In light of several street canyon studies cited above, it appears that the street canyon may confine resuspended roadway dust and may not be a suitable location for ambient monitors. However, since roads with lower traffic (less than approximately 3,000 vehicles per day) generally do not generate

a concentrated particulate plume, monitors located in Zone B should not be adversely influenced. Therefore, for those cases where the traffic is less than approximately 3,000 vehicles per day, the monitor must be located greater than 5 meters from the edge of the nearest traffic lane and 2 to 15 meters above ground level (either Zone A or Zone B).

In the case of elevated roadways where the monitor must be placed below the level of the roadway, then the monitor should be located no closer than approximately 25 meters from the edge of the nearest traffic lane. This separation distance applies for those situations where the road is elevated greater than 5 meters above the ground level, and applies to all traffic volumes.

2.4 Other Considerations

Stations should not be located in an unpaved area unless there is vegetative ground cover year round so that the impact of reentrained or fugitive dusts will be kept to a minimum. Addtional information on TSP probe siting may be found in reference 10.

3. SULFUR DIOXIDE (SO2)

3.1 Horizontal and Vertical Probe Placement

As with TSP monitoring, the most desirable height for an SO, monitor inlet probe is near the breathing height. Various factors enumerated before may require that the inlet probe be elevated. Therefore, the inlet probe must be located 3 to 15 meters above ground level. If the inlet probe is located on the side of a building, then it should be located on the windward side of the building relative to the prevailing winter wind direction. The inlet probe must also be located more than 1 meter vertically or horizontally away from any supporting structure and also away from dirty, dusty

areas.

3.2 Spacing from Obstructions

No furnace or incineration flues, or other minor sources of SO2 should be nearby. The separation distance is dependent on the height of the flues, type of waste or fuel burned, and the quality of the fuel (sulfur content). If the inlet probe is located on a roof or other structure, it must be at least 1 meter from walls, parapets, penthouses, etc. The inlet probe should be placed more than 20 meters from trees and must be located away from obstacles and buildings. The distance between the obstacles and the inlet probe must be at least twice the height that the obstacle protrudes above the inlet probe. Sampling stations that are located

closer to obstacles than this criterion allows should not be classified as a neighborhood scale, since the measurements from such a station would closely represent middle scale stations. Therefore, stations not meeting the criterion should be classified as middle scale. Airflow must also be unrestricted in an arc of at least 270° around the inlet probe, and the predominant wind direction for the season of greatest pollutant concentration potential must be included in the 270° arc. If the probe is located on the side of a building, 180° clearance is required. Additional information on SO, probe siting criteria may be found in reference 11.

4. CARBON MONOXIDE (CO)

4.1 Horizontal and Vertical Probe Placement

Because of the importance of measuring population exposure to CO concentrations, air should be sampled at average breathing heights. However, practical factors require that the inlet probe be higher. The required height of the inlet probe for CO monitoring is therefore 3+2 meter for a microscale site, which is a compromise between representative breathing height and prevention of vandalism. The recommended 1 meter range of heights is also a compromise to some extent. For consistency and comparability, it would be desirable to have all inlets at exactly the same height, but practical considerations often prevent this. Some reasonable range must be specified and 1 meter provides adequate leeway to meet most requirements.

For the middle and neighborhood scale stations, the vertical concentration gradients are not as great as for the microscale station. This is because the diffusion from roads is greater and the concentrations would represent larger areas than for the microscale. Therefore, the required height of the inlet probe is 3 to 15 meters for middle and neighborhood scale stations. The inlet probe must be located more than 1 meter in the vertical or horizontal direction from any supporting structure.

4.2 Spacing from Obstructions Airflow must also be unrestricted in an arc of at least 270° around the inlet probe, and the predominant wind direction for the season of greatest pollutant concentration potential must be included in the 270° arc. If the probe is located on the side of a building, 180° clearance is required.

4.3 Spacing from Roads

Street canyon and traffic corridor stations (microscale) are intended to provide a measurement of the influence of the immediate source on the pollution exposure of the population. In order to provide some reasonable consistency and comparability in the air quality data from such stations, a minimum distance of 2 meters and a maximum dis

tance of 10 meters from the edge of the nearest traffic lane must be maintained for these CO monitor inlet probes. This should give consistency to the data, yet still allow flexibility of finding suitable locations.

