obtained, and aerosol condensation and coagulation sinks can be calculated. Using this growth-rate and condensation sink, the concentration of condensable vapours and their source rate can be estimated. Then, combining the coagulation sink together with measured number concentrations and apparent source rates of 3 nm particles, 1 nm particle nucleation rates and concentration can be estimated. To estimate nucleation rates and vapour concentration source rates producing new particle bursts over the Boreal forest regions, three cases from the BIOFOR project were examined using this analytical tool. In this environment, the nucleation mode growth-rate was observed to be 2–3 nm hour−1, which required a condensable vapour concentration of 2.5–4×107 cm−3 and a source rate of approximately 7.5–11×104 cm−3 s−1 to be sustained. The formation rate of 3 nm particles was =1 particle cm−3 s−1 in all three cases. The estimated formation rate of 1 nm particles was 10–100 particles cm−3 s−1, while their concentration was estimated to be between 10,000 and 100,000 particles cm−3. Using hygroscopicity data and mass flux expressions, the mass flux of insoluble vapour is estimated to be of the same order of magnitude as that of soluble vapour, with a soluble to insoluble vapour flux ratio ranging from 0.7 to 1.4 during these nucleation events.

Can new particle formation occur in the clean marine boundary layer?

An analysis of new particle formation probability in the marine boundary layer (MBL) is conducted using a detailed aerosol dynamics and gas‐phase chemistry model, thermodynamically correct classical binary (H2O‐H2SO4) nucleation theory, and recently developed ternary (H2O‐H2SO4‐NH3) nucleation theory. Additionally, the effect of boundary‐layer meteorology (i.e., adiabatic cooling, small scale fluctuations, and entrainment) in enhancing nucleation is also examined. The results indicate that for typical marine conditions, binary nucleation does not occur for any realistic conditions regardless of adiabatic cooling, turbulent fluctuations, or entrainment. For polar marine conditions, binary nucleation does occur due to lower temperatures, and is enhanced due to turbulent fluctuations. An increase in detectable particle sizes (N3>3 nm), is only seen after multiple boundary layer circulations for conditions of high dimethyl sulphide (DMS) concentrations (400 ppt). Under extreme conditions of entrainment of free‐troposphere layers containing very low aerosol condensation sinks and extraordinary high sulphuric acid concentrations (>108 molecules cm−3), increases in detectable particles up to 10,000 cm−3 are predicted only in polar marine air, but are viewed as unlikely to occur in reality. Comparison of model simulations with observed values of DMS and sulphuric acid in polar marine air masses suggest that binary nucleation may lead to an enhancement of ≈ 1000 cm−3 in N3 particle concentration, but not to enhancements of ≈ 10,000 cm−3. Ternary nucleation is predicted to occur under realistic sulphuric acid (1.2×107 molecules cm−3) and ammonia (>5 ppt) concentrations; however, significant growth to detectable sizes (N3) only occurs for DMS concentrations of the order of 400 ppt and very low aerosol condensation sinks, but these conditions are thought to be very infrequent in the MBL and are unlikely to make a significant contribution to the general MBL aerosol concentration. It is plausible that the background MBL aerosol concentration could be maintained by a slow, almost undetectable production rate, and not by noticeable nucleation events where large enhancements in N3 concentrations are observed. The former requires sustained DMS concentrations of the order of 100 ppt which seems unlikely. In summary, the occurrence of new particles in the unperturbed MBL would be difficult to explain by DMS emissions alone. DMS emissions can explain the occurrence of thermodynamically stable sulphate clusters, but under most conditions, to grow these clusters to detectable sizes before they are scavenged by coagulation, an additional condensable species other than DMS‐derived sulphuric acid would be required. In the event, however, of significant removal of the preexisting aerosol due to precipitation, the MBL aerosol can be replenished through growth of new particle formed through ternary nucleation under moderately high DMS concentrations.

