Additional information about the DRX Aerosol Monitors used on this project. +
Tappan Zee Constructors, LLC (TZC) will be monitoring particulate matter (PM) concentrations in areas adjacent to the New NY Bridge project construction site.
Engineers will use the DRX Aerosol Monitor manufactured by TSI Incorporated to measure on-site PM concentrations. The data collected from the device is used to evaluate work practices and dust control measures to ensure minimal impact to the surrounding community.
The DRX is a portable, rugged light-scattering laser photometer that detects particles by measuring the total amount of light scattered as the unit pulls air in through the optical chamber. It provides real-time particle mass concentrations in micrograms per cubic meter (µg/m3). Photometer-type meters like the DRX are commonly used at construction and environmental remediation sites to measure on-site PM concentrations.
PM is generally measured in two size categories: particles with a diameter of less than or equal to 2.5 µg/m3 (PM2.5) and particles with a diameter of less than or equal to 10 µg/m3 (PM10). By comparison, a shaft of human hair is about 60 µg/m3.
The useful data collected from the DRX is limited to upwind and downwind comparisons. The wind comparisons provide a net impact of the construction activity regardless of the background level at any given time. Rapid increases in PM concentrations can also indicate effects from other nearby local sources of dust or engine exhaust that are not construction-related.
The monitoring stations are located at ground level (approximately 5 to 6 feet above the ground) in the area of the project site, which includes near-field sources such as lawn mowing or leaf blowing activities, motor vehicle traffic, a rail line, and local building heating systems. All of these sources will at times have an impact on monitor data.
The PM values recorded by the devices should not be used to compare to the National Ambient Air Quality Standard (NAAQS). Sampling stations that monitor for the NAAQS, such as the sites operated by the New York State Department of Environmental Conservation (NYSDEC), are positioned away from such local sources so they are able to measure regional air pollution concentrations.
The bridge construction area is impacted by humidity due to local weather and the proximity to the Hudson River and there are significant non-construction related particulate or aerosol sources in the area. These conditions impact the operation of the DRX and therefore the data collected is expected to be higher than those monitored by the NYSDEC at ambient monitoring stations in Rockland and Westchester counties.
The NAAQS for PM10 is based on a 24-hour average concentration (up to one exceedance per year allowed). The PM2.5 NAAQS are based on an annual average and the annual 98th percentile of the 24-hr average concentration (i.e., 8th highest value in a calendar year), averaged over 3 years. Annual and 24-hour collection periods are unsuitable for monitoring the impact of construction activities, which requires real-time feedback.
When monitoring construction sites for PM concentrations, a shorter sample collection interval is beneficial because it provides quick feedback regarding fugitive dust or unusually high engine emissions that may be generated from project activity.
Photometers are not approved by the United States Environmental Protection Agency as either a Federal Reference Method (FRM) or a Federal Equivalent Method (FEM) for monitoring pollutant concentrations in the context of the NAAQS. However, published reports have shown a positive correlation under controlled conditions when PM2.5 and PM10 data collected with a photometer is compared with data collected using other monitoring system, such as Tapered Element Oscillating Microbalance (TEOM) monitor or a Beta Attenuation Monitor (BAM) which are often used in fixed stations to measure ambient particulate concentrations. Some models of TEOM and the BAM samplers are FEMs; however, these are generally designed for a permanent installation and require AC power and a temperature-controlled room or enclosure.