Primary sources for benzene include motor vehicle exhaust, gasoline storage and filling stations, oil and gas refineries, and chemical manufacturing facilities. Exposure to benzene has been associated with a range of acute and chronic health effects. The high toxicity of benzene means its level in ambient air is strictly regulated by various agencies, including the US EPA under the National Emission Standards for Hazardous Air Pollutants (NESHAP).

Toluene, ethylbenzene, and xylene are important industrial solvents used in a variety of processes and products. All are ozone precursors.

Aeroqual offers two options for measuring benzene. The Aeroqual benzene module is designed to measure ambient benzene (at ppb levels) at both industrial fenceline and urban sites. The BTEX module is designed to measure BTEX at fenceline and urban sites, at levels relevant to human health and ozone precursor studies.

The Benzene module works via temperature programmed desorption. Air is sampled through the gas inlet into a proprietary adsorbent bed, where VOCs, including benzene, are adsorbed. The temperature of the bed is increased at a known and controlled rate. VOCs are desorbed at different temperatures, allowing separation of benzene from other compounds. A PID sensor then measures the benzene concentration. The benzene module provides continuous measurement of benzene, with a 15-minute measurement interval. The Benzene inlet samples at approximately 400 mL min^-1.

The BTEX module is a carrier gas-free miniature gas chromatograph (GC), manufactured by Pollution Analytical Equipment. It uses a MEMS (Micro Electro-Mechanical System) microfluidics design for selective pre-concentration and GC separation. Analyte detection is via a 10.6 eV photoionization detector. Peak quantification is performed automatically using built-in curve fitting algorithms. The BTEX module measures a sample approximately every 15 minutes. This measurement sequence comprises a 10-minute sample pre-concentration, followed by injection and an approximately 5-minute chromatographic separation. The BTEX inlet samples at approximately 400 mL min^-1.

The Benzene and BTEX module performance specifications are given in the table below.

Both the Benzene and BTEX modules can be field calibrated using standard calibration equipment and reference gases. This ensures the module calibration is fully traceable to NIST primary standards.

Benzene and BTEX module factory calibration is achieved using a zero-air source, a certified calibration gas standard and a certified gas dilution calibrator. The Aeroqual AirCal 1000 or 8000 systems can be used to calibrate the Benzene and BTEX module in the field.

Calibration frequency will be dependent on the user’s data quality objectives and Quality Assurance Project Plan (QAPP), but based on the low rate of field drift, the recommended calibration frequency is 1-3 months.

The Benzene and BTEX modules contain several components that need to be maintained for optimum performance.

In the Benzene module, the adsorbent bed has an expected lifetime of 18-24 months of continuous operation. The expected life of the lamp in the PID sensor is approximately 200 days of continuous operation. The Benzene module can be returned to Aeroqual for these replacements.

In the BTEX module, the carrier gas filter and MEMS pre-concentrator both have an expected life of 1 year of continuous operation. The expected life of the lamp in the PID sensor is approximately 200 days of continuous operation. The MEMS chromatographic column should be replaced after 2 years of continuous operation. The BTEX module can be returned to Aeroqual for these replacements.