Loss of particle nitrate from teflon sampling filters: effects on measured gravimetric mass in California and in the IMPROVE network

The extent of mass loss on Teflon filters caused by ammonium nitrate volatilization can be a substantial fraction of the measured particulate matter with an aerodynamic diameter less than 2.5 microm (PM2.5) or 10 microm (PM10) mass and depends on where and when it was collected. There is no straightforward method to correct for the mass loss using routine monitoring data. In southern California during the California Acid Deposition Monitoring Program, 30-40% of the gravimetric PM2.5 mass was lost during summer daytime. Lower mass losses occurred at more remote locations. The estimated potential mass loss in the Interagency Monitoring of Protected Visual Environments network was consistent with the measured loss observed in California. The biased mass measurement implies that use of Federal Reference Method data for fine particles may lead to control strategies that are biased toward sources of fugitive dust, other primary particle emission sources, and stable secondary particles (e.g., sulfates). This analysis clearly supports the need for speciated analysis of samples collected in a manner that preserves volatile species. Finally, although there is loss of volatile nitrate (NO3-) from Teflon filters during sampling, the NO3- remaining after collection is quite stable. We found little loss of NO3- from Teflon filters after 2 hr under vacuum and 1 min of heating by a cyclotron proton beam.     

Direct detection of sulfate and nitrate layers on sampled marine aerosols by laser microprobe mass analysis

The direct detection of sulfate and nitrate layers on sampled marine aerosols was carried out by laser microprobe mass analysts.     

Spatial and temporal variability of outdoor coarse particulate matter mass concentrations measured with a new coarse particle sampler during the Detroit Exposure and Aerosol Research Study

The Detroit Exposure and Aerosol Research Study (DEARS) provided data to compare outdoor residential coarse particulate matter (PM_10–2.5) concentrations in six different areas of Detroit with data from a central monitoring site. Daily and seasonal influences on the spatial distribution of PM_10–2.5 during Summer 2006 and Winter 2007 were investigated using data collected with the newly developed coarse particle exposure monitor (CPEM). These data allowed the representativeness of the community monitoring site to be assessed for the greater Detroit metro area. Multiple CPEMs collocated with a dichotomous sampler determined the precision and accuracy of the CPEM PM_10–2.5 and PM_2.5 data.CPEM PM_2.5 concentrations agreed well with the dichotomous sampler data. The slope was 0.97 and the R² was 0.91. CPEM concentrations had an average 23% negative bias and R² of 0.81. The directional nature of the CPEM sampling efficiency due to bluff body effects probably caused the negative CPEM concentration bias.PM_10–2.5 was observed to vary spatially and temporally across Detroit, reflecting the seasonal impact of local sources. Summer PM_10–2.5 was 5 μg m^?3 higher in the two industrial areas near downtown than the average concentrations in other areas of Detroit. An area impacted by vehicular traffic had concentrations 8 μg m^?3 higher than the average concentrations in other parts of Detroit in the winter due to the suspected suspension of road salt. PM_10–2.5 Pearson Correlation Coefficients between monitoring locations varied from 0.03 to 0.76. All summer PM_10–2.5 correlations were greater than 0.28 and statistically significant (p-value < 0.05). Winter PM_10–2.5 correlations greater than 0.33 were statistically significant (p-value < 0.05). The PM_10–2.5 correlations found to be insignificant were associated with the area impacted by mobile sources during the winter. The suspected suspension of road salt from the Southfield Freeway, combined with a very stable atmosphere, caused concentrations to be greater in this area compared to other areas of Detroit. These findings indicated that PM_10–2.5, although correlated in some instances, varies sufficiently across a complex urban airshed that that a central monitoring site may not adequately represent the population's exposure to PM_10–2.5.     

Variation of mass scattering efficiencies in IMPROVE

The Interagency Monitoring of Protected Visual Environments (IMPROVE) equation used to assess compliance under the U.S. Environmental Protection Agency (EPA) Haze Rule assumes that dry mass scattering efficiencies for aerosol chemical components are constant. However, examination of aerosol size distributions and chemical composition during the Big Bend Regional Aerosol and Visibility Observational Study and the Southeastern Aerosol and Visibility Study suggests that volume and mass scattering efficiencies vary directly with increasing particle light scattering and aerosol mass concentration. This is consistent with the observation that particle distributions were shifted to larger sizes under more polluted conditions and appears to be related to aging of the aerosol during transport to remote locations.     

