Modeling particle loss in ventilation ducts

Empirical equations were developed and applied to predict losses of 0.01–100 μm airborne particles making a single pass through 120 different ventilation duct runs typical of those found in mid-sized office buildings. For all duct runs, losses were negligible for submicron particles and nearly complete for particles larger than 50 μm. The 50th percentile cut-point diameters were 15 μm in supply runs and 25 μm in return runs. Losses in supply duct runs were higher than in return duct runs, mostly because internal insulation was present in portions of supply duct runs, but absent from return duct runs. Single-pass equations for particle loss in duct runs were combined with models for predicting ventilation system filtration efficiency and particle deposition to indoor surfaces to evaluate the fates of particles of indoor and outdoor origin in an archetypal mechanically ventilated building. Results suggest that duct losses are a minor influence for determining indoor concentrations for most particle sizes. Losses in ducts were of a comparable magnitude to indoor surface losses for most particle sizes. For outdoor air drawn into an unfiltered ventilation system, most particles smaller than 1 μm are exhausted from the building. Large particles deposit within the building, mostly in supply ducts or on indoor surfaces. When filters are present, most particles are either filtered or exhausted. The fates of particles generated indoors follow similar trends as outdoor particles drawn into the building.     

The effects of ozone/limonene reactions on indoor secondary organic aerosols

An indoor air quality model was used to predict dynamic particle mass concentrations based on homogeneous chemical mechanisms and partitioning of semi-volatile products to particles. The ozone–limonene reaction mechanism was combined with gas-phase chemistry of common atmospheric organic and inorganic compounds and incorporated into the indoor air quality model. Experiments were conducted in an environmental chamber to investigate secondary particle formation resulting from ozone/limonene reactions. Experimental results indicate that significant fine particle growth occurs due to the interaction of ozone and limonene and subsequent intermediate by-products. Secondary particle mass concentrations were estimated from the measured particle size distribution. Predicted particle mass concentrations were in good agreement with experimental results—generally within ∼25% at steady-state conditions. Both experimental and predicted results suggest that air exchange rate plays a significant role in determining secondary fine particle levels in indoor environments. Secondary particle mass concentrations are predicted to increase substantially with lower air exchange rates, an interesting result given a continuing trend toward more energy efficient buildings. Lower air exchange rates also shifted the particle size distribution toward larger particle diameters. Secondary particle mass concentrations are also predicted to increase with higher outdoor ozone concentrations, higher outdoor particle concentrations, higher indoor limonene emission rates, and lower indoor temperatures.     

Characteristics of Indoor Aerosols in Residential Homes in Urban Locations: A Case Study in Singapore

Abstract

As part of a major study to investigate the indoor air quality in residential houses in Singapore, intensive aerosol measurements were made in an apartment in a multistory building for several consecutive days in 2004. The purpose of this work was to identify the major indoor sources of fine airborne particles and to assess their impact on indoor air quality for a typical residential home in an urban area in a densely populated country. Particle number and mass concentrations were measured in three rooms of the home using a real-time particle counter and a low-volume particulate sampler, respectively. Particle number concentrations were found to be elevated on several occasions during the measurements. All of the events of elevated particle concentrations were linked to indoor activities based on house occupant log entries. This enabled identification of the indoor sources that contributed to indoor particle concentrations. Activities such as cooking elevated particle number concentrations ≤2.05 × 10⁵ particles/cm³. The fine particles collected on Teflon filter substrates were analyzed for selected ions, trace elements, and metals, as well as elemental and organic carbon (OC) contents. To compare the quality of air between the indoors of the home and the outdoors, measurements were also made outside the home to obtain outdoor samples. The chemical composition of both outdoor and indoor particles was determined. Indoor/outdoor (I/O) ratios suggest that certain chemical constituents of indoor particles, such as chloride, sodium, aluminum, cobalt, copper, iron, manganese, titanium, vanadium, zinc, and elemental carbon, were derived through migration of outdoor particles (I/O<1 or ≈1), whereas the levels of others, such as nitrite, nitrate, sul-fate, ammonium, cadmium, chromium, nickel, lead, and OC, were largely influenced by the presence of indoor sources (I/O >1).     

