Evaluating isolation of a closed space to suspended particulate matters

4. Results and Discussion

Fig.1 presents the concentration of PM2.5 of the inside and the outside of the apartment during 170-hours evaluation. The concentration is averaged into a 10-minute interval. The vertical lines in red marks the time when all windows and the main door are shut and the time they are opened after the residents returned. The number surrounded by the circle marks some episodes for later analysis.

PM2.5 10 minutes interval
Fig. 1: An entire PM2.5 concentration during 5-day monitoring. Data is aggregated into 10-minute interval.

Fig.2 presents the same data in the Fig. 2. The data is aggregated into 1-hour interval for a clear look.

PM2.5 1 hour interval
Fig 2: PM2.5 concentration with data is aggregated into 1-hour interval.

Fig. 3 presents the concentration of PM10 similars to Fig. 1.

PM10 10 minutes interval
Fig. 3: An entire PM100 concentration during 5-day monitoring. Data is aggregated into 10-minute interval.

Similar to Fig. 2, Fig 4 presented aggregated PM10 concentration over 1-hour period.

PM10 1 hour interval
Fig. 4: PM10 concentration with data is aggregated into 1-hour interval.

Basic ratios of PM2.5 and PM10 are presented in Table 2. The numbers are averaged by the entire period which is slightly different than if only between the period marked by two vertical lines in red. The authors decided to do so because of the convenience

Table 1: Ratios of PM2.5 to PM10 over the entire experiment (170 hours)
# Inside(PM2.5/PM10) Outside(PM2.5/PM10) PM2.5(In/Out) PM10(In/Out)
Mean(in %) 85 82 87 85
Std (in %) 5 4 14 15
Table 2: Ratios of PM2.5 to PM10 over the period that the apartment is isolated (129 hours)
# Inside(PM2.5/PM10) Outside(PM2.5/PM10) PM2.5(In/Out) PM10(In/Out)
Mean(in %) 86 83 86 84
Std (in %) 5 4 14 16

The averaged ratios of PM2.5/PM10 in Table 1 and Table 2 were presented in Figs. 5 and 6 in a time series graph.

Fig. 5: Ratios of PM2.5/PM10 of the inside and the outside environments
Fig. 6: Ratios of PM(s) of the inside to PM(s) of the outside environments
Fig. 7: Temperature of the outside environment

The apartment area is about 70m2 with clear space around the building. The entrance door and the main windows are in the North-South direction. When both the door and windows are open, strong wind imposes a heavy advective flow through the apartment. However, when either the door or the windows are closed, there is no distinct flow in the apartment. The author hypothesized, based on the observation of apparent advective flow, with the door and the windows are shut, the internal environment is relatively isolated from the outside environment.

The data presented in Fig. 1 supported the author's hypothesis partly. The internal environment contains consistently lower PM2.5 and PM10 concentration compared to the outside environment or the I/O < 1. However, the overall reduction is only marginal, about 13-17%. In the total PM10, PM2.5 is the dominant species inside and outside environments. Dominant fine particles (PM2.5) in the suspended particles supports the data that the sliding windows provide a little reduction of PM2.5 and PM10.

Examining closer the episodes (E) marked by the numbers in Fig. 1, E1 is the start-up period which contains an abnormal lower PM(s) concentration. The author did not recall where the fan was turned off or the intake was placed against the wall. E1 is discarded for future discussion.

E2 shows a typical pattern during the experiment period. The interval PM2.5 concentration responded to PM2.5 concentration with a delay. The magnitude of change was smaller for the interval concentration. When the outside concentration increased, the interval one also increased with delay and a smaller value. When the outside concentration decreased, the inside one reduced but the final value is slightly higher than the concentration in the external environment.

For E3, heavy rains with 30-60mm/24h [1, 2] during the night of April 29 and the morning of April 30 was marked by a sharp drop of PM2.5 in the outside environment. The internal value responses to the drop but only about one-third of the reduction and followed by the increment during the day of April 30. Two peaks during April 30 showed the correlation between two values. A slower drop in the early of May 1st followed by the drop of PM2.5concentration. In this instance, the PM2.5 inside remained lower than the outside one to E4.

E4 was when the residents returned to the apartment after the holiday. With the windows opened, the response of the concentration is almost instantly and the magnitude is close, in which the internal one is slightly smaller than the PM2.5 concentration outside.

Through these episodes, the data indicated with the door closed and with small spaces between the sliding windows and under the entrance door resulted in marginal isolation of PM2.5. The delay in the pattern of PM2.5 concentration is the evidence of active transport with the speed between advection and diffusion. The minimal isolation of the apartment suggested a more stringent construction if the residents prefer a better solution such as using sealants in fixed windows and positive pressure by filtered air flow to overcome advective flows from the outside environment.

The second significant finding is a clear diurnal cycle of ratios of PM(s) from the inside to the outside as presented on Fig. 6. The I/O of PM2.5 ratios are the lowest during early mornings and the highest in late afternoons. This pattern indicated a strong correlation of the temperature of the outside to the transport of PM(s) to and from the inside environment. Comparing Figs. 6 & 7 presents the correlation for most of the day, except the April 30 (E4). From this initial findings, the author recognized the correlation but not conclusively suggests a direct effect of the temperature to the I/O ratios. The temperature also has a compound effect to the volume of air and making the same number of particles becomes more dense with a lower temperature and thus, presenting as a higher concentration.

The effectiveness of building isolation to PM(s) found in this study is less expected to the author but in agreement with other studies. Challoner & Gill (2000)[3] evaluated 10 buildings in Ireland and found the ratios of I/O of PM2.5 and NO2 close or above 1. Massey at el. (2008)[4] evaluated I/O ratios of residential homes in central India concluded that I/O ratios closed to from 0.76 to 1.46. The I/O average of residental home by the roadside is close to 1, in the rural area is smaller to 1, and in the urbance is larger than 1 suggesting a different mix of emission sources. A study found a different outcome [5] in which the to I/O ratios are lower in the range 0.5-0.8 and the peak is during the daytime.