Relationships between cloud condensation nuclei spectra and aerosol particles on a south‐north transect of the Indian Ocean

Do anthropogenic emissions affect cloud microphysical properties? One parameter needed to answer the preceding question is a link between the aerosol number distribution and the cloud condensation nuclei (CCN) supersaturation spectrum for polluted and unpolluted air masses. On a south‐north cruise in the Indian Ocean in March of 1998, we found that measured CCN spectra and CCN spectra calculated from the ambient aerosol number distributions disagreed by as much as a factor of 2. The aerosol soluble fraction inferred from the relationship between measured and calculated CCN spectra ranged from 0.2 to 0.05 and exhibited a broad minimum at the Intertropical Convergence Zone (ITCZ). The concentration of CCN, non‐sea‐salt SO42−, NH4+, black carbon, and organic carbon increased by an order of magnitude as we passed through the ITCZ.

Microstructural rearrangement of sodium chloride condensation aerosol particles on interaction with water vapor

The interaction of water vapor with submicron NaCl condensation aerosols has been investigated with a tandem differential mobility analyzer (TDMA) and compared to experiments with NaCl dispersion and PbS condensation aerosols. The mobility equivalent diameters of the NaCl condensation aerosol particles decreased by up to 50% when exposed to relative humidities (RH) below the deliquescence threshold. The extent of shrinking was determined by relative humidity, initial particle size (19–200 nm) and aerosol generation parameters. Above ∼75% RH hygroscopic growth was observed in agreement with Köhler theory. Dynamic shape factors for the NaCl condensation aerosol particles before and after humidification were calculated, indicating a transformation from sparse branched-chain to more compact agglomerates. The investigated PbS particles shrank much less but still significantly when exposed to water vapor, which is discussed in view of the microstructural rearrangement processes at work.

Cloud condensation nuclei and cloud droplet measurements during ACE-2

During the 2nd Aerosol Characterization Experiment (ACE-2), relationships between stratocumulus cloud properties and aerosols were examined. Here, the relevant measurements including the cloud condensation nuclei (CCN) activation spectrum, updraft velocity, cloud microphysical and aerosol properties are presented. It is shown that calculations of droplet concentration based on updraft velocity and the CCN activation spectrum are consistent with direct observations. Also discussed is an apparent disparity among measurements of the CCN activation spectrum, the accumulation mode size distribution, and the composition of the submicrometric aerosol. The observed consistency between CCN, updraft and cloud droplets is a necessary refinement; however, extended analyses of the ACE-2 data set are needed to guide improvements in model simulations of the interaction between aerosols and cloud microphysics. In particular, there is need for an examination of aerosol size spectra and chemical composition measurements with a view towards validating droplet activation schemes which relate the aerosol and cloud dynamical properties to cloud albedo.

Six‐moment representation of multiple aerosol populations in a sub‐hemispheric chemical transformation model

This letter describes the first application of the Quadrature Method of Moments (QMOM) [McGraw, 1997] in a 3‐D chemical transformation and transport model. The QMOM simultaneously tracks an arbitrary (even) number of moments of a particle size distribution directly in space and time without the need for explicitly representing the distribution itself. The host 3‐D model, the Global Chemistry Model driven by Observation‐derived meteorological data (GChM‐O), has been previously described [Benkovitz et al., 1994]. The present implementation evolves the six lowest‐order radial moments for each of several externally‐mixed aerosol populations. From these moments we report modeled geographic distributions of several aerosol properties, including a shortwave radiative forcing obtained using the Multiple Isomomental Distribution Aerosol Surrogate (MIDAS) technique [Wright, 2000]. These results demonstrate the capabilities of these moment‐based techniques to simultaneously represent aerosol nucleation, condensation, coagulation, dry deposition, wet removal, cloud activation, and transport processes in a large scale model, and to yield aerosol optical properties and radiative influence from the modeled aerosol.