A New Mathematical Model of Air Pollution Concentration

Abstract

Submicron particles play a major role in soiling processes and contribute to corrosion, current leakage and shorts in electronic equipment. For more than a year, optical particle counters have been used to continuously measure the concentrations of submicron particles at a telecommunications facility in Southern California. Separate instruments have simultaneously sampled at four locations: the outdoor air intake, immediately upstream of the HVAC filters, immediately downstream of the HVAC filters, and inside the office. The indoor concentrations can be explained in the context of a one-compartment mass balance model. Key parameters in the model (e.g., the air exchange rate) were monitored throughout the sampling period. In the latter part of this study, the particle counters were used as feedback elements in the HVAC system. An estimate of the concentration of indoor submicron particles, based on measurements of outdoor submicron particles, has been used as a control variable. When this variable exceeds a preset value, the outdoor air damper is partially closed, reducing the amount of outdoor air entering the building. That is, the position of the damper is based on the concentration of outdoor particles as well as the outdoor temperature. As a consequence, the average indoor concentration of submicron particles has been significantly reduced within this facility.     

Characteristics of indoor aerosols in residential homes in urban locations: a case study in Singapore

As part of a major study to investigate the indoor air quality in residential houses in Singapore, intensive aerosol measurements were made in an apartment in a multistory building for several consecutive days in 2004. The purpose of this work was to identify the major indoor sources of fine airborne particles and to assess their impact on indoor air quality for a typical residential home in an urban area in a densely populated country. Particle number and mass concentrations were measured in three rooms of the home using a real-time particle counter and a low-volume particulate sampler, respectively. Particle number concentrations were found to be elevated on several occasions during the measurements. All of the events of elevated particle concentrations were linked to indoor activities based on house occupant log entries. This enabled identification of the indoor sources that contributed to indoor particle concentrations. Activities such as cooking elevated particle number concentrations < or =2.05 x 10(5) particles/cm3. The fine particles collected on Teflon filter substrates were analyzed for selected ions, trace elements, and metals, as well as elemental and organic carbon (OC) contents. To compare the quality of air between the indoors of the home and the outdoors, measurements were also made outside the home to obtain outdoor samples. The chemical composition of both outdoor and indoor particles was determined. Indoor/outdoor (I/O) ratios suggest that certain chemical constituents of indoor particles, such as chloride, sodium, aluminum, cobalt, copper, iron, manganese, titanium, vanadium, zinc, and elemental carbon, were derived through migration of outdoor particles (I/O <1 or - 1), whereas the levels of others, such as nitrite, nitrate, sulfate, ammonium, cadmium, chromium, nickel, lead, and OC, were largely influenced by the presence of indoor sources (I/O >1).     

Continuous Monitoring of Ultrafine,Fine, and Coarse Particles in a Residence for 18 Months in 1999-2000

Abstract

Continuous monitors were employed for 18 months in an occupied townhouse to measure ultrafine, fine, and coarse particles; air change rates; wind speed and direction; temperature; and relative humidity (RH). A main objective was to document short-term and long-term variation in indoor air concentrations of size-resolved particles (0.01-20 μm) caused by (1) diurnal and seasonal variation of outdoor air concentrations and meteorological variables, (2) indoor sources such as cooking and using candles, and (3) activities affecting air change rates such as opening windows and using fans. A second objective was to test and compare available instruments for their suitability in providing real-time estimates of particle levels and ancillary variables.

Despite different measuring principles, the instruments employed in this study agreed reasonably well for particles less than 10 μm in diameter. The three instruments measuring fine and coarse particles (aerodynamic diameter between 0.3 and 20 μm) agreed to within 30% in their overall estimates of total volume. Two of these instruments employed optical scattering, and the third used an aerodynamic acceleration principle. However, several lines of evidence indicated that the instrument employing aerodynamic acceleration overestimated concentrations for particle diameters greater than 10 μm. A fourth instrument measuring ultrafine and accumulation-mode particles (0.01-1 μm) was operated with two different inlets providing somewhat different particle size ranges. The two inlets agreed in the ultrafine region (<0.1 μm) but diverged increasingly for larger particles (up to 0.445 μm).

Indoor sources affecting ultrafine particle concentrations were observed 22% of the time, and sources affecting fine and coarse particle concentrations were observed 12 and 15% of the time, respectively. When an indoor source was operating, particle concentrations for different sizes ranged from 2 to 20 times the average concentrations when no indoor source was apparent. Indoor sources, such as cooking with natural gas, and simple physical activities, such as walking, accounted for a majority (50-90%) of the ultrafine and coarse particle concentrations, whereas outdoor sources were more important for accumulation-mode particles between 0.1 and 1 um in diameter. Averaged for the entire year and including no periods when indoor sources were apparent, the number distribution was bimodal, with a peak at ~10 nm (possibly smaller), a shallow minimum at ~14 nm, and a second broad peak at ~68 nm. The volume distribution was also bimodal, with a broad peak at ~200 nm, a minimum at ~1.2 μm, and then an upward slope again through the remaining size fractions.

