Ambient Air Monitoring for Sulfur Compounds

The role of sulfur compounds in our environment has been the subject of much speculation during the past two decades. An evaluation of the effect of man-made contributions to the biogeochemical sulfur cycle requires a comprehensive examination of source magnitudes, atmospheric concentrations and removal processes. Many voids exist in our present knowledge of these parameters despite the contributions of numerous researchers. Adequate information is needed concerning the various forms of sulfur as well as other critical constituents which exist in our biosphere so that their interrelationship and role in the mechanisms of the sulfur cycle may be more fully understood.     

Sampling Frequency Guidance for Ambient Air Toxics Monitoring

Abstract

The U.S. Environmental Protection Agency (EPA) is in the process of designing a national network to monitor hazardous air pollutants (HAPs), also known as air toxics. The purposes of the expanded monitoring are to (1) characterize ambient concentrations in representative areas; (2) provide data to support and evaluate dispersion and receptor models; and (3) establish trends and evaluate the effectiveness of HAP emission reduction strategies. Existing air toxics data, in the form of an archive compiled by EPA’s Office of Air Quality Planning and Standards (OAQPS), are used in this paper to examine the relationship between estimated annual average (AA) HAP concentrations and their associated variability. The goal is to assess the accuracy, or bias and precision, with which the AA can be estimated as a function of ambient concentration levels and sampling frequency. The results suggest that, for several air toxics, a sampling schedule of 1 in 3 days (1:3) or 1:6 days may be appropriate for meeting some of the general objectives of the national network, with the more intense sampling rate being recommended for areas expected to exhibit relatively high ambient levels.     

Analysis of VOCs in Ambient Air Using Multisorbent Packings for VOC Accumulation and Sample Drying

Abstract

Solid multisorbent packings have been characterized for trapping and release efficiency of trace (10-20 ppbv in humidified zero air) volatile organic compounds (VOCs). The use of a two-stage trapping system reduces sample water content typically by more than 95.5% while maintaining a trapping and release efficiency of 100% for 49 VOCs, including eight water-soluble VOCs. Three combinations of primary tube and focusing tube are examined in detail by using an atomic emission detector to monitor hydrogen as an indication of residual water vapor, and to monitor either chlorine, bromine, or carbon for target VOCs. Linearity of response to individual VOCs, the presence of artifacts, and a laboratory monitoring application are also discussed.     

Monitoring of Benzene in Ambient Air with Organic Vapor Badges

The original Texas Intersection Model (TEXIN) for air quality near street intersections has been widely adopted across the nation. At the request of transportation agencies from several states, the TEXIN model was revised to improve its performance and flexibility. The new capabilities include T-intersections, one-way streets, four-way stops, inspection/maintenance capabilities, anti-tampering programs and a short-cut emissions algorithm. TEXIN2 uses the CMA procedures for estimating traffic flow parameters, MOBILE3 to determine free flowing traffic cruise emissions, and CALINE3 to model the pollutant distribution downwind of an intersection. The new model, TEXIN2, offers the user more accurate simulations with enhanced versatility while still requiring a minimal amount of input data.     

Regional Monitoring of Ambient Air Carbon Monoxide in Fairbanks,Alaska

A method employing the timed fill of Mylar bags was used to obtain average carbon monoxide concentration values for ten locations in the Fairbanks, Alaska, area. The method is shown to be accurate, reliable, and inexpensive. The correlation coefficient between the bag sampling method and a continuous carbon monoxide analyzer was 0.945; correlation of carbon monoxide data from several locations to a reference in downtown Fairbanks showed a general decrease with distance from the reference analyzer. Analysis of data from Fairbanks shows that carbon monoxide concentrations during the winter months may have to be reduced 50-75% to achieve the State and National Ambient Air Quality Standards; the high levels of carbon monoxide are the result of emissions from automobile traffic during periods of severe inversion in the Fairbanks basin.     

Solid Absorbent Method for Sampling and Analysis of Nitrogen Dioxide in Ambient Air

A sampling and analysis system was developed for measuring concentrations of nitrogen dioxide in ambient air. Copper shot was found to be an effective absorbent for collecting samples at ambient levels. The analytical system was based on the desorption of the sample from the copper by heating in a hydrogen carrier stream. The desorbed sample was then determined by a combination of catalytic pyrolysis, which converted it to ammonia, and a Coulson Conductivity Detector. Data are presented showing overall recovery, effects of storage of collected samples, results for 24-hour sampling and interferences. Studies indicated the feasibility of the method for short time sampling as well as for 24-hour sampling periods. The detection limit was shown to be as little as 100 ng NO₂ in laboratory studies. This solid absorbent method provides a simple, convenient, and compact means for air sampling.     

