Concentrations of mobile source air pollutants in urban microenvironments

Human exposures to criteria and hazardous air pollutants (HAPs) in urban areas vary greatly due to temporal-spatial variations in emissions, changing meteorology, varying proximity to sources, as well as due to building, vehicle, and other environmental characteristics that influence the amounts of ambient pollutants that penetrate or infiltrate into these microenvironments. Consequently, the exposure estimates derived from central-site ambient measurements are uncertain and tend to underestimate actual exposures. The Exposure Classification Project (ECP) was conducted to measure pollutant concentrations for common urban microenvironments (MEs) for use in evaluating the results of regulatory human exposure models. Nearly 500 sets of measurements were made in three Los Angeles County communities during fall 2008, winter 2009, and summer 2009. MEs included in-vehicle, near-road, outdoor, and indoor locations accessible to the general public. Contemporaneous 1- to 15-min average personal breathing zone concentrations of carbon monoxide (CO), carbon dioxide (CO₂), volatile organic compounds (VOCs), nitric oxide (NO), nitrogen oxides (NO_x), particulate matter (<2.5 μm diameter; PM_2.5) mass, ultrafine particle (UFP; <100 nm diameter) number, black carbon (BC), speciated HAPs (e.g., benzene, toluene, ethylbenzene, xylenes [BTEX], 1,3-butadiene), and ozone (O₃) were measured continuously. In-vehicle and inside/outside measurements were made in various passenger vehicle types and in public buildings to estimate penetration or infiltration factors. A large fraction of the observed pollutant concentrations for on-road MEs, especially near diesel trucks, was unrelated to ambient measurements at nearby monitors. Comparisons of ME concentrations estimated using the median ME/ambient ratio versus regression slopes and intercepts indicate that the regression approach may be more accurate for on-road MEs. Ranges in the ME/ambient ratios among ME categories were generally greater than differences among the three communities for the same ME category, suggesting that the ME proximity factors may be more broadly applicable to urban MEs.

Implications:Estimates of population exposure to air pollutants extrapolated from ambient measurements at ambient fixed site monitors or exposure surrogates are prone to uncertainty. This study measured concentrations of mobile source air toxics (MSAT) and related criteria pollutants within in-vehicle, outdoor near-road, and indoor urban MEs to provide multipollutant ME measurements that can be used to calibrate regulatory exposure models.     

Potential Interference Bias in Ozone Standard Compliance Monitoring

Abstract

The U.S. Environmental Protection Agency has established a federal reference method (FRM) for ozone (O₃) and allowed for designation of federal equivalent methods (FEMs). However, the ethylene‐chemiluminescence FRM for O₃ has been replaced by the UV photometric FEM by most state and local monitoring agencies because of its relative ease of operation. Accumulating evidence indicates that the FEM is prone to bias under the hot, humid, and stagnant conditions conducive to high O₃ formation. This bias may lead to overreporting hourly O₃ concentrations by as much as 20–40 ppb. Measurement bias is caused by contamination of the O₃ scrubber, a problem that is not detected by dry air calibration. An adequate wet test has not been codified, although a procedure has been proposed for agency consideration. This paper includes documentation of laboratory tests quantifying specific interferant responses, collocated ambient FRM/FEM monitoring results, and smog chamber comparisons of the FRM and FEMs with alternative scrubber designs. As the numbers of reports on monitor interferences have grown, interested parties have called for agency recognition and correction of these biases.     

A re-examination of ambient air ozone monitor interferences

Attaining the National Ambient Air Quality Standard (NAAQS) for ozone (O3) could cost billions of dollars nationwide. Attainment of the NAAQS is judged on O3 measurements made by the Federal Reference Method (FRM), ethylene chemiluminescence, or a Federal Equivalent Method (FEM), predominantly ultraviolet (UV) absorption. Starting in the 1980s, FRM monitors were replaced by FEMs so that today virtually all monitoring in the United States uses the UV methodology. This report summarizes a laboratory and collocated ambient air monitoring study of interferences in O3 monitors. Potential interferences examined in the laboratory included water vapor, mercury, o-nitrophenol, naphthalene, p-tolualdehyde, and mixed reaction products from smog chamber simulations of urban atmospheric photochemistry. UV absorption O3 monitors modified for humidity equilibration were also collocated with UV FEM O3 monitors at six sites in Houston, TX, during the 2007 summer O3 season. The results suggest that humidity and interfering species can positively bias (overestimate) O3 measured by FEM monitors used to determine compliance with the O3 standards. The results also suggest that humidity equilibration can mitigate this bias.     

Quantification of personal exposure concentrations to gasoline vehicle emissions in high-end exposure microenvironments: Effects of fuel and season

Mobile-source air toxic (MSAT) levels increase in confining microenvironments (MEs) with numerous emission sources of vehicle exhaust or evaporative emissions or during high-load and cold-start conditions. Reformulated fuels are expected to reduce MSAT and ozone precursor emissions. This study, required under the Clean Air Act Section 211b, evaluated high-end exposures in cities using reformulated (methyl tertiary-butyl ether [MTBE] or ethanol [EtOH]) fuels and conventional gasoline blends. The study investigates 13 high-end MEs, sampling under enhanced exposure conditions expected to result in maximal fuel and exhaust component exposures to carbon monoxide (CO), carbon dioxide (CO₂), BTEX (benzene, toluene, ethylbenzene, xylenes), MTBE, 1,3-butadiene (1,3-BD), EtOH, formaldehyde (HCHO), and acetaldehyde (CH₃CHO). The authors found that day-to-day ME variations in high-end benzene, 1,3-BD, HCHO, and CO concentrations are substantial, but independent of gasoline composition and season, and related to the activity and emission rates of ME sources, which differ from day to day.