A trajectory-grid approach for solving the condensation and evaporation equations of aerosols

The trajectory-grid method (Chock et al., Atmospheric Environment 30 (1996) 857–868) for solving the transport equation in a grid model can be easily adapted to solve the condensation/evaporation equations of internally mixed multicomponent aerosols based on the sectional approach. Compared to the Bott scheme (Monthly Weather Review 117 (1998) 1006–1015, 2633–2636) often used for solving these equations, the method is significantly more accurate, of comparable speed, but gaining in speed as the number of aerosol components increases. Most notably, there is no mismatch in size change of the different components of the same aerosol. The method is quite flexible, can handle an abrupt change in aerosol size due to evaporation of fog particles, and is ideal for multiply-parallel processors.

Aerosol particle size growth by simultaneous coagulation and condensation with diffusion losses in laminar flow tubes

The growth process of evaporation–condensation aerosol particles by simultaneous Brownian coagulation and condensation in a laminar flow tube downstream the particle generation unit has been studied. Size growth has been modeled assuming that vapor concentration in the tube just after the generator is already very low (negligible self-nucleation). The model, which also accounts for particle diffusion losses, permits an easy estimation of the coagulation and condensation rates. The effect of the generator cooling temperature on the relative importance of coagulation and condensation in the after-generation particle growth process has been examined. Experiments have shown that aerosols generated by traditional evaporation–condensation methods still grow by condensation much after they have left the generator.

Experiments and computational models for aerosol behaviour in the containment

Aerosols generated by condensation of volatile fission products during nuclear reactor core meltdown accidents represent a major fraction of the accidental airborne radioactivity. A comprehensive experimental research programme was performed at Battelle to investigate the transport and deposition behaviour of aerosols in the containment, in order to support the development of computer models which estimate the fission product behaviour in the containment and the source term for potential radionuclide releases to the environment. Important steps in the investigations were: (1) DEMONA experiments. The first large scale aerosol test series performed in the Battelle model containment (BMC) (total volume 640 m 3), using an open (quasi one-room) geometry and condensation aerosols from a plasma torch generator. (2) VANAM experiments. Advanced aerosol tests in the BMC, using a multi-room geometry, mixed hygroscopic/non hygroscopic condensation aerosols, a double injection period, and varying thermohydraulic conditions. One of the experiments was subject of the International Standard Problem ISP 37. (3) KAEVER experiments. A systematic investigation of aerosol materials and mixtures and the related deposition behaviour, using a simplified one-room test vessel (10 m 3 volume) and advanced instrumentation. Important computer codes developed and/or validated in connection with the experiments are FIPLOC and NAUA; aerosol codes CONTAIN, MELCOR and GOTHIC-MAEROS were also applied. Some important results from the investigations and code developments are: (1) significant local aerosol concentration differences can occur in a multi-room geometry; (2) concentration differences can be caused by atmospheric stratification; and (3) deposition is strongly affected by material hygroscopicity and atmospheric humidity. (4) Satisfactory prediction requires a consistent treatment of multi-room thermal hydraulics, aerosol transport and steam condensation on particles. (5) Prediction results can be affected by numerical stability and nodalization (user experience). This paper presents a number of results of the experimental investigations and the present state of code modelling, with special reference to the findings of ISP37.

Vapor Condensation and Thermophoretic Aerosol Deposition of Cesium Iodide in Horizontal Thermal Gradient Pipes

The aerosol deposition test series is being performed to investigate the deposition of FP vapor and aerosol onto the inner surface of reactor coolant piping during a severe accident of a light water reactor. Vapor and aerosol of CsI as an FP simulant was introduced into the horizontal test section pipes. No substantial decomposition of CsI was identified to occur both in the high temperature inert and superheated steam environments. The comparison between the results of the aerosol deposition test series and the thermo-fluiddynamic analysis with WINDFLOW implied that a profile of the CsI deposition due to vapor condensation and thermophoretic aerosol deposition was influenced by local thermo-fluiddynamic conditions. Deposition velocities were evaluated for CsI vapor and aerosol based on the deposition characteristics of CsI and the thermo-fluiddynamic analysis.