Street canyon/corridor (microscale) inlet probes must be located at least 10 meters from an intersection and preferably at a midblock location. Midblock locations are preferable to intersection locations because intersections represent a much smaller portion of downtown space than do the streets between them. Pedestrian exposure is probably also greater in street canyon/corridors than at intersections. Finally, the practical difficulty of positioning sampling inlets is less at midblock locations than at the intersection.

In determining the minimum separation between a neighborhood scale monitoring station and a specific line source, the presumption is made that measurements should not be unduly influenced by any one roadway. Computations were made to determine the separation distances, and table 1 provides the required minimum separation distance between roadways and neighborhood scale stations. Sampling stations that are located closer to roads than this criterion allows should not be classified as a neighborhood scale, since the measurements from such a station would closely represent the middle scale. Therefore, stations not meeting this criterion should be classified as middle scale. In some cases, such a monitoring station would be acceptable for SLAMS purposes, but not NAMS since no middle scale NAMS stations are required. Additional information on CO probe siting may be found in reference 12.

The inlet probe for ozone monitors should be as close as possible to the breathing zone. The complicating factors discussed previously, however, require that the probe be elevated. The height of the inlet probe must be located 3 to 15 meters above ground level. The probe must also be located more than 1 meter vertically or horizontally away from any supporting structure.

5.2 Spacing from Obstructions

The probe must be located away from obstacles and buildings such that the distance between the obstacles and the inlet probe is at least twice the height that the obstacle protrudes above the sampler. The probe should also be located at least 20 meters from trees. Since the scavenging effect of trees is greater for ozone than for TSP, SO2, CO, and NO2, strong consideration should be used in locating the inlet probe to avoid this effect. Airflow must be unrestricted in an arc of at least 270° around the inlet probe, and the predominant wind direction for the season of greatest pollutant concentration potential must be included in the 270° arc. If the probe is located on the side of a building, 180° clearance is required. 5.3 Spacing from Roads

It is important in the probe siting process to minimize destructive interferences from sources of nitric oxide (NO) since NO readily reacts with ozone. Table 2 provides the required minimum separation distances between roadways and ozone monitoring stations. These distances were based on recalculations using the methodology in reference 13 and validated using more recent ambient data collected near a major roadway. Sampling stations that are located closer to roads than this criterion allows should not be classified as neighborhood or urban scale, since the measurements from such stations would more closely represent the middle scale. Accordingly, such stations should be classified as middle scale. In some cases, a middle scale station would be acceptable for SLAMS purposes, but not for NAMS since no middle scale NAMS are required. The minimum separation distance must also be maintained between an ozone station and other similar volumes of automotive traffic, such as parking lots. Additional information on ozone probe siting criteria may be found in reference 13.

'Distances should be interpolated based on traffic flow.

5. Ozone (O1)

5.1 Vertical and Horizontal Probe Place

ment

6. NITROGEN DIOXIDE (NO2)

6.1 Vertical and Horizontal Probe Placement

The height of the NO, inlet probe must be 3 to 15 meters above the ground. This is a compromise between measuring in the breathing zone and avoidance of vandalism, finding suitable sites, etc. For NO2, the height does not appear to be a critical factor since the NO, should be fairly well mixed and somewhat uniform in the vertical direction. The distance of the inlet probe from any supporting structure must be greater than 1 meter vertically or horizontally.

6.2 Spacing from Obstructions

Buildings, trees, and other obstacles may possibly scavenge NO2. In order to avoid this kind of interference, the station must be located well away from such obstacles so that the distance between obstacles and the inlet probe is at least twice the height that the obstacle protrudes above the probe. Sampling stations that are located closer to obstacles than this criterion allows should not be classified in the neighborhood or urban scales, since the measurements from such stations would more closely represent the middle scale. Such stations should be classified as middle scale. For similar reasons, a probe inlet along a vertical wall is undesirable because air moving along that wall may be subject to possible removal mechanisms. The inlet probe should also be at least 20 meters from trees. There must be unrestricted airflow in an arc of at least 270° around the inlet probe, and the predominant wind direction for the season of greatest pollutant concentration potential must be included in the 270° arc. If the probe is located on the side of the building, 180° clearance is required.