Comparison among filter-based,impactor-based and continuous techniques for measuring atmospheric fine sulfate and nitrate

Filter-based methods for sampling aerosols are subject to great uncertainty if the gas–particle interactions on filter substrates are not properly handled. Sampling artifacts depend on both meteorological conditions and the chemical mix of the atmosphere. Despite numerous of studies on the subject, very few have evaluated filter-based methods in the Asian environments. This paper reports the results of a comparison of the performances of two filter-based samplers, including a Thermo Anderson Chemical Speciation Monitor (RAAS) and a honeycomb denuder filter-pack system, a Micro Orifice Uniform Deposit Impactor (MOUDI) and a real-time ambient ion monitor (AIM, URG9000B) in measuring atmospheric concentrations of PM_2.5 sulfate and nitrate. Field studies were conducted at an urban site in Jinan, Shandong province, during the winter of 2007 and at a rural site near Beijing in the summer of 2008. The AIM was first compared with the honeycomb denuder filter-pack system which was considered to have minimal sampling artifacts. After some modifications made to it, the AIM showed good performance for both sulfate and nitrate measurement at the two sites and was then used to evaluate other instruments. For the un-denuded RAAS, the extent of sampling artifacts for nitrate on quartz filters was negligible, while that on Teflon filters was also minimal at high nitrate concentrations (>10 μgm^?3); however, loss through evaporation was significant (～75%) at low nitrate concentrations under hot summer conditions. The MOUDI using aluminum substrates suffered a significant loss of nitrate (50–70%) under summer conditions due to evaporation. Considering that the aluminum substrates are still being widely used to obtain size-resolved aerosol compositions because of their low cost and accurate mass weighed, caution should be taken about the potential significant under determination of semi-volatile components such as ammonium nitrate.     

Potential contribution of sulfate production in cumulus cloud droplets to ground level particle sulfur concentrations

The relationships have been examined between the presence or absence of cumulus clouds and 3rd quarter fine particle sulfur concentrations in St Louis. An association between the presence of cumulus clouds with SO₂ conversions in droplets and incrementally higher fine particle sulfur concentrations can be demonstrated. However, diurnal patterns of fine particle sulfur concentrations in the presence of cumulus clouds are not consistent with local contributions from sulfate formation in cumulus clouds. Morning fog often occurs on the same days on which cumulus clouds form later. Reactions of SO₂ in fog droplets appear to make a contribution, but do not account for the major part of the increments in fine particle sulfur concentrations associated with cumulus clouds. The variations in fine particle sulfur concentrations observed can be explained if a substantial part of the sulfate formed in cumulus is transported upwards from the planetary boundary layer into the lower free troposphere. Subsequent multiday regional scale horizontal transport with concurrent gradual vertical transport of sulfate down to the surface would be consistent with the observed results.     

Effects of changes in sulfate,ammonia, and nitric acid on particulate nitrate concentrations in the southeastern United States

A thermodynamic equilibrium model, Simulating Composition of Atmospheric Particles at Equilibrium (SCAPE2), was used to investigate the response of fine particulate NO3(-) to changes in concentrations of HNO3, NH3, and SO4(2-) in the southeastern United States. The data consisted of daily, 24-hr time resolution measurements from the Aerosol Research Inhalation Epidemiology Study (ARIES) Jefferson Street (Atlanta) site and five other sites of the Southeastern Aerosol Research and Characterization Project (SEARCH). Reductions of total NH3 (gas-phase NH3 plus particulate NH4(+)), total NO3(-) (HNO3 plus particulate NO3(-)), SO4(2-), or combined total NO3(-) (HNO3 plus particulate NO3(-)) with SO4(2-) were used to estimate the effects of changing emission levels. The conversion of SO2 to SO4(2-) and NO2 to HNO3 involves additional nonlinear reactions not incorporated into the model. For all sites, fine particulate NO3(-) concentrations decreased in response to reductions of either NH3 or total NO3(-), but the particulate NO3(-) decreases were greater for the NH3 reductions than for the total NO3(-) reductions. Particulate NO3(-) concentrations increased in response to reductions of SO4(2-). For the combined reduction (total NO3(-) plus SO4(2-)), the resulting particulate NO3(-) concentrations were on average no different than the base-case NO3(-) levels. Measurements of fine particulate NO3(-) and HNO3 support the modeling conclusions and indicate that particulate NO3(-) formation is limited by the availability of NH3 at most times at all SEARCH sites.     

