Nanoparticle and ultrafine particle events at the Fresno supersite

Continuous measurements of particle size distributions of 3-407 nm were collected from August 2002 to July 2004 at the Fresno Supersite to understand their number concentrations, size distributions, and formation processes. Measurements for fine particulate matter (PM2.5) mass, sulfate (SO4(2-)), nitrate (NO3-), black carbon (BC), particle-bound polycyclic aromatic hydrocarbons (PAHs), nitrogen oxides (NOx), carbon monoxide (CO), ozone (O3), and meteorological data (wind speed, wind direction, temperature [T], relative humidity [RH], and solar radiation) were used to determine the causes of nanoparticle (3-10 nm) and ultrafine (10-100 nm) particle events. These events were found to be divided into four types: (1) 3- to 10-nm morning nucleation; (2) 10- to 30-nm morning traffic; (3) 10- to 30-nm afternoon photochemical; and (4) 50- to 84-nm evening home heating, including residential wood combustion. Intense examples of the first type (>10(4) number [#]/cm3) were observed on 29 days, nearly always during the summer. The second type of event was observed on more than 73 days and occurred throughout the year. The third type was observed on 36 days, from spring through summer. The fourth type was found on 109 days, all of them during the winter. Although sulfur dioxide (SO2) emissions in Central California are low, the small residual amounts in gasoline and diesel fuel are apparently sufficient to initiate nucleation events. These were measured in the morning, soon after the shallow surface inversion coupled with layers aloft where nucleation probably was initiated. PM2.5 concentrations were poorly correlated with nanoparticle number.     

Size distribution and diurnal and seasonal trends of ultrafine particles in source and receptor sites of the Los Angeles basin

This paper presents results from a study conducted in two urban areas of southern California, Downey and Riverside, to examine the effect of different sources and formation mechanisms on the size distribution and temporal trends of ultrafine particles. Near-continuous data were collected for 5 months at each location. Our data clearly identified Downey as a source site, primarily affected by vehicular emissions from nearby freeways, and Riverside as a receptor site, where photochemical secondary reactions form a substantial fraction of particles, along with local vehicular emissions. In Downey, the diurnal trends of total particle number concentration and elemental carbon (EC) appear to be almost identical throughout the day and irrespective of season, thereby corroborating the role of primary emissions in the formation of these particles. This agreement between EC and particle number was not observed in Riverside during the warmer months of the year, while very similar trends to Downey were observed during the winter months in that area. Similarly, the size distribution of ultrafine particles in Downey was generally unimodal with a mode diameter of 30-40 nm and without significant monthly variations. The number-based particle size distributions obtained in Riverside were bimodal, with a significant increase in accumulation mode as the season progressed from winter to summer. During the warmer months, there was also an increase in sub-100-nm particles in the afternoon hours, between 2:00 p.m. and 4:00 p.m., that also increased with the temperature. The differences observed in the ultrafine particle distribution and temporal trends clearly demonstrated that mechanisms other than direct emissions play an important role in the formation of ultrafine particles in receptor sites of the Los Angeles Basin.     

Aerosol particle number concentration measurements in five European cities using TSI-3022 condensation particle counter over a three-year period during health effects of air pollution on susceptible subpopulations

In this study, long-term aerosol particle total number concentration measurements in five metropolitan areas across Europe are presented. The measurements have been carried out in Augsburg, Barcelona, Helsinki, Rome, and Stockholm using the same instrument, a condensation particle counter (TSI model 3022). The results show that in all of the studied cities, the winter concentrations are higher than the summer concentrations. In Helsinki and in Stockholm, winter concentrations are higher by a factor of two and in Augsburg almost by a factor of three compared with summer months. The winter maximum of the monthly average concentrations in these cities is between 10,000 cm(-3) and 20,000 cm(-3), whereas the summer min is approximately 5000-6000 cm(-3). In Rome and in Barcelona, the winters are more polluted compared with summers by as much as a factor of 4-10. The winter maximum in both Rome and Barcelona is close to 100,000 cm(-3), whereas the summer minimum is > 10,000 cm(-3). During the weekdays the maximum of the hourly average concentrations in all of the cities is detected during the morning hours between 7 and 10 a.m. The evening maxima were present in Barcelona, Rome, and Augsburg, but these were not as pronounced as the morning ones. The daily maxima in Helsinki and Stockholm are close or even lower than the daily minima in the more polluted cities. The concentrations between these two groups of cities are different with a factor of about five during the whole day. The study pointed out the influence of the selection of the measurement site and the configuration of the sampling line on the observed concentrations.     