A database was created on a 5-min averaging time basis. It contains more than 90,000 measurements by two of the instruments and approximately 30,000 by the two optical scattering instruments. About 4500 hour-long average air change rates were also calculated throughout the year using a dedicated gas chromatograph with electron capture detection (GC/ECD). At high air change rates [>0.8 air changes per hour (hr^?1)], particle concentrations were either elevated (when no source was present) or depressed (when an indoor source was operating) by factors of up to 2 compared with low air change rates.     

Continuous monitoring of ultrafine,fine, and coarse particles in a residence for 18 months in 1999-2000

Continuous monitors were employed for 18 months in an occupied townhouse to measure ultrafine, fine, and coarse particles; air change rates; wind speed and direction; temperature; and relative humidity (RH). A main objective was to document short-term and long-term variation in indoor air concentrations of size-resolved particles (0.01-20 microm) caused by (1) diumal and seasonal variation of outdoor air concentrations and meteorological variables, (2) indoor sources such as cooking and using candles, and (3) activities affecting air change rates such as opening windows and using fans. A second objective was to test and compare available instruments for their suitability in providing real-time estimates of particle levels and ancillary variables. Despite different measuring principles, the instruments employed in this study agreed reasonably well for particles less than 10 microm in diameter. The three instruments measuring fine and coarse particles (aerodynamic diameter between 0.3 and 20 microm) agreed to within 30% in their overall estimates of total volume. Two of these instruments employed optical scattering, and the third used an aerodynamic acceleration principle. However, several lines of evidence indicated that the instrument employing aerodynamic acceleration overestimated concentrations for particle diameters greater than 10 microm. A fourth instrument measuring ultrafine and accumulation-mode particles (0.01-1 microm) was operated with two different inlets providing somewhat different particle size ranges. The two inlets agreed in the ultrafine region (< 0.1 microm) but diverged increasingly for larger particles (up to 0.445 microm). Indoor sources affecting ultrafine particle concentrations were observed 22% of the time, and sources affecting fine and coarse particle concentrations were observed 12 and 15% of the time, respectively. When an indoor source was operating, particle concentrations for different sizes ranged from 2 to 20 times the average concentrations when no indoor source was apparent. Indoor sources, such as cooking with natural gas, and simple physical activities, such as walking, accounted for a majority (50-90%) of the ultrafine and coarse particle concentrations, whereas outdoor sources were more important for accumulation-mode particles between 0.1 and 1 microm in diameter. Averaged for the entire year and including no periods when indoor sources were apparent, the number distribution was bimodal, with a peak at approximately 10 nm (possibly smaller), a shallow minimum at approximately 14 nm, and a second broad peak at approximately 68 nm. The volume distribution was also bimodal, with a broad peak at approximately 200 nm, a minimum at approximately 1.2 microm, and then an upward slope again through the remaining size fractions. A database was created on a 5-min averaging time basis. It contains more than 90,000 measurements by two of the instruments and approximately 30,000 by the two optical scattering instruments. About 4500 hour-long average air change rates were also calculated throughout the year using a dedicated gas chromatograph with electron capture detection (GC/ECD). At high air change rates [> 0.8 air changes per hour (hr(-1))], particle concentrations were either elevated (when no source was present) or depressed (when an indoor source was operating) by factors of up to 2 compared with low air change rates.     