Vapor Condensation and Thermophoretic Aerosol Deposition of Cesium Iodide in Horizontal Thermal Gradient Pipe.

The aerosol deposition test series is being performed to investigate the deposition of FP vapor and aerosol onto the inner surface of reactor coolant piping during a severe accident of a light water reactor. Vapor and aerosol of CsI as an FP simulant was introduced into the horizontal test section pipes. No substantial decomposition of CsI was identified to occur both in the high temperature inert and superheated steam environments. The comparison between the results of the aerosol deposition test series and the thermo-fluiddynamic analysis with WINDFLOW implied that a profile of the CsI deposition due to vapor condensation and thermophoretic aerosol deposition was influenced by local thermo-fluiddynamic conditions. Deposition velocities were evaluated for CsI vapor and aerosol based on the deposition characteristics of CsI and the thermo-fluiddynamic analysis.

Cloud condensation nuclei (CCN) and aerosol mass size distributions at an European background and an urban site

Cloud condensation nuclei (CCN) and aerosol mass size distributions at an European background and an urban site

A Continuous Monitoring System for Strong Acidity in Aerosols

Inhalation of acid aerosols is believed to be a major source of respiratory ailments. A fully automated instrument that can measure acidity levels meaningful to real situations is described. A 5−10 L/min sampling rate is used. First, coarse particles are removed by a cyclone, followed by the removal of acid and basic gases with a parallel plate wet denuder. Aerosols are then collected with a vapor condensation aerosol collection system (VCACS). The VCACS provides an aqueous liquid effluent in which all soluble constituents are present in the dissolved form and the undissolved material remains suspended. The effluent is concentrated sequentially on a cation exchanger and an anion exchanger, which respectively constitute the injection loops of cation and anion analysis subsystems. In the first system, non-H+-cations (primarily NH4+) are conductometrically determined as the corresponding hydroxide by elution with a strong acid plug and conductivity suppression with a hydroxide-form anion exchanger. In the se...

In situ analysis of ash deposits from black liquor combustion

Aerosols formed during combustion of black liquor cause a significant fire-side fouling problem in pulp mill recovery boilers. (Black liquor is a recycled by-product formed during the pulping of wood in the paper-making industry). The ash deposits reduce heat transfer effectiveness, plug gas passages, and contribute to corrosion. Both vapors and condensation aerosols lead to the formation of such deposits. The high ash content of the fuel and the low dew point of the condensate salts lead to a high aerosol and vapor concentration in most boilers. In situ measurements of the chemical composition of these deposits is an important step in gaining a fundamental understanding of the deposition process. Infrared emission spectroscopy is used to characterize the composition of thin film deposits resulting from the combustion of black liquor and the deposition of submicron aerosols and vapors. New reference spectra of Na 2SO 4, K 2SO 4, Na 2CO 3 and K 2CO 3 pure component films were recorded and compared with the spectra of the black liquor deposit. All of the black liquor emission bands were identified using our new reference spectra as well as literature data and ab initio calculations. The ab initio calculations predict the locations and intensities of infrared bands for the alkali-containing vapors of interest.

A balloon‐borne cloud condensation nuclei counter

A balloon‐borne instrument was constructed for observations of vertical profiles of cloud condensation nucleus (CCN) concentrations, active at 1% supersaturation. Droplet concentration in the static thermal‐gradient diffusion chamber is deduced from the amount of scattered laser light detected by a photodetector. The photodetector is calibrated using a video camera and computer system to count the number of droplets produced from NaCl aerosol. Preliminary data are available from nine early morning profiles obtained at Laramie, Wyoming, between June 1995 and January 1997. To complement the CCN measurements, instruments that measure condensation nuclei (CN) and aerosols with diameter greater than 0.30 μm (D0.3) were also included on the balloon package. CCN concentrations exhibited a general decrease from the surface to the top of the boundary layers, were generally uniform through well‐mixed layers, and show variability above well‐mixed layers. In general, the structure of the CCN profile appears to be closely related to the structure in the CN and D0.3 profiles. Summer profiles generally have CCN concentration greater than 200 cm−3 up to 500 mbar, whereas winter profiles are less than 200 cm−3 at all levels.

