2.4. Measurements and Quality Control

The field samplings in three categories of sub-MEs (i.e., thermal environment, air quality, and lighting environment) were conducted in station halls, platforms and subway cabins (Figure 2). This research follows the current standard of subway acoustic environment, viz. the Acoustical Requirement and Measurement on Station Platform of Urban Rail Transit (GB14227-2006) and concentrates on platform situation when monitoring acoustic environment. Besides, the noises on platforms are found in the pilot study that they are commonly composed of e.g., subway trains arriving or leaving, out screen broadcasting, talking noises, and even shoes clattering of passengers.

Sampling Sites of Nanjing Metro Line X (NMLX).

The whole process of sampling point selection and measurements in sub-MEs are rigorously in line with the latest national standards, including the Examination methods for public places—Part 1: physical parameters (GB/{"type":"entrez-nucleotide","attrs":{"text":"T18204","term_id":"462990","term_text":"T18204"}}T18204.1-2013), the Examination methods for public places—Part 2: Chemical pollutant (GB/{"type":"entrez-nucleotide","attrs":{"text":"T18204","term_id":"462990","term_text":"T18204"}}T18204.2-2014), the Examination methods for public places scrutiny—Part 3: Airborne microorganism (GB/{"type":"entrez-nucleotide","attrs":{"text":"T18204","term_id":"462990","term_text":"T18204"}}T18204.3-2013), the Examination methods for public places—Part 6: Technical specifications of health monitoring (GB/{"type":"entrez-nucleotide","attrs":{"text":"T18204","term_id":"462990","term_text":"T18204"}}T18204.6-2013), and the Acoustical requirement and measurement on station platform of urban transit (GB14227-2006), where the measurements are incompliance with those of the limit values selected.

According to the standards above, at least three sampling points are required with an air monitoring area over 1000 m². Hence, the points based on indoor depth were selected, and those of station halls and platforms were aligned vertically. Same points were distributed with regard to illuminance monitored, while they were separately designed for acoustic measurement. When the noise was monitored, it was positioned at the middle of the platform with one meter away from screen doors. As for sub-MEs in cabins, we experimented in two head carriages as well as a body carriage of each train, where sampling points were similarly located to those in stations halls or platforms. Three points were then selected based on the length of carriage. Detailed distributions of the sampling points are shown in Figure 3.

Distributions of sampling points in subway station halls, platforms and cabins.

According to Figure 3, we conducted the sampling in point coding order, and monitored ME-indicators simultaneously at each point. Subway cabins were monitored in time intervals before we arrived at the next sampling location. Since the experiments were conducted in ventilation season, it was ventilation systems rather than cooling systems of air conditioning that were operated at the time of sampling. Besides, no air curtain was opened at the entrance of each station hall. All platforms were equipped with screen doors.

Measurement at each point was tested via collocated sampling. Side-by-side comparisons were made twice for samplings in sub-ME of thermal environment, air quality, and lighting environment. Following the standard operation procedures in the Acoustical Requirement and Measurement on Station Platform of Urban Rail Transit (GB14227-2006), noise was monitored individually regarding acoustic environment. It was sampled ten separate times when trains were arriving and leaving platforms. Moreover, equivalent consecutive sound level A (LAeq) under the circumstance of no trains passing was monitored as fundamental noise levels on platforms. Specifically, it was monitored five times when screen broadcast was on, and another five with broadcast off. Mean value was finally calculated for sampling measurement of each sub-ME indicator.

The sampling experiment was conducted without impact on daily passengers commuting in NMLX. It was assisted by specialists from Nanjing Municipal Center for Disease Control and Prevention during the whole sampling process. We collected all air samples at the height of breathing zone (0.5–1.5 m) [40]. Similarly, when illuminance and noise levels were monitored, the sampling inlets of instruments were located at roughly 1.5 m above the ground. For validity of air samples, instruments were kept off the zones with strong wind, such as ventilation shafts, air outlets, and screen door-sides, and kept at least one meter away from walls or carriage bodies. Moments of passengers on and off were shunned in terms of cabin samplings. Furthermore, the sampling time selected was reasonable, as it avoided rainy days and daily rush hours (7.00–9.00 a.m. and 17.00–19.00 p.m.). All instruments were seriously zero-calibrated before each measurement carried out at sampling locations. Monitors at one point were set to around 6-s sampling intervals [37]. The data obtained were timely logged by trained lab assistants. All measurements of sub-MEs were completed on the same day in order to avoid ambient influence on the daily basis.