Development of a Long-Term Sampling Method for Determination of NMHCs in Indoor Air

Abstract

Vapor intrusion is detrimental for indoor air quality. One of the most common sources of vapor intrusion is soil contaminated with petroleum hydrocarbons. To evaluate the long-term risk from individual exposure to hydrocarbons it is necessary to measure quantitively and reliably an average concentration level of individual pollutants on a monthly or yearly basis. Temporal variability of vapor intrusion from hydrocarbons poses a significant challenge to determination of average exposure and there is a need for reliable long-term integrative sampling. To this end, an analytical method for determination of 10 selected nonmethane hydrocarbons (NMHCs), including hexane, heptane, octane, decane, benzene, toluene, ethyl-benzene, m,p-xylene, o-xylene, and naphthalene, sampled on active triple-bed tubes filled with Carbograph 2, Carbograph 1, and Carboxen 1003 adsorbents was developed and validated. Extensive laboratory studies proved the absence of breakthrough at 50% HR and ambient temperature for experiments lasting up to 28 days and established a safe sampling time/volume of 20 days/114 L when sampling at a low flow rate of around 4 mL min^-1. In addition, the developed method includes detailed uncertainty calculations for determination of concentrations. Finally, the method was tested by measuring NMHC concentrations in indoor air at a former industrial site during a 2-month-long field campaign in Lyon. The results of the field campaign suggest that 4-week integrated concentration measurements can be achieved by using active sampling on triple-bed tubes at 4.5 mL min^-1.

Keywords: breakthrough studies; gas sampling; indoor air; method development; method validation; non-methane hydrocarbons (NMHCs); uncertainty calculations; vapor intrusion.

Figure 1

Stability of generated concentrations of (A) hexane and (B) benzene in breakthrough studies. In most cases, each point represents an average of 24 measurements. Sometimes, an average of 5 to 23 measurements was taken, depending on availability of analytical data.

Figure 2

Breakthrough curves for (A) hexane and (B) benzene at concentrations of 1, 10, and 70 µg m⁻³ sampled at a flow rate ranging from 2.5 to 4.0 mL min⁻¹. Green circles represent the masses measured in the TB tubes by GC-MS analysis. Blue triangles represent theoretical doped mass values.

Figure 3

Relationship between measured and theoretical values for benzene and hexane. In orange (barely visible due to low values) are the values from breakthrough experiments performed at 1 µg m⁻³, in blue are the values from breakthrough experiments at 10 µg m⁻³, in green are the values from breakthrough experiments at 70 µg m⁻³. A dashed black line is a linear fit through the points obtained when doping the tubes with 1, 10, and 70 µg m⁻³. A dashed red line represents the 1:1 line.

Figure 4

Typical profiles of hexane and benzene concentrations calculated using masses measured by portable GC analyzer. Measurements were taken continuously every hour from 1 February 2021 to 27 April 2021. Expanded uncertainty of the measurements for benzene is 25% and for hexane is 30%. Uncertainty is not applied to the graphics to facilitate observance of the trends.

Figure 5

Comparison of average weekly, bi-weekly, 4-weekly, and 6-weekly concentrations for (A) hexane and (B) benzene. Red error bars represent the results that are not in accordance with the measurements obtained from portable GC.

Figure 6

Comparison of average weekly concentrations for (A) hexane and (B) benzene when measured by GC-MS, two TB tubes connected in series, and two Tenax tubes connected in series.

Figure 7

Experimental laboratory setup to evaluate breakthrough volume of ten selected NMHCs.

Figure 8

Experimental setup for the Lyon campaign to measure indoor concentrations of NMHCs. In blue is the experimental room with dimensions of 4 × 4 × 2.6 m (L × W × H); flooring is 25 cm thick. Adjacent room measures 4 × 2 m (L × W) and has no ceiling. Five yellow cylinders represent openings with area of 4.9 cm² and 25 cm deep made in the floor of the experimental room to increase permeability. Four green cylinders represent gas probes with 20 cm bottom openings depicted in grey, which are installed to sample soil gas at different depths: 1, 2, 3, and 4 m below the slab.