Household insecticides: evaluation and assessment of inhalation toxicity: a workshop summary

Particularly in tropical countries household insecticides are used on a day-by-day basis to control mosquitoes, other crawling and flying insects to prevent the spread of vector-borne diseases. The products used most often are spray-cans, oil-sprays, mosquito coils as well as slow-release vaporising systems such as mats and liquid vaporiser. The extent and duration of exposure of humans is highly dependent on the type of product used. The objective of this workshop was to analyse the necessity and feasibility of inhalation studies with household insecticides taking into account the specific constrains associated with each type of end-use product. The standardisation of inhalation studies with regard to the generation of test atmospheres, mode and duration of exposure, and selection of adequate toxicological endpoints were addressed. Due to the complex nature of exposure atmospheres generated by some household insecticides, viz. mosquito coils, it is scientifically challenging to characterise the pathomechanism of most concern, since irritant combustion gases, volatile and semi-volatile organic substances, particulates (soot), condensation aerosols and re-condensed substances onto particulates may act independently, synergistically or mixture specific. It has been concluded that for the comparative safety evaluation and risk assessment of indoor insecticide end-use products generally recognised guidance for harmonised inhalation testing is required: 1) For high-dose release products, such as spray-cans, acute inhalation testing appears to be most relevant. 2) For low-dose, slow-release devices, subchronic inhalation studies of 13-weeks, duration of exposure 6 hours/day for 5 consecutive days per week, should be performed on rats preferably with the end-use product. A dose-range finding study of 2-weeks duration, daily exposure, should be available for the justification of dose selection and to demonstrate that the findings of 5 days/week exposure is not different from that utilising a daily exposure. 3) In certain instances, inhalation testing of shorter duration, for example 2 or 4 weeks, may be appropriate if the inhalation toxicity of the basic formula has already been characterised or the major constituents contained in the formulation have been examined in extant studies. The duration of study should also depend on actual use and exposure pattern, the accumulation potential and the mode of action of the ingredients of concern. 4) Particularly for mosquito coil smokes, the possible physico-chemical interactions of particulates, potentially irritant water soluble or nonsoluble vapours and aerosol or vapours of the active ingredient(s) are virtually impossible to predict without appropriate inhalation testing. 5) Physiological measurements may be useful for the detection and quantification of the respiratory tract sensory irritation potency. Biochemical endpoints in lung lavage and conventional histopathology of the lungs and extrathoracic airways are considered to be important. 6) It has been agreed that the nose-only exposure technique provides the most cost-effective and controlled means to expose small laboratory animals to well defined atmospheres as long as the limitations of this technique are taken into account.

Vertical profiles of Volcanic Aerosol and Polar Stratospheric Clouds Above Kiruna, Sweden: Winters 1993 and 1995

Vertical profiles of aerosol were measured in February 1993, and January - March 1995 using balloon-borne particle counters released from Kiruna, Sweden. Condensation nuclei (CN) and aerosol with radii ≥ 0.15 - 10.0 µm were measured in 8-12 size classes. The three flights in 1993 were within the polar vortex. Temperatures were below polar stratospheric cloud (PSC) threshold temperatures on one flight and a thin PSC was observed. The volcanic aerosol in the 1993 vortex was similar to that in 1992. In 1993, surface areas were 10 - 20 µm2 cm-3 and volumes 1 - 3 µm3 cm-3. In 1995 three of five flights were within the polar vortex. The volcanic aerosol had decreased to 3 - 7 µm2 cm-3 and 0.1 - 0.4 µm3 cm-3. The top of the volcanic aerosol layer in both years was near 500 K potential temperature (~20 km). A thick nitric acid and water PSC was observed in January 1995. In the thickest region of this PSC nearly all CN were observed to be activated, and surface areas of 5 - 10 µm2 cm-3 were calculated. The volumes observed in this PSC were closer to what would be expected for particles composed of nitric acid trihydrate than for ternary solution droplets. In 1993 the opposite was observed, the volumes in the thin PSC were closer to what would be expected for ternary solution droplets.