Measurements of the scavenging of sulfate and nitrate in clouds

Measurements are presented on the scavenging of sulfate and nitrate by cumulus, stratus and strato-cumulus clouds. Assuming that all of the particulate sulfate was in the size range 0.1–1.0 μm radius and that sulfate was scavenged with the same efficiency as sub-micrometer particles in general, the nucleation scavenging coefficient of sulfate in these clouds was deduced to be 0.7 ±0.2 and evidence for sulfate production (1.0±0.3 μgm⁻³) within cloud water was also obtained. Evidence for nitrate scavenging, by nitrates serving as cloud condensation nuclei or by the absorption of HNO₃ by cloud droplets, is also presented. The data suggest that either gaseous nitrogen compounds in the air other than HNO₃ can dissolve and contribute to the nitrate concentration in cloud water or that nitrate can be produced within cloud droplets.     

Variations in exposure to in-vehicle particle mass and number concentrations in different road environments

Road environments significantly affect in cabin concentration of particulate matter (PM). This study conducted measurements of in-vehicle and on-road concentrations of PM10, PM2.5, PM1, and particle number (PN) in size of 0.02–1 µm, under six ventilation settings in different urban road environments (tunnels, surface roads and elevated roads). Linear regression was then used to analyze the contributions of multiple predictor variables (including on-road concentrations, temperature, relative humidity, time of day, and ventilation settings) to measured variations. On-road measurements of PM2.5, PM1, and PN concentrations from the open surface roads were 5.5%, 3.7%, and 16% lower, respectively, than those measured in tunnels, but 7.6%, 7.1% and 24% higher, respectively, than those on elevated roads. The highest on-road PM10 concentration was observed on surface roads. The time series pattern of in-vehicle particle concentrations closely tracked the on-road concentrations outside of the car and exhibited a smoother profile. Irrespective of road environment, the average I/O ratio of particles was found to be the lowest when air conditioning was on with internal recirculation, the highest purification efficiency via ventilation was obtained by switching on external air recirculation and air conditioning. Statistical models showed that on-road concentration, temperature, and ventilation setting are common factors of significance that explained 58%-80%, 64%-97%, and 87%-98% of the variations in in-vehicle PM concentrations on surface roads, on elevated roads, and in tunnels, respectively.

Implications: Inside vehicles, both driver and passengers will be exposed to elevated particle concentrations. However, for in-vehicle particles, there has been no comprehensive comparative study of the three-dimensional traffic environment including tunnels surface roads and elevated roads. This study focuses on the analysis of the trends and main influencing factors of particle concentrations in different road environments. The results can provide suggestions for the driver's behavior, and provide data support for the environmental protection department to develop pollutant concentration limits within the vehicle.     

Accuracy of measured ammonium nitrate equilibrium values

The use of previously measured ammonium nitrate equilibrium constants in atmospheric work involves a large extrapolation to ambient conditions. Given the many uncertainties in the original measurements confidence in this procedure is unwarranted.     

PM2.5 mass and light extinction reconstruction in IMPROVE

Compliance under the Regional Haze Rule of 1999 is based on Interagency Monitoring of Protected Visual Environments (IMPROVE) protocols for reconstructing aerosol mass and light extinction from aerosol chemical concentrations measured in the IMPROVE network. The accuracy, consistency, and potential biases in these formulations were examined using IMPROVE aerosol chemistry and light extinction data from 1988-1999. Underestimation of particulate matter with an aerodynamic diameter < 2.5 microm (PM2.5) by the IMPROVE mass reconstruction formula by 12%, on average, appears to be related to the exclusion of sodium, chlorine, and other elements and to artifacts associated with the measurement of organic carbon, but not to absorption of water by sulfates and nitrates on IMPROVE Teflon filters during weighing. Light scattering measured by transmissometry is not consistent with nephelometer scattering or single-scatter albedos expected for remote locations. Light scattering was systematically overestimated by 34%, on average, with the IMPROVE particle scattering (Bsp) reconstruction formula. The use of climatologically based hygroscopic growth factors f(RH) suggested for compliance with the Haze Rule contributes significantly to this overestimation and increases the amount of light extinction attributable to sulfates for IMPROVE samples between 1993 and 1999 by 5 percentage points.     