Implications: Mobile-source air toxic (MSAT) levels increase in confining microenvironments (MEs) in the presence of vehicular exhaust or evaporative emissions. This study, required under the Clean Air Act Section 211b, evaluated high-end exposures in cities using oxygenated (methyl tertiary-butyl ether or ethanol) and conventional gasoline blends. Personal exposure concentrations were quantified in selected MEs representing the upper end of the frequency distribution of potential population exposures. This work presents the first systematic look at high-end/maximal exposures to multiple contaminants, in multiple microenvironments, in multiple cities, over two seasons, for multiple fuels, making it a very complete evaluation of reformulated fuel impacts on MSAT concentrations in confined microenvironments. The study found that day-to-day ME variations of high-end pollutant concentrations are substantial, but independent of gasoline composition and season, and related to the variable daily activity and emission rates of ME sources. The data collected in this study may be used in bounding exposure modeling estimates that account for time spent in similar confining MEs.     

Field evaluations of newly available “interference-free” monitors for nitrogen dioxide and ozone at near-road and conventional National Ambient Air Quality Standards compliance sites

Long-standing measurement techniques for determining ground-level ozone (O₃) and nitrogen dioxide (NO₂) are known to be biased by interfering compounds that result in overestimates of high O₃ and NO₂ ambient concentrations under conducive conditions. An increasing near-ground O₃ gradient (NGOG) with increasing height above ground level is also known to exist. Both the interference bias and NGOG were investigated by comparing data from a conventional Federal Equivalent Method (FEM) O₃ photometer and an identical monitor upgraded with an “interference-free” nitric oxide O₃ scrubber that alternatively sampled at 2 m and 6.2 m inlet heights above ground level (AGL). Intercomparison was also made between a conventional nitrogen oxide (NO_x) chemiluminescence Federal Reference Method (FRM) monitor and a new “direct-measure” NO₂ NO_x 405 nm photometer at a near-road air quality measurement site. Results indicate that the O₃ monitor with the upgraded scrubber recorded lower regulatory-oriented concentrations than the deployed conventional metal oxide–scrubbed monitor and that O₃ concentrations 6.2 m AGL were higher than concentrations 2.0 m AGL, the nominal nose height of outdoor populations. Also, a new direct-measure NO₂ photometer recorded generally lower NO₂ regulatory-oriented concentrations than the conventional FRM chemiluminescence monitor, reporting lower daily maximum hourly average concentrations than the conventional monitor about 3 of every 5 days.

Implications: Employing bias-prone instruments for measurement of ambient ozone or nitrogen dioxide from inlets at inappropriate heights above ground level may result in collection of positively biased data. This paper discusses tests of new regulatory instruments, recent developments in bias-free ozone and nitrogen dioxide measurement technology, and the presence/extent of a near-ground O₃ gradient (NGOG). Collection of unbiased monitor inlet height–appropriate data is crucial for determining accurate design values and meeting National Ambient Air Quality Standards.     

Continuous monitoring of particle emissions during showering

Particle formation from showering may be attributed to dissolved mineral aerosols remaining after evaporation of micron-sized satellite droplets produced by the showerhead or from splashing of larger shower water droplets on surfaces. Duplicate continuous particle monitors measured particle size distributions in a ventilated residential bathroom under various showering conditions, using a full-size mannequin in the shower to simulate splashing effects during showering. Particle mass concentrations were estimated from measured shower particle number densities and used to develop emission factors for inhalable particles. Emission source strengths of 2.7-41.3 microg/ m3/min were estimated under the various test conditions using residential tap water in Columbus, OH. Calculated fine particulate matter (PM2.5) concentrations in the bathroom reached several hundred micrograms per cubic meter; calculated coarse particulate matter (PM10) levels approached 1000 microg/m3. Rates of particle formation tended to be highest for coarse shower spray settings with direct impact on the mannequin. No consistent effects of water temperature, water pressure, or spray setting on overall emission rates were apparent, although water temperature and spray setting did have an effect when varied within a single shower sampling run. Salt solutions were injected into the source water during some tests to assess the effects of total dissolved solids on particle emission rates. Injection of salts was shown to increase the PM2.5 particle formation rate by approximately one third, on average, for a doubling in tap water-dissolved solids content; PM10 source strengths approximately doubled under these conditions, because very few particles >10 microm were formed.     

Potential interference bias in ozone standard compliance monitoring

The U.S. Environmental Protection Agency has established a federal reference method (FRM) for ozone (O3) and allowed for designation of federal equivalent methods (FEMs). However, the ethylene-chemiluminescence FRM for O3 has been replaced by the UV photometric FEM by most state and local monitoring agencies because of its relative ease of operation. Accumulating evidence indicates that the FEM is prone to bias under the hot, humid, and stagnant conditions conducive to high O3 formation. This bias may lead to overreporting hourly O3 concentrations by as much as 20-40 ppb. Measurement bias is caused by contamination of the O3 scrubber, a problem that is not detected by dry air calibration. An adequate wet test has not been codified, although a procedure has been proposed for agency consideration. This paper includes documentation of laboratory tests quantifying specific interferant responses, collocated ambient FRM/FEM monitoring results, and smog chamber comparisons of the FRM and FEMs with alternative scrubber designs. As the numbers of reports on monitor interferences have grown, interested parties have called for agency recognition and correction of these biases.