Experiments probing the influence of air exchange rates on secondary organic aerosols derived from indoor chemistry

Reactions between ozone and terpenes have been shown to increase the concentrations of submicron particles in indoor settings. The present study was designed to examine the influence of air exchange rates on the concentrations of these secondary organic aerosols as well as on the evolution of their particle size distributions. The experiments were performed in a manipulated office setting containing a constant source of d-limonene and an ozone generator that was remotely turned “on” or “off” at 6 h intervals. The particle number concentrations were monitored using an optical particle counter with eight-channels ranging from 0.1–0.2 to>2.0 μm diameter. The air exchange rates during the experiments were either high (working hours) or low (non-working hours) and ranged from 1.6 to>12 h⁻¹, with intermediate exchange rates. Given the emission rates of ozone and d-limonene used in these studies, at an air exchange rate of 1.6 h⁻¹ particle number concentration in the 0.1–0.2 μm size-range peaked 1.2 h after the ozone generator was switched on. In the ensuing 4.8 h particle counts increased in successive size-ranges up to the 0.5–0.7 μm diameter range. At higher air exchange rates, the resulting concentrations of total particles and particle mass (calculated from particle counts) were smaller, and at exchange rates exceeding 12 h⁻¹, no excess particle formation was detectable with the instrument used in this study. Particle size evolved through accretion and, in some cases, coagulation. There was evidence for coagulation among particles in the smallest size-range at low air exchange rates (high particle concentrations) but no evidence of coagulation was apparent at higher air exchange rates (lower particle concentrations). At higher air exchange rates the particle count or size distributions were shifted towards smaller particle diameters and less time was required to achieve the maximum concentration in each of the size-ranges where discernable particle growth occurred. These results illustrate still another way in which ventilation affects human exposures in indoor settings. However, the ultimate effects of these exposures on health and well being remain to be determined.     

Effect of outside air ventilation rate on volatile organic compound concentrations in a call center

A study of the relationship between outside air ventilation rate and concentrations of volatile organic compounds (VOCs) generated indoors was conducted in a call center office building. The building, with two floors and a total floor area of 4600 m², is located in the San Francisco Bay Area, CA. Ventilation rates were manipulated with the building's four air handling units (AHUs). VOC and CO₂ concentrations in the AHU returns were measured on 7 days during a 13-week period. VOC emission factors were determined for individual zones on days when they were operating at near steady-state conditions. The emission factor data were subjected to principal component (PC) analysis to identify groups of co-varying compounds. Potential sources of the PC vectors were ascribed based on information from the literature. The per occupant CO₂ generation rates were 0.0068–0.0092 l s⁻¹. The per occupant isoprene generation rates of 0.2–0.3 mg h⁻¹ were consistent with the value predicted by mass balance from breath concentration and exhalation rate. The relationships between indoor minus outdoor VOC concentrations and ventilation rate were qualitatively examined for eight VOCs. Of these, acetaldehyde and hexanal, which likely were associated with material sources, and decamethylcyclopentasiloxane, associated with personal care products, exhibited general trends of higher concentrations at lower ventilation rates. For other compounds, a clear inverse relationship between VOC concentrations and ventilation was not observed. The net concentration of 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate isomers, examples of low-volatility compounds, changed very little with ventilation likely due to sorption and re-emission effects. These results illustrate that the efficacy of ventilation for controlling VOC concentrations can vary considerably depending upon the operation of the building, the pollutant sources and the physical and chemical processes affecting the pollutants. Thus, source control measures, in addition to adequate ventilation, are required to limit concentrations of VOCs in office buildings.     

The transformation of outdoor ammonium nitrate aerosols in the indoor environment

Recent studies associate particulate air pollution with adverse health effects; however, the exposure to indoor particles of outdoor origin is not well characterized, particularly for individual chemical species. We conducted a field study in an unoccupied, single-story residence in Clovis, California to provide data and analyses to address issues important for assessing exposure. We used real-time particle monitors both outdoors and indoors to quantify nitrate, sulfate, and carbon particulate matter of particle size 2.5 μm or less in diameter (PM-2.5). The results show that measured indoor ammonium nitrate concentrations were significantly lower than would be expected based solely on penetration and deposition losses. The additional reduction can be attributed to the transformation indoors of ammonium nitrate into ammonia and nitric acid gases, which are subsequently lost by deposition and sorption to indoor surfaces. A mass balance model that accounts for the kinetics of ammonium nitrate evaporation was able to reproduce measured indoor ammonium nitrate and nitric acid concentrations, resulting in a fitted value of the deposition velocity for nitric acid of 0.56 cm s⁻¹. The results indicate that indoor exposure to outdoor ammonium nitrate in Central Valley of California are small, and suggest that exposure assessments based on total particle mass measured outdoors may obscure the actual causal relationships for indoor exposure to particles of outdoor origin.     