Comparison of particle light scattering and fine particulate matter mass in central California

Particle light scattering (Bsp) from nephelometers and fine particulate matter (PM2.5) mass determined by filter samplers are compared for summer and winter at 35 locations in and around California's San Joaquin Valley from December 2, 1999 to February 3, 2001. The relationship is described using particle mass scattering efficiency (sigmasp) derived from linear regression of Bsp on PM2.5 that can be applied to estimated PM2.5 from nephelometer data within the 24-hr filter sampling periods and between the every-6th-day sampling frequency. An average of sigmaSp = 4.9 m2/g was found for all of the sites and seasons; however, sigmasp averaged by site type and season provided better PM2.5 estimates. On average, the sigmasp was lower in summer than winter, consistent with lower relative humidities, lower fractions of hygroscopic ammonium nitrate, and higher contributions from fugitive dust. Winter average sigmasp were similar at non-source-dominated sites, ranging from 4.8 m2/g to 5.9 m2/g. The sigmasp was 2.3 m2/g at the roadside, 3.7 m2/g at a dairy farm, and 4.1 m2/g in the Kern County oilfields. Comparison of Bsp from nephelometers with and without a PM2.5 inlet at the Fresno Supersite showed that coarse particles contributed minor amounts to light scattering. This was confirmed by poorer correlations between Bsp and coarse particulate matter measured during a fall sampling period.     

Relationships between size segregated mass concentration data and ultrafine particle number concentrations in urban areas

Mass concentration data derived from samples collected with a micro-orifice uniform deposit impactor (MOUDI) in six Australian urban centers during periods of significant particle loading have been used to investigate the relationships between TSP, PM10, PM2.5, PM1 and ultrafine particles. While PM10 and PM2.5 display a clear relationship, the lack of correlation between PM10 and the coarse fraction of PM10 (PM10–PM2.5) suggests that variation in PM10 is dominated by variance in PM2.5. Given that particles of less than 2.5 μm are suspected to have adverse health effects, increasing the extent of PM2.5 monitoring may improve detection of relationships between air pollution and human health. A lack of correlation between both PM10 and PM2.5 with ultrafine mass concentrations indicates that PM10 and PM2.5 cannot be used as a surrogate for ultrafine mass concentration. Similarly, ultrafine number concentrations cannot be inferred from mass concentration information determined by the MOUDI.     

Dry deposition of airborne sulfate and nitrate to soybeans

Dry deposition contributes a substantial part of the total deposition of acidic pollutants and acid precursors to agricultural systems. However, because of the relative intractability of measurement of dry deposition fluxes, little work has been done to directly quantify dry inputs of pollutants to crops. In this research, foliar surface sampling ('leaf-washing') methods were developed and shown to be a practical and fairly precise means of monitoring the accumulation of dry-deposited SO4(2-) and NO3- on plant surfaces. Leaching of these ions from plant tissues was shown to be negligible; however, uptake by plants (e.g. stomatal gas exchange of SO2 or HNO3 and/or assimilation of surface accumulations of materials) is not accounted for by the sampling method. The significance of dry deposition to modification of the chemical microenvironment of leaf surfaces appears to be a factor of 3 to 20 or more greater than that of wet deposition alone. This is due to the cyclic reactivation of accumulated materials by dew and light rains, which may dissolve and mobilize, but not remove, the pollutant surface deposit. Therefore, while dry deposition of SO2 and SO4(2-) containing particles may contribute only part of the total mass of sulfur inputs to crop systems, the exposure of plant surface tissue to pollutants can be dominated by the dry-deposited material. The alteration of leaf surface chemistry may contribute to possible stress-producing mechanisms such as reduction of cuticular integrity, cellular injury and death, enhanced leaching of primary and secondary metabolites, and changes in pathogen infection efficiency.     

Peroxyacetyl nitrate (PAN) concentrations in the Antarctic troposphere measured during the photochemical experiment at Neumayer (PEAN'99)

Because investigations of PAN at higher southern latitudes are very scarce, we measured surface PAN concentrations for the first time in Antarctica. During the Photochemical Experiment at Neumayer (PEAN'99) campaign mean surface PAN mixing ratios of 13±7 pptv and maximum values of 48 pptv were found. When these PAN mixing ratios were compared to the sum of NO_x and inorganic nitrate they were found to be equal or higher. Low ambient air temperatures and low PAN concentrations caused a slow homogeneous PAN decomposition rate of approximately 5×10⁻² pptv h⁻¹. These slow decay rates were not sufficient to firmly establish the simultaneously observed NO_x concentrations. In addition, low concentration ratios of [HNO₃]/[NO_x] imply that the photochemical production of NO_x within the snow pack can influence surface NO_x mixing ratios in Antarctica. Alternate measurements of PAN mixing ratios at two different heights above the snow surface were performed to derive fluxes between the lower troposphere and the underlying snow pack using calculated friction velocities. Most of the concentration differences were below the precision of the measurements. Therefore, only an upper limit for the PAN flux of ±1×10¹³ molecules m⁻² s⁻¹ without a predominant direction can be estimated. However, PAN fluxes below this limit can still influence both the transfer of nitrogen compounds between atmosphere and ice, and the PAN budget in higher southern latitudes.     