Winter and summer characteristics of airborne particles inside emperor Qin's Terra-Cotta Museum, China: a study by scanning electron microscopy-energy dispersive X-ray spectrometry

Day- and nighttime total suspended particulate matter was collected inside and outside Emperor Qin's Terra-Cotta Museum in winter and summer 2008. The purpose was to characterize the winter and summer differences of indoor airborne particles in two display halls with different architectural and ventilation conditions, namely the Exhibition Hall and Pit No. 1. The morphology and elemental composition of two season samples were investigated using scanning electron microscopy and energy dispersive X-ray spectrometry. It is found that the particle size, particle mass concentration, and particle type were associated with the visitor numbers in the Exhibition Hall and with the natural ventilation in Pit No. 1 in both winter and summer. Evident winter and summer changes in the composition and physicochemical properties of the indoor suspended particulate matters were related to the source emission and the meteorological conditions. Particle mass concentrations in both halls were higher in winter than in summer. In winter, the size of the most abundant particles at the three sites were all between 0.5 and 1.0 microm, whereas in summer the peaks were all located at less than 0.5 microm. The fraction of sulfur-containing particles was 2-7 times higher in winter than in summer. In addition to the potential soiling hazard, the formation and deposition of sulfur-containing particles in winter may lead to the chemical and physical weathering of the surfaces of the terra-cotta statues.     

Characteristics of regional nucleation events in urban East St. Louis

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

Regional transport and urban contributions to fine particle concentrations in southeastern Canada

The impact of various atmospheric transport directions on ambient fine particle (PM2.5) concentrations at several sites in southeastern Canada was estimated (for May-September) using back-trajectory analysis. Three-day back trajectories (four per day) were paired with 6-hr average PM2.5 mass concentrations measured using tapered element oscillating microbalances (TEOM). PM2.5 concentrations at rural locations in the region were affected by nonlocal sources originating in both Canada and the United States. Comparison of sites revealed that, on average, the local contribution to total PM2.5 in the greater Toronto area (GTA) is approximately 30-35%. At each location, average PM2.5 concentrations under south/southwesterly flow conditions were 2-4 times higher than under the corresponding northerly flow conditions. The chemical composition of both urban and rural PM2.5 was determined during two separate 2-week spring/summer measurement campaigns. Components identified included SO4(2-) NO3-, NH4+, black carbon and organic carbon (OC), and trace elements. Higher particle mass at the urban Toronto site was composed of a higher proportion of all components. However, black carbon, NO3-, NaCl, and trace elements were found to be the most enriched over the rural/regional background levels.     

Elk River Completes Second Year With Zero Bag Failures Reported

Abstract

This paper presents results from a study conducted in two urban areas of southern California, Downey and Riverside, to examine the effect of different sources and formation mechanisms on the size distribution and temporal trends of ultrafine particles. Near-continuous data were collected for 5 months at each location. Our data clearly identified Downey as a source site, primarily affected by vehicular emissions from nearby freeways, and Riverside as a receptor site, where photochemical secondary reactions form a substantial fraction of particles, along with local vehicular emissions. In Downey, the diurnal trends of total particle number concentration and elemental carbon (EC) appear to be almost identical throughout the day and irrespective of season, thereby corroborating the role of primary emissions in the formation of these particles. This agreement between EC and particle number was not observed in Riverside during the warmer months of the year, while very similar trends to Downey were observed during the winter months in that area. Similarly, the size distribution of ultrafine particles in Downey was generally unimodal with a mode diameter of 30–40 nm and without significant monthly variations. The number-based particle size distributions obtained in Riverside were bimodal, with a significant increase in accumulation mode as the season progressed from winter to summer. During the warmer months, there was also an increase in sub-100-nm particles in the afternoon hours, between 2:00 p.m. and 4:00 p.m., that also increased with the temperature. The differences observed in the ultrafine particle distribution and temporal trends clearly demonstrated that mechanisms other than direct emissions play an important role in the formation of ultrafine particles in receptor sites of the Los Angeles Basin.     