A Method for Using Air Pollution Measurements and Public Opinion to Establish Ambient Air Quality Standards

Air quality was comprehensively evaluated by means of sampling at 21 locations over metropolitan Birmingham during a period of one year. Thousands of integrated samples of three common atmospheric gaseous pollutants and two common particulate pollutants were collected and analyzed. Following the year of sampling in 1964, a random household survey was completed by conducting personal interviews at more than 7200 households over metropolitan Birmingham. Statistical reduction of household survey results by census tract and by neighborhood area provided domestic fuel and waste burning emission data as well as public (resident) opinion on specific air pollution effects. The relationship between ambient air quality and neighborhood opinion of air pollution effects on health and property are evaluated statistically. Ambient standards are suggested which are based upon those air pollution levels shown to have adverse effects on approximately one-third of the people.     

Evaluation of the Sodium Arsenite Method for Measurement of NO2 in Ambient Air

The sodium arsenite method for measurement of nitrogen dioxide in ambient air was evaluated. The method has a constant-high collection efficiency (82%) for nitrogen dioxide, and is insensitive to normal variations in operating parameters. Nitric oxide and carbon dioxide are positive and negative interferents, respectively. The combined average effect of these interferents, over ambient levels, is a positive bias of 9.9 µg/m³. This bias, although statistically significant, is minor ( 10 % ) in relation to the ambient air standard of 100 µg NO₂/m³ and does not warrant modification of the method to remove the interference.     

The National Ambient Air Monitoring Strategy: Rethinking the Role of National Networks

Abstract

A current re-engineering of the United States routine ambient monitoring networks intended to improve the balance in addressing both regulatory and scientific objectives is addressed in this paper. Key attributes of these network modifications include the addition of collocated instruments to produce multiple pollutant characterizations across a range of representative urban and rural locations in a new network referred to as the National Core Monitoring Network (NCore). The NCore parameters include carbon monoxide (CO), sulfur dioxide (SO₂), reactive nitrogen (NO_y), ozone (O₃), and ammonia (NH₃) gases and the major fine particulate matter (PM_2.5) aerosol components (ions, elemental and organic carbon fractions, and trace metals). The addition of trace gas instruments, deployed at existing chemical speciation sites and designed to capture concentrations well below levels of national air quality standards, is intended to support both long-term epidemiological studies and regional-scale air quality model evaluation. In addition to designing the multiple pollutant NCore network, steps were taken to assess the current networks on the basis of spatial coverage and redundancy criteria, and mechanisms were developed to facilitate incorporation of continuously operating particulate matter instruments.     

A Cyclone for Size-Selective Sampling of Ambient Air

A cyclone with a 47 mm after-filter has been developed for ambient air size-selective monitoring. It has been extensively evaluated with laboratory-generated aerosol. Variation of the pressure drop and 50% cut point with flow rate show that the cyclone operates in a single flow regime with a vortex in the outlet flow. The particle size cutoff curve is comparable in sharpness to a cascade impactor and is the same for solid or liquid particles. At 21.7 L/min, D ₅₀ is 2.5μm and at 15.4 L/min, D ₅₀ is 3.5 μm. Collection efficiency data for flow rates from 8 to 27 L/min fit a universal curve when plotted vs. the normalized particle diameter, (D-D ₅₀)/D ₅₀ Reentrainment of previously deposited particles is less than 1 % of the loading per day. In field tests the cyclone has proved to be a very satisfactory size-selective sampler.     

Are We Ready for Ambient Air Quality Standards

The legal basis for the control of air pollution has progressed from nuisance law to the statutory regulation of specific substances as the sophistication of the sciences involved has progressed. But, the control of air pollution by pursuing air pollutants one by one as evidence accumulates against them seems clearly inadequate to a technology producing new pollutants at an almost geometric rate and inappropriate to the gathering body of evidence relating air pollution to health effects. To understand the need for changed control concepts it is necessary to understand the biological problem and the evidence that has been accumulated.     

Distribution of Light Hydrocarbons in Ambient Air

The diversity of hydrocarbons which are present in ambient polluted air provide a potentially rich source of information concerning the nature of this type of pollution. Measurements of the relative amounts of various hydrocarbons can be correlated with the various possible sources. Since hydrocarbon reactivities vary widely it is also possible to estimate the extent to which various individual hydrocarbons have reacted. Except for samples taken deliberately near sources of hydrocarbon pollution these air samples invariably resemble auto exhaust with an addition of natural gas and of C₃–C₅ paraffins which resemble gasoline vapor. Samples taken in industrial areas and near the smoke plume from a brush fire showed distinctive differences in composition. During the smog season in the fall of 1968 good data were obtained of “typical” or “representative” samples of light, medium and heavy smog. These show the expected depletion of more reactive hydrocarbons in a much more convincing way than before. By comparing these distributions with composition in unreacted samples and by making use of data from bottle irradiations, it was possible to estimate the contribution of the various hydrocarbons in terms of “amount reacted.” The amounts of higher hydrocarbons present and reacted were also estimated from gasoline composition.     