The impact of recirculation,ventilation and filters on secondary organic aerosols generated by indoor chemistry

This study examined the impact of recirculation rates (7 and 14 h^?1), ventilation rates (1 and 2 h^?1), and filtration on secondary organic aerosols (SOAs) generated by ozone of outdoor origin reacting with limonene of indoor origin. Experiments were conducted within a recirculating air handling system that serviced an unoccupied, 236 m³ environmental chamber configured to simulate an office; either no filter, a new filter or a used filter was located downstream of where outdoor air mixed with return air. For otherwise comparable conditions, the SOA number and mass concentrations at a recirculation rate of 14 h^?1 were significantly smaller than at a recirculation rate of 7 h^?1. This was due primarily to lower ozone concentrations, resulting from increased surface removal, at the higher recirculation rate. Increased ventilation increased outdoor-to-indoor transport of ozone, but this was more than offset by the increased dilution of SOA derived from ozone-initiated chemistry. The presence of a particle filter (new or used) strikingly lowered SOA number and mass concentrations compared with conditions when no filter was present. Even though the particle filter in this study had only 35% single-pass removal efficiency for 100 nm particles, filtration efficiency was greatly amplified by recirculation. SOA particle levels were reduced to an even greater extent when an activated carbon filter was in the system, due to ozone removal by the carbon filter. These findings improve our understanding of the influence of commonly employed energy saving procedures on occupant exposures to ozone and ozone-derived SOA.     

The relationship between indoor and outdoor airborne particles in the residential environment

The relationship between indoor and outdoor airborne particles was investigated for 16 residential houses located in a suburban area of Brisbane, Australia. The submicrometer particle numbers were measured using the Scanning Mobility Particle Sizer, the larger particle numbers using the Aerodynamic Particle Sizer and an approximation of PM_2.5 was also measured using a DustTrak. The measurements were conducted for normal and minimum ventilation conditions using simultaneous and non-simultaneous measurement methods designed for the purpose of the study. Comparison of the ratios of indoor to outdoor particle concentrations revealed that while temporary values of the ratio vary in a broad range from 0.2 to 2.5 for both lower and higher ventilation conditions, average values of the ratios were very close to one regardless of ventilation conditions and of particle size range. The ratios were in the range from 0.78 to 1.07 for submicrometer particles, from 0.95 to 1.0 for supermicrometer particles and from 1.01 to 1.08 for PM_2.5 fraction. Comparison of the time series of indoor to outdoor particle concentrations shows a clear positive relationship existing for many houses under normal ventilation conditions (estimated to be about and above 2 h⁻¹), but not under minimum ventilation conditions (estimated to be about and below 1 h⁻¹). These results suggest that for normal ventilation conditions, outdoor particle concentrations could be used to predict instantaneous indoor particle concentrations but not for minimum ventilation, unless air exchange rate is known, thus allowing for estimation of the “delay constant”.     

Transient analysis of volatile organic compound concentrations for estimating emission rates

While emission rates of volatile organic compounds (VOCs) have been obtained for building materials, furnishings and processes in chambers, field measurements are more difficult. Procedures to estimate emission rates using transient analysis of VOC concentrations are described and applied in a two-story classroom/office building. The analysis employs semi-real-time VOC concentrations determined with a portable GC/FID and simultaneous air change rate measurements using tracer gas decay. The results of the analysis yield consistent values of emission rates for building materials ranging from 0.20 to 0.40 mg m⁻² h⁻¹ when normalized by floor area. Occupancy-related emissions were more difficult to estimate and covered a wider range from roughly 0.1 to 1.5 mg m⁻² h⁻¹. The test data were also analyzed in an attempt to determine sink parameters, but these efforts were not particularly successful. Furthermore, in these tests, the inclusion of sink effects did not significantly impact the estimated emission rates. While this paper offers a transient analysis approach that may lead to improved field estimates of VOC emission rates, it is not presented as a definitive methodology. Nevertheless, transient analysis has potential for use in other buildings, but simultaneous air change rate measurements are critical in its application in estimating VOC emission rates in the field.     