Measurement of ultrafine particle size distributions from coal-, oil-, and gas-fired stationary combustion sources

Currently, we have limited knowledge of the physical and chemical properties of emitted primary combustion aerosols and the changes in those properties caused by nucleation, condensation growth of volatile species, and particle coagulations under dilution and cooling in the ambient air. A dilution chamber was deployed to sample exhaust from a pilot-scale furnace burning various fuels at a nominal heat input rate of 160 kW/h(-1) and 3% excess oxygen. The formation mechanisms of particles smaller than 420 nm in electrical mobility diameter were experimentally investigated by measurement with a Scanning Mobility Particle Sizer (SMPS) as a function of aging times, dilution air ratios, combustion exhaust temperatures, and fuel types. Particle formation in the dilution process is a complex mixture of nucleation, coagulation, and condensational growth, depending on the concentrations of available condensable species and solid or liquid particles (such as soot, ash) in combustion exhausts. The measured particle size distributions in number concentrations measured show peaks of particle number concentrations for medium sulfur bituminous coal, No. 6 fuel oil, and natural gas at 40-50 nm, 70-100 nm, and 15-25 nm, respectively. For No. 6 fuel oil and coal, the particle number concentration is constant in the range of a dilution air ratio of 50, but the number decreases as the dilution air ratio decreases to 10. However, for natural gas, the particle number concentration is higher at a dilution air ratio of 10 and decreases at dilution air ratios of 20-50. At a dilution air ratio of 10, severe particle coagulation occurs in a relatively short time. Samples taken at different combustion exhaust temperatures for these fuel types show higher particle number concentrations at 645 K than at 450 K. As the aging time of particles increases, the particles increase in size and the number concentrations decrease. The largest gradient of particle number distribution occurs within the first 10 sec after dilution but shows only minor differences between 10 and 80 sec. The lifetimes of the ultrafine particles are relatively short, with a scale on the order of a few seconds. Results from this study suggest that an aging time of 10 sec and a dilution air ratio of 20 are sufficient to obtain representative primary particle emission samples from stationary combustion sources.     

Health Effects and Energy Choices

ABSTRACT

Day- and nighttime total suspended particulate matter was collected inside and outside Emperor Qin's Terra-Cotta Museum in winter and summer 2008. The purpose was to characterize the winter and summer differences of indoor airborne particles in two display halls with different architectural and ventilation conditions, namely the Exhibition Hall and Pit No. 1. The morphology and elemental composition of two season samples were investigated using scanning electron microscopy and energy dispersive X-ray spectrometry. It is found that the particle size, particle mass concentration, and particle type were associated with the visitor numbers in the Exhibition Hall and with the natural ventilation in Pit No. 1 in both winter and summer. Evident winter and summer changes in the composition and physicochemical properties of the indoor suspended particulate matters were related to the source emission and the meteorological conditions. Particle mass concentrations in both halls were higher in winter than in summer. In winter, the size of the most abundant particles at the three sites were all between 0.5 and 1.0 μm, whereas in summer the peaks were all located at less than 0.5 μm. The fraction of sulfur-containing particles was 2–7 times higher in winter than in summer. In addition to the potential soiling hazard, the formation and deposition of sulfur-containing particles in winter may lead to the chemical and physical weathering of the surfaces of the terra-cotta statues.

IMPLICATIONS Relatively few microanalyses of indoor airborne particles inside museums in China have been performed. The mass concentrations of winter and summer indoor total suspended particulate (TSP) are presented for the Emperor Qin's Terra-Cotta Museum along with the characterization of individual particles via computerized scanning electron microscopy and an energy dispersive X-ray spectrometry. The results indicate that both the high level of particle mass concentrations in winter and the predominant fine particles in summer were responsible for the soiling of the statues over a long period of time. The irreplaceable statues were also facing with weathering hazards posed by abundant sulfur-containing particles, especially in winter. Data provided in this study suggested that mechanical ventilation, air filtration, and air conditioning systems should be installed in Pit No. 1 display hall and proper display strategy should be adopted in Exhibition Hall.     