Size mass distribution of water-soluble ionic species and gas conversion to sulfate and nitrate in particulate matter in southern Taiwan

A Micro-Orifice Uniform Deposition Impactor (MOUDI) and a Nano-MOUDI were employed to determine the size-segregated mass distributions of ambient particulate matter (PM) and water-soluble ionic species for particulate constituents. In addition, gas precursors, including HCl, HONO, HNO₃, SO₂, and NH₃ gases, were analyzed by an annular denuder system. PM size mass distribution, mass concentration, and ionic species concentration were measured during the day and at night during episode and non-episode periods in winter and summer. Average total suspended particle (TSP) concentrations during episode days in winter were as high as 153?±?33 μg/m³, and PM mass concentrations in summer were as low as one-third of that in winter. Generally, PM concentration at night was higher than that in the daytime in southern Taiwan during the sampling periods. In winter during the episode periods, the size-segregated mass distribution of PM mass concentration was mostly in the 0.32–3.2-μm range, and the PM concentration increased significantly in the range of 0.32–3.2 μm at night. Ammonium, nitrate, and sulfate were the dominant water-soluble ionic species in PM, contributing 34–48 % of TSP mass. High concentrations of ammonia (12.9–49 μg/m³) and SO₂ (2.6–27 μg/m³) were observed in the gas precursors. The conversion ratio was high in the PM size range of 0.18–3.2 μm both during the day and at night in winter, and the conversion ratio of episode days was 20 % higher than that of non-episode days. The conversion factor was high for both nitrogen and sulfur species at nighttime, especially on episode days.     

Artifact particulate sulfate and nitrate formation on filter media

Laboratory and atmospheric sampling studies were performed to evaluate glass fiber, quartz and Teflon filters for their abilities to form artifact particulate sulfate and nitrate with SO₂and HNO₃, respectively. The glass fiber filters were the types employed by the U.S. EPA as well as many State and local agencies for hi-vol particle sampling in the period 1977–1982.In 24-h laboratory trials, the sulfate artifact with the glass fiber filters ranged from 32 to 59μgcm⁻² (8–15μg m⁻³ at simulated hi-vol sampling rates). With the exception of Whatman QMA, the ‘quartz’ and Teflon filters collected very low levels of SO₂. At nitric acid dosages representative of those in atmospheric sampling, the glass fiber filters retained > 94% of the HNO₃; the ‘quartz’ filters, 33 to > 99%; and Teflon filters, <2 % of the HNO₃,. Because of competing acidic species in ambient air, these represent upper limits to the values to be expected in atmospheric sampling. Over the dosage range evaluated, artifact particulate sulfate and nitrate formation on ‘quartz’ and Teflon filters were highly correlated to each other from one filter to another, with a 4:1 molar ratio of nitrate: sulfate.The sulfate artifact in atmospheric sampling with Pallflex ‘quartz’ filters was consistent with laboratory findings while the nitrate artifact was much lower than predicted.     

Estimation of ambient PM2.5 concentrations in Maryland and verification by measured values

In 1997, Maryland had no available ambient Federal Reference Method data on particulate matter less than 2.5 microm in aerodynamic diameter (PM23), but did have annual ambient data for PM smaller than 10 microm (PM10) at 24 sites. The PM10 data were analyzed in conjunction with local annual and seasonal zip-code-level emission inventories and with speciated PM2.5 data from four nearby monitors in the IMPROVE network (located in the national parks, wildlife refuges, and wilderness areas) in an effort to estimate annual average and seasonal high PM2.5 concentrations at the 24 PM10 monitor sites operating from 1992 to 1996. All seasonal high concentrations were estimated to be below the 24-hr PM2.5 National Ambient Air Quality Standards (NAAQS) at the sites operating in Maryland between 1992 and 1996. The estimates also indicated that 12 monitor sites might exceed the 3-year annual average PM2.5 NAAQS of 15 microg/m3, but Maryland's air quality shows signs that it has been improving since 1992. The estimates also were compared with actual measurements after the PM2.5 monitor network was installed. The estimates were adequate for describing the chemical composition of the PM2.5, forecasting compliance status with the 24-hr and annual standards, and determining the spatial variations in PM2.5 across central Maryland.