A study of the horizontal and vertical profile of submicrometer particles in relation to a busy road

Epidemiological studies are consistently reporting an association between fine particulate pollution and ill-health. Motor vehicle emissions are considered to be the main source of fine particles in ambient urban air of cities which are not directly influenced by industrial emissions. The aim of this work was to assess the influence of a major arterial road on concentration levels of airborne fine particles in its vicinity. Measurements of over 500 particle size distributions in the particle size range 16–626 nm, were made using two scanning mobility particle sizers (SMPS). A subsequent comparison of the recorded values from differing locations is discussed, with reference made to topographic and climatic influences. Both horizontal and vertical profile measurements of fine particle number size distributions are described; the combination of the two yielding information as to the relative exposures of occupants of buildings in the vicinity of a major arterial route. With the exception of measurements in close proximity to the freeway (about 15 m), the horizontal profile measurements did not provide any evidence of a statistically significant difference in fine particle number concentration with respect to distance at ground level up to a distance of 200 m within the study area. The vertical profile measurements also revealed no significant correlation between particle concentration and height. However, for buildings in the immediate proximity to the arterial road (about 15 m) concentrations around the building envelope are very high, comparable to those in the immediate vicinity of the road, indicating undiluted concentrations drawn directly from the freeway. This finding has a significant implication for management of indoor air quality in the buildings located in the immediate vicinity of major roads.     

Concentration, size distribution, and dry deposition rate of particle-associated metals in the Los Angeles region

Daily averaged atmospheric concentrations and dry deposition fluxes of particulate metals were measured seasonally at six urban sites and one non-urban coastal site in the Los Angeles region using a conventional total suspended particulate matter (TSP) filter, surrogate surface deposition plates, and a Noll Rotary Impactor (NRI), which provides information about particle size distribution in four size ranges above 6 μm. With the exception of the non-urban site, particulate metal concentrations and deposition fluxes were remarkably uniform spatially and temporally. At all sites there were significant metal concentrations on particles greater than 10 μm, a commonly used upper limit for many air quality monitoring studies, and these large particles were estimated to be responsible for most of the deposited mass of metals. Annual averaged values of deposition rates measured with a surrogate surface were in good agreement with values estimated using theoretical deposition velocities in conjunction with measured size-segregated particle concentrations. Image analysis of particles deposited on NRI stage A, which collects all particles greater than 6 μm, indicated nighttime metal concentrations and deposition at the non-urban coastal site was higher than in the day time due to offshore advection of urban air associated with the diurnal land breeze. Measured enrichments of crustal elements and metals were correlated, indicating efficient mixing of natural and anthropogenic material from different sources, hypothesized to be the result of cyclical resuspension and deposition of dust by moving vehicles and wind.     

Relationships between indoor and outdoor air pollution by carcinogenic PAHs and PCBs

PAHs and PCBs were collected simultaneously indoors and outdoors at eight non-smoking homes located in four buildings in high-traffic areas of Rome. The purpose was to evaluate the relevance of indoor air in contributing to the overall exposure of the urban population. The vertical distribution was also investigated by collecting outdoor samples at both road and roof level, and indoor samples in both a high and a low floor flat of each building. At one coal-heated building, samples were collected during both the heating and the non-heating season. No evident PAH source was present indoors. Indoor and outdoor daily concentrations of benzo[a]pyrene (BaP) ranged, respectively, 0.1–4.6 ng m⁻³ and 0.7–2.3 ng m⁻³. With the heating on, indoor PAH concentrations equalled or exceeded those outdoors, with BaP indoor/outdoor ratios up to 4; during the warm season, ratios decreased to 0.2–0.6. Indoor PAHs at the low floors exceeded the high-floor ones when the heating was off (vehicle exhausts being the dominant source), while being equal or lower with the heating on; the vertical gradient of indoor PAHs between different floors was within a factor of 2. Outdoor PAHs at roof level were 20–70% of those at road level, which in turn exceeded those at the medium-traffic station up to a factor of 4. The outdoor concentrations of Σ6 indicator PCBs ranged 0.1–1.6 ng m⁻³. Indoor PCB concentrations exceeded those outdoors by an approximate factor of 2–50. No vertical gradient was observed. The results indicated that indoor air may contribute to the overall exposure to PAHs and PCBs more than the urban air. They were also consistent with recent findings suggesting that indoor air can be a relevant source of PCBs for outdoor air.     