Investigation of the motor vehicle exhaust contribution to primary fine particle organic carbon in urban air

Motor vehicle (MV) emissions and ambient particle concentrations under a variety of situations were studied in Toronto and Vancouver, Canada. Petroleum biomarkers (i.e., hopanes and steranes) were used to determine the fraction of fine particle organic carbon (OC) attributed to primary particles in MV exhaust. Source profiles obtained from a tunnel and from direct tailpipe emissions were applied to ambient measurements at locations ranging from rush hour traffic to a regional background site. The greatest amount of MV OC, 4.0 μgC m⁻³ out of 9.1 μgC m⁻³ or 43%, was observed 75 m south of a commuter highway during a period that included morning rush hour. Monthly estimates of MV-OC were determined for a downtown Toronto monitoring site for 2 years. Total OC concentrations were greater in the summer, due to secondary OC, but the amount of MV-OC did not exhibit a strong seasonal pattern. However, on a per cent basis, MV contributions from primary OC emissions were greatest in the winter (15–20%) and smallest in the summer (10–15%) with a two-year average of 14% of the OC or about 5% of the PM_2.5.     

Particulate matter in California: part 2--Spatial, temporal, and compositional patterns of PM2.5, PM10-2.5, and PM10

Geographic and temporal variations in the concentration and composition of particulate matter (PM) provide important insights into particle sources, atmospheric processes that influence particle formation, and PM management strategies. In the nonurban areas of California, annual-average PM2.5 and PM10 concentrations range from 3 to 10 microg/m3 and from 5 to 18 microg/m3, respectively. In the urban areas of California, annual-averages for PM2.5 range from 7 to 30 microg/m3, with observed 24-hr peaks reaching levels as high as 160 microg/m3. Within each air basin, exceedances are a mixture of isolated events as well as periods of elevated PM2.5 concentrations that are more prolonged and regional in nature. PM2.5 concentrations are generally highest during the winter months. The exception is the South Coast Air Basin, where fairly high values occur throughout the year. Annual-average PM2.5 mass, as well as the concentrations of major components, declined from 1988 to 2000. The declines are especially pronounced for the sulfate (SO4(2-)) and nitrate (NO3-) components of PM2.5 and PM10) and correlate with reductions in ambient levels of oxides of sulfur (SOx) and oxides of nitrogen (NOx). Annual averages for PM10-2.5 and PM10 exhibited similar downwind trends from 1994 to 1999, with a slightly less pronounced decrease in the coarse fraction.     

Effect of the fine fraction of particulate matter versus the coarse mass and other pollutants on daily mortality in Santiago, Chile

Daily counts of non-accidental deaths in Santiago, Chile, from 1988 to 1996 were regressed on six air pollutants--fine particles (PM2.5), coarse particles (PM10-2.5), CO, SO2, NO2, and O3. Controlling for seasonal and meteorological conditions was done using three different models--a generalized linear model, a generalized additive model, and a generalized additive model on previously filtered data. Single- and two-pollutant models were tested for lags of 1-5 days and the average of the previous 2-5 days. The increase in mortality associated with the mean levels of air pollution varied from 4 to 11%, depending on the pollutants and the way season of the year was considered. The results were not sensitive to the modeling approaches, but different effects for warmer and colder months were found. Fine particles were more important than coarse particles in the whole year and in winter, but not in summer. NO2 and CO were also significantly associated with daily mortality, as was O3 in the warmer months. No consistent effect was observed for SO2. Given particle composition in Santiago, these results suggest that combustion-generated pollutants, especially from motor vehicles, may be associated with increased mortality. Temperature was closely associated with mortality. High temperatures led to deaths on the same day, while low temperatures lead to deaths from 1 to 4 days later.     

The influences of ambient particle composition and size on particle infiltration in Los Angeles, CA, residences