Pulmonary cystic keratinizing squamous cell lesions of rats after inhalation/instillation of different particles

SummaryCystic keratinizing squamous cell lesions from three inhalation studies (Study A, B, C) and one intratracheal instillation study (Study D) in rats were reclassified and a certain number of lesions examined immunohistochemically for PCNA (proliferating cell nuclear antigen) as a marker of cellular proliferation. The following classification was used: squamous cell metaplasia with marked keratinization, keratinizing cyst, cystic keratinizing epithelioma, cystic keratinizing squamous cell carcinoma, keratinizing squamous cell carcinoma and non-keratinizing squamous cell carcinoma. In study A (inhalation of coal oven exhaust and subcutaneous injection of a high dose of DB (ah)A) 49.3 % of rats developed cystic keratinizing squamous cell carcinomas. Inhalation of coal oven exhaust gas together with intratracheal instillation of crocidolite or subcutaneous injection of a low dose DB(ah)A (dibenz(ah)anthracene) resulted in cystic keratinizing squamous cell carcinomas in 23 % to 24 % of the rats. High incidences of cystic squamous cell carcinomas in the range of 31.9 % to 76.4 % were observed in rats of Study B1 after a 10-months exposure to tar/pitch condensation aerosol (different B(a)P (benzo(a)pyrene) concentrations) with added carbon black in some groups. After a 20-months exposure period to the same inhalation atmospheres (Study B2) the incidence of squamous cell carcinomas was increased up to 95.8 %. Exposure of rats to various concentrations of unfiltered diesel exhaust (Study C) resulted in incidences of cystic keratinizing epitheliomas ranging from 2.5 % (2.5 mg/m3) to 10.7 % (7.5 mg/m3). Epitheliomas were also observed in 16.2 % of carbon black and 16.0 % of titanium dioxide exposed rats. Only a few cystic keratinizing squamous cell carcinomas occurred. In the intratrachel instillation study (Study D) increased incidences of cystic keratinizing epitheliomas occurred in rats exposed to native diesel exhaust particles (16.7 %), high dose of extracted diesel exhaust particles (14.6 %), extracted printex 90-carbon black particles (18.8 %), and extracted printex 90-carbon black particles + B(a)P (18.8 %). High indicences of cystic keratinizing squamous cell carcinomas were noted in rats that received 15 mg B(a)P (14.6 %) or 30 mg B(a)P (72.7 %) intratracheally. Immunohistochemical labeling of nuclei with PCNA demonstrated proliferative activity in one or two (and focally more than two) peripheral cell layers of cystic keratinizing epitheliomas and in more than three peripheral cell layers of cystic keratinizing squamous cell carcinomas and keratinizing squamous cell carcinomas. The wall of keratinizing cysts showed no or a weak reaction.

Correlation of Plume Opacity with Particles and Sulfates from Boilers

The effects of emission concentrations of particulate matters and sulfates on plume opacity are investigated by in situ measurements. The studies are conducted for three processes of two coal-fired plants and one oil-fired that are all equipped with electrostatic precipitators. Flue-gas sampling and analysis include the concentrations of particles and total water soluble sulfates, particle size distribution, and flue-gas composition; while in-stack and out-of-stack opacities are determined by a transmissometer and certified smoke inspectors, respectively. Experimental results show that plume opacity outside the stack linearly correlates well with the in-stack opacity. The mixing of hot flue gas with cold ambient air would result in the condensation of hygroscopic sulfuric acid aerosols and an increase about 1.6% out of typical 15--25% measured opacity. An empirical equation similar to the Beer-Lambert-Bouger form is derived for predicting the plume opacity in terms of the stack diameter and the concentrations of particles and total water soluble sulfates. Good comparisons are achieved between predictions by the empirical equation and other available field data.