Spatial variation of particle number and mass over four European cities

The number of ultrafine particles may be a more health relevant characteristic of ambient particulate matter than the conventionally measured mass. Epidemiological time series studies typically use a central site to characterize human exposure to outdoor air pollution. There is currently very limited information how well measurements at a central site reflect temporal and spatial variation across an urban area for particle number concentrations (PNC).The main objective of the study was to assess the spatial variation of PNC compared to the mass concentration of particles with diameter less than 10 or 2.5 μm (PM₁₀ and PM_2.5).Continuous measurements of PM₁₀, PM_2.5, PNC and soot concentrations were conducted at a central site during October 2002–March 2004 in four cities spread over Europe (Amsterdam, Athens, Birmingham and Helsinki). The same measurements were conducted directly outside 152 homes spread over the metropolitan areas. Each home was monitored during 1 week. We assessed the temporal correlation and the variability of absolute concentrations.For all particle indices, including particle number, temporal correlation of 24-h average concentrations was high. The median correlation for PNC per city ranged between 0.67 and 0.76. For PM_2.5 median correlation ranged between 0.79 and 0.98. The median correlation for hourly average PNC was lower (range 0.56–0.66). Absolute concentration levels varied substantially more within cities for PNC and coarse particles than for PM_2.5. Measurements at the central site reflected the temporal variation of 24-h average concentrations for all particle indices at the selected homes across the urban area. A central site could not assess absolute concentrations across the urban areas for particle number.     

Characterization of Indoor Particle Sources Using Continuous Mass and Size Monitors

ABSTRACT

A comprehensive indoor particle characterization study was conducted in nine Boston-area homes in 1998 in order to characterize sources of PM in indoor environments. State-of-the-art sampling methodologies were used to obtain continuous PM_2.5 concentration and size distribution particulate data for both indoor and outdoor air. Study homes, five of which were sampled during two seasons, were monitored over week-long periods. Among other data collected during the extensive monitoring efforts were 24hr elemental/organic carbon (EC/OC) particulate data as well as semi-continuous air exchange rates and time-activity information.

This rich data set shows that indoor particle events tend to be brief, intermittent, and highly variable, thus requiring the use of continuous instrumentation for their characterization. In addition to dramatically increasing indoor PM₂₅ concentrations, these data demonstrate that indoor particle events can significantly alter the size distribution and composition of indoor particles. Source event data demonstrate that the impacts of indoor activities are especially pronounced in the ultrafine (d_a < 0.1 um) and coarse (2.5 < d_a < 10 |um) modes. Among the sources of ultrafine particles characterized in this study are indoor ozone/terpene reactions. Furthermore, EC/OC data suggest that organic carbon is a major constituent of particles emitted during indoor source events. Whether exposures to indoor-generated particles, particularly from large short-term peak events, may be associated with adverse health effects will become clearer when biological mechanisms are better known.     

Development of a low-cost air sensor package and indoor air quality monitoring in a California middle school: Detection of a distant wildfire

A low-cost air sensor package was used to monitor indoor air quality (IAQ) in a classroom at the Albany Middle School in the San Francisco Bay Area of California. A rapid increase in carbon dioxide (CO₂) was observed in the classroom as soon as it is occupied. When the classroom is unoccupied, the CO₂ levels decay slowly toward the outdoor background level. All high CO₂ concentrations observed inside the classroom, above the outdoor background, was due to exhaling of the occupants. The CO₂ concentrations generally exceed the recommended level of 1000 ppb towards the end of the school day. The exceedances and slow decay may suggest that the ventilation rate in this school is not sufficient. The particulate level in the classroom was low until a distant wildfire advected large amount of particulate matter to the San Francisco Bay Area. Very high (10–15 times compared to the background) particle numbers (per m³ of particles with diameter >0.3 µm) were observed in the classroom during the wildfire. These particles were relatively small (0.3–1.0 µm) and the filters (MERV 8) of the ventilation system were unable to filter them out. Therefore, the measurements made by low-cost particle counters can inform the school administrators of adverse IAQ during future wildfire (or other combustion) events. The particle number was independent of the occupation before and during the wildfire suggesting that all observed particles were infiltrated into the classroom from outside. Consistent with previous studies, no appreciable increase in the local ambient CO₂ background was observed during this distant wildfire event.

Implications: Low-cost air sensors are effective in monitoring indoor air quality in classrooms. The CO₂ levels in classrooms are mainly generated indoors due to exhalation of occupants. Concentration of CO₂ generally exceed the recommended level of 1000 ppb towards the end of the school day. In contrast, the particulate matter mostly comes from outdoors and small particles penetrate though the filters normally used at schools. Distant wildfires do not increase the local CO₂ background appreciably, but significantly increase the particulate matter concentrations both indoors and outdoors. Further investigations are needed to assure that ventilation rates in classrooms are sufficiently health protective.