Particle infiltration is a key determinant of the indoor concentrations of ambient particles. Few studies have examined the influence of particle composition on infiltration, particularly in areas with high concentrations of volatile particles, such as ammonium nitrate (NH4NO3). A comprehensive indoor monitoring study was conducted in 17 Los Angeles-area homes. As part of this study, indoor/outdoor concentration ratios during overnight (nonindoor source) periods were used to estimate the fraction of ambient particles remaining airborne indoors, or the particle infiltration factor (FINF), for fine particles (PM2.5), its nonvolatile (i.e., black carbon [BC]) and volatile (i.e., nitrate [NO3-]) components, and particle sizes ranging between 0.02 and 10 microm. FINF was highest for BC (median = 0.84) and lowest for NO3- (median = 0.18). The low FINF for NO3- was likely because of volatilization of NO3- particles once indoors, in addition to depositional losses upon building entry. The FINF for PM2.5 (median = 0.48) fell between those for BC and NO3-, reflecting the contributions of both particle components to PM25. FINF varied with particle size, air-exchange rate, and outdoor NO3- concentrations. The FINF for particles between 0.7 and 2 microm in size was considerably lower during periods of high as compared with low outdoor NO3- concentrations, suggesting that outdoor NO3- particles were of this size. This study demonstrates that infiltration of PM2.5 varies by particle component and is lowest for volatile species, such as NH4NO3. Our results suggest that volatile particle components may influence the ability for outdoor PM concentrations to represent indoor and, thus, personal exposures to particles of ambient origin, because volatilization of these particles causes the composition of PM2.5 to differ indoors and outdoors. Consequently, particle composition likely influences observed epidemiologic relationships based on outdoor PM concentrations, especially in areas with high concentrations of NH4NO3 and other volatile particles.     

Aerosols in a Mediterranean forest: sulfates, particle size distribution, and growth rates

Particle size distribution measurements in 16 nonlinear intervals covering the 0.1-7.5 microns range and concurrent sulfate concentrations were continuously recorded in September 1996 over the period of two weeks in a Mediterranean forest. Sulfate size distribution was derived from a linear correlation fit between the concentrations and the number of particles recorded at each particle size interval. The results revealed two modal diameters for sulfates, with typical diameters at 0.3 and 0.675 micron. These results were associated with two different dominant chemical mechanisms governing sulfate formation. In order to describe the dominant chemical mechanism, the growth law approach was applied. Growth rates were determined using the parameter estimates of the fitted particle size distribution function. By matching these data with sulfate concentrations, the dominant chemical reactions were identified. The results have shown that sulfate formation is governed by both homogeneous and heterogeneous reactions and that the latter process was dominant. Condensation reactions prevailed in the early morning and late afternoon, and volume reactions at night, particularly in high-moisture conditions. From the observational data, the gas-to-particle conversion rate for sulfur dioxide (SO2) at nighttime was also derived, yielding a 2.18%/hr-1.     

Measurement of aerosol number size distributions in the Yangtze River delta in China: Formation and growth of particles under polluted conditions

Particle size distribution is important for understanding the sources and effects of atmospheric aerosols. In this paper we present particle number size distributions (10 nm–10 μm) measured at a suburban site in the fast developing Yangtze River Delta (YRD) region (near Shanghai) in summer 2005. The average number concentrations of ultrafine (10–100 nm) particles were 2–3 times higher than those reported in the urban areas of North America and Europe. The number fraction of the ultrafine particles to total particle count was also 20–30% higher. The sharp increases in ultrafine particle number concentrations were frequently observed in late morning, and the particle bursts on 5 of the 12 nucleation event days can be attributed to the homogeneous nucleation leading to new particle formation. The new particle formation events were characterized with a larger number of nucleation-mode particles, larger particle surface area, and larger condensational sink than usually reported in the literature. These suggest an intense production of sulfuric acid from photo-oxidation of sulfur dioxide in the YRD. Overall, the growth rate of newly formed particles was moderate (6.4 ± 1.6 nm h^?1), which was comparable to that reported in the literature.     

Variation of particle number and mass concentration in various size ranges of ambient aerosols in Eastern Germany

There is an ongoing debate on the question which size fraction of particles in ambient air may be responsible for short-term responses of the respiratory system as observed in several epidemiological studies. However, the available data on ambient particle concentrations in various size ranges are not sufficient to answer this question.Therefore, on 180 days during the winter 1991/92 daily mean size distributions of ambient particles were determined in. Erfurt, a city in Eastern Germany. In the range 0.01–0.3 μm particles were classified by an electrical mobility analyzer and in the range 0.1–2.5 μm by an optical particle counter. From the derived size distributions, number and mass concentrations were calculated.The mean number concentration over this period of time was governed by particles smaller than 0.1 μm (72%), whereas the mean mass concentration was governed by particles in the size range 0.1–0.5 pm (83%). The contribution of particles larger than 0.5 μm to the overall number concentration was negligible and so was the contribution of particles smaller than 0.1 μm to the overall mass concentration. Furthermore, total number and mass concentrations in the range 0.01–2.5 μm were poorly correlated.The results suggest that particles larger than 2.5 μm (or even larger than 0.5 μm) are rare in the European urban environment so that the inhalation of these particles is probably not relevant for human health. Since particle number and mass concentrations can be considered poorly correlated variables, more insight into health-related aspects of particulate air pollution will be obtained by correlating respiratory responses with mass and number concentrations of ambient particles below 0.5 μm.     