6.3 Spacing from Roads

It is important that the monitoring probe be removed from oxides of nitrogen sources to avoid measurements being dominated by any one source and to allow time for conver

sion (reactions) of NO emissions to NO2. Further, the effects of roadway sources must be minimized by using separation distances for neighborhood and urban scale stations found in Table 3. These distances were based on recalculations using the methodology in reference 13 and validated using more recent ambient data collected near a major roadway. The minimum ṣeparation distance must also be maintained between an NO, probe and any other similar volume of automotive traffic such as parking lots. Sampling stations that are located closer to roads than this criterion allows should not generally be classified as neighborhood or urban scales, since the measurements from such stations would more closely represent middle scale stations. Such stations should generally be classified as middle scale. In some cases, such a monitoring station would be acceptable for SLAMS purposes, but not for NAMS since no middle scale NAMS are acceptable. Additional information on NO, probe siting criteria may be found in reference 13.

7. PROBE MATERIAL AND POLLUTANT SAMPLE RESIDENCE TIME

For the reactive gases, SO2, NO2, and O3, special probe material must be used. Studies 14-18 have been conducted to determine the suitability of materials such as polypropylene, polyethylene, polyvinylchloride, tygon, aluminum, brass, stainless steel, copper, pyrex glass and teflon for use as intake sampling lines. Of the above materials, only pyrex glass and teflon have been found to be acceptable for use as intake sampling lines for all the reactive gaseous pollutants. Furthermore, EPA 19 has specified borosilicate glass or FEP teflon as the

14-18 See References at end of this Appendix. 19 See References at end of this Appendix.

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only acceptable probe materials for delivering test atmospheres in the determination of reference or equivalent methods. Therefore, borosilicate glass, FEP teflon, or their equivalent must be used for existing and new NAMS or SLAMS.

No matter how nonreactive the sampling probe material is initially, after a period of use reactive particulate matter is deposited on the probe walls. Therefore, the time it takes the gas to transfer from the probe inlet to the sampling device is also critical. Ozone in the presence of NO will show significant losses even in the most inert probe material when the residence time exceeds 20 seconds. 20 Other studies 21-22 indicate that a 10-second or less residence time is easily achievable. Therefore, sampling probes for reactive gas monitors at SLAMS or NAMS must have a sample residence time less than 20 seconds.

8. WAIVER PROVISIONS

It is believed that most sampling probes or monitors can be located so that they meet the requirements of this appendix. New stations with rare exceptions, can be located within the limits of this appendix. However, some existing stations may not meet these requirements and yet still produce useful data for some purposes. EPA will consider a written request from the State Agency to waive one or more siting criteria for some monitoring stations providing that the State can adequately demonstrate the need (purpose) for monitoring or establishing a monitoring station at that location. For establishing a new station. a waiver may be granted only if both of the following criteria are met:

• The site can be demonstrated to be as representative of the monitoring area as it

would be if the siting criteria were being met.

• The monitor or probe cannot reasonably be located so as to meet the siting criteria because of physical constraints (e.g., inability to locate the required type of station the necessary distance from roadways or obstructions).

However, for an existing station, a waiver may be granted if either of the above criteria are met.

Cost benefits, historical trends, and other factors may be used to add support to the above, however, they in themselves, will not be acceptable reasons for granting a waiver. Written requests for waivers must be submitted to the Regional Administrator. For those SLAMS also designated as NAMS, the request will be forwarded to the Administrator.

9. DISCUSSION AND SUMMARY

Table 4 presents a summary of the requirements for probe siting criteria with respect to distances and heights. It is apparent from Table 4 that different elevation distances above the ground are shown for the various pollutants. The discussion in the text for each of the pollutants described reasons for elevating the monitor or probe. The differences in the specified range of heights are based on the vertical concentration gradients. For CO, the gradients in the vertical direction are very large for the microscale, so a small range of heights has been specified. For SO2, NO2, TSP, and O, (except near roadways), the vertical gradients are smaller and thus a larger range of heights can be used. The upper limit of 15 meters was specified for consistency between pollutants and to allow the use of a single manifold for monitoring more than one pollutant.