Potential Impact on Standards for Toxic Air Contaminants

Since the Bhopal incident, the public has placed pressure on regulatory agencies to set community exposure limits for the dozens of chemicals that may be released by manufacturing facilities. More or less objective limits can be established for the vast majority of these chemicals through the use of risk assessment. However, each step of the risk assessment process (i.e., hazard identification, dose-response assessment, exposure assessment, and risk characterization) contains a number of pitfalls that scientists need to avoid to ensure that valid limits are established. For example, in the hazard identification step there has been little discrimination among animal carcinogens with respect to mechanism of action or the epidemiology experience. In the dose-response portion, rarely is the range of “plausible” estimated risks presented. Physiologically based pharmacokinetic (PB-PK) models should be used to understand the difference between the tissue doses and the administered dose, as well as the difference in target tissue concentrations of the toxicant between rodents and humans. Biologically-based models like the Moolgavkar-Knudson-Venzon (MKV) should be developed and used, when appropriate. The exposure assessment step can be significantly improved by using more sensitive and specific sampling and analytical methods, more accurate exposure parameters, and computer models that can account for complex environmental factors. Whenever possible, model predictions of exposure and uptake should be validated by biological monitoring of exposed persons (urine, blood, adipose) or by field measurements of plants, soil, fish, air, or water. In each portion of an assessment, the weight of evidence approach should be used to identify the most defensible value. In the risk characterization, the best estimate of the potential risk as well as the highest plausible risk should be presented. Future assessments would be much improved if quantitative uncertainty analyses were conducted. Procedures are currently available for making future assessments. By correcting some of these shortcomings in how health risk assessments have been conducted, scientists and risk managers should be better able to identify scientifically appropriate ambient air standards and emission limits.     

The Need for Representative Ambient Air Carbon Monoxide Sampling

Recent investigations have indicated that ambient air CO measurements may not reflect population exposure to CO. The lack of correlation may be due to improper siting of CO instruments, improper interpretation of air quality data, or both. Studies of population carboxy-hemoglobin levels are evaluated and compared with ambient air data.,

No significant correlation was found between median population COHb levels and reductions in CO concentrations required to meet ambient air standards when calculations used to estimate reductions were based on the second highest 8 hour average. However, calculated reductions based on annual average concentrations and a trend analysis technique correlated significantly with COHb levels in five cities from which both CAMP and COHb data were available.

Studies to determine the nature of the relationship between ambient air CO concentrations and population COHb levels are needed. The differences between the Occupational Safety and Health Act Regulations and the National Ambient Air Standards for carbon monoxide should be scrutinized to determine if a redefinition of the standards or their applicability is warranted. A reevaluation of the controls necessary to make reductions in population COHb burden may be necessary.     

Methodology for Siting Ambient Air Monitors at the Neighborhood Scale

Abstract

In siting a monitor to measure compliance with U.S. National Ambient Air Quality Standards (NAAQS) for par-ticulate matter (PM), there is a need to characterize variations in PM concentration within a neighborhood-scale region to achieve monitor siting objectives. A simple methodology is provided here for the selection of a neighborhood-scale site for meeting either of the two objectives identified for PM monitoring. This methodology is based on analyzing middle-scale (from 100 to 500 m) data from within the area of interest. The required data can be obtained from widely available dispersion models and emissions databases.

The performance of the siting methodology was evaluated in a neighborhood-scale field study conducted in Hudson County, NJ, to characterize the area’s inhalable particulate (PM₁₀) concentrations. Air monitors were located within a 2- by 2-km area in the vicinity of the Lincoln Tunnel entrance in Hudson County. Results indicate the siting methodology performed well, providing a positive relationship between the predicted concentration rank at each site and the actual rank experienced during the field study. Also discussed are factors that adversely affected the predictive capabilities of the model.     

Field Method Comparison between Passive Air Samplers and Continuous Monitors for VOCs and NO2 in El Paso,Texas

Abstract

This study evaluates the performance of Model 3300 Ogawa Passive Nitrogen Dioxide (NO₂) Samplers and 3M 3520 Organic Vapor Monitors (OVMs) by comparing integrated passive sampling concentrations to averaged hourly NO₂ and volatile organic compound (VOC) measurements at two sites in El Paso, TX. Sampling periods were three time intervals (3-day weekend, 4-day weekday, and 7-day weekly) for three consecutive weeks. OVM concentrations were corrected for ambient pressure to account for higher elevation. Precise results (<5% relative standard deviation, RSD) were found for NO₂ measurements from collocated Ogawa samplers. Reproducibility was lower from duplicate OVMs for BTEX (benzene, toluene, ethylbenzene, and xylene isomers) VOCs (≥7% RSD for 2-day samples) with better precision for longer sampling periods. Comparison of Ogawa NO₂ samplers with chemiluminescence measurements averaged over the same time period suggested potential calibration problems with the chemiluminescence analyzer. For BTEX species, generally good agreement was obtained between OVMs and automated-gas chromatograph (auto-GC) measurements. The OVMs successfully tracked increasing levels of VOCs recorded by the auto-GCs.