Composition of wet and bulk deposition in Erzurum, Turkey

Seasonal variations in the chemical characteristics of wet and bulk deposition samples collected in Erzurum were investigated for the period March 2002-January 2003. Major cations (Ca2+, K+, Mg2+) and major anions (SO4(2-),NO3-) were determined in bulk and wet deposition samples; pH was also measured in wet deposition. The average pH of the wet deposition at Erzurum was 6.6 due to extensive neutralization of the acidity. A strong relationship between pH and SO4(2-) concentrations was observed in all seasons; however, only a weak relationship was found between pH and NO3-. On a seasonal basis, the correlation between Ca2+ and SO4(2-) concentrations was stronger in winter than in summer. Seasonal variations of ions were examined in both wet and bulk deposition samples. Although maximum concentrations of anions generally occurred during winter and spring, cation concentrations peaked in summer for both wet and bulk deposition. Results indicated that Ca2+ was the dominant cation and SO4(2-) the dominant anion in all deposition samples at Erzurum. Even though correlations among the crustal ions (calcium, magnesium and potassium) were high, the relationship between anthropogenic ions (sulfate and nitrate) was less clear in bulk deposition.     

Concentrations of ultrafine,fine and PM2.5 particles in three European cities

Total number concentrations, number concentrations of ultrafine (0.01–0.1 μm) and accumulation (0.1–0.5 μm) particles, as well as mass concentration of PM_2.5 particles and blackness of PM_2.5 filters, which is related to Black Smoke were simultaneously monitored in three European cities during the winter period for three and a half months. The purpose of the study was to describe the differences in concentration levels and daily and diurnal variations in particle number and mass concentrations between European cities. The results show statistically significant differences in the concentrations of PM_2.5 and the blackness of the PM_2.5 filters between the cities, but not in the concentrations of ultrafine particles. Daily PM_2.5 levels were found to be poorly correlated with the daily total and ultrafine number concentrations but better correlated with the number concentration of accumulation particles. According to the principal component analysis airborne particulate pollutants seem to be divided into two major source categories, one identified with particle number concentrations and the other related to mass-based information. The present results underline the importance of using both particle number and mass concentrations to evaluate urban air quality.     

The impact of biomass burning on the environmental aerosol concentration in Gaborone,Botswana

Biomass burning, in the form of savanna fires and firewood for cooking and warmth, is widespread during the dry winter months in Southern Africa. This study was carried out to investigate its impact on the environment in Gaborone, Botswana, which is a small-sized city with very little pollution from industrial sources. Measurements of aerosol size and number concentrations were carried out at the University of Botswana campus in Gaborone from September 1999 to July 2000 using two automatic laser scattering particle counters. Particles were monitored in eight size ranges from 0.1 to 5.0 μm. The mean daily particle concentrations were found to vary from about 200 cm⁻³ on clear visibility days during the summer to a high of over 9000 cm⁻³ on cold winter evenings, when there was a significant smoke haze over the city. Particle concentrations were noticeably higher during the winter than in the summer. During a typical winter day, the total particle concentration peaked between 18 and 23 h, often showing an increase of over four-fold from mid-morning minimum values. The aerosol number size distributions under various conditions were investigated and the corresponding surface area and volume distributions were derived. In general, both the surface and volume distributions were bimodal with peaks close to 0.2 μm and at 5.0 μm or greater. A hand-held counter with a minimum detectable particle size of 0.3 μm was used to monitor the size and number concentrations of aerosols across the city. The results indicate a consistent pattern of maximum concentration in the highly populated areas close to the city centre, falling significantly in the sparsely populated outlying areas by up to an order of magnitude during peak biomass burning, suggesting that much of the smoke particles in the city are removed by wind.