[Concentration level and characteristics of indoor particle matter ≤ 2.5 µm in aerodynamic diameter under extreme weather condition in Beijing winter in 2013].

Objective: To study the concentration level and characteristics of indoor particle matter ≤ 2.5 µm in aerodynamic diameter (PM2.5) under extreme weather condition.

Methods: During the period of haze in January 2013 and the fireworks and firecracker setting off in the Spring Festival of February 2013, three monitoring sites located indoor and outdoor were respectively selected by Beijing CDC, considering the vertical and horizontal distance, windows tightness and human activity indoor. PM2.5 samples were collected by filters and measured by laboratory weight method. At the same time, the meteorological data was also collected.

Results: The median (quartiles) of overall concentration level of indoor PM2.5 was 87.76 µg/m(3) (52.05-174.48 µg/m(3)) and was lower than that of outdoor PM2.5 (128.79 µg/m(3), 95.14-221.88 µg/m(3); Z = -4.13, P < 0.01). The concentration of three indoor monitoring sites was different (χ(2) = 23.09, P < 0.01). The PM2.5 concentrations of monitoring point B in poor sealing window was the highest (94.05 µg/m(3); 63.46-189.17 µg/m(3)) and point C in sealed and less human activity, which was the lowest (77.89 µg/m(3), 51.19-144.40 µg/m(3)). The concentration level of indoor PM2.5 in the haze period (273.22 µg/m(3), 223.44-308.47 µg/m(3)) was higher than the overall concentration level of indoor PM2.5 (Z = -5.20, P < 0.01). The concentration level of indoor PM2.5 in the fireworks and firecracker period (167.90 µg/m(3), 129.15-187.90 µg/m(3)) was higher than that in the Spring Festival period (7 days, 72.76 µg/m(3), 36.97-145.30 µg/m(3), Z = -2.34, P < 0.05) and the overall concentration level of indoor PM2.5 (Z = -1.98, P < 0.05); however, it was lower than the concentration level of indoor PM2.5 in the haze period (Z = -3.43, P < 0.01). The I/O ratio (indoor concentration/outdoor concentration) was all less than 1.00 except 4, which was between 1.00-1.09. The mean I/O ratio in monitoring site B, monitoring site A and monitoring site C was 0.69 ± 0.21, 0.64 ± 0.23 and 0.58 ± 0.18, respectively, show significant bias (F = 22.85, P < 0.01). During the period of haze, the fireworks and firecracker and fine weather (when ambient PM2.5 concentration was lower than the standard value of 75 µg/m(3)), the mean I/O ratio was 0.87 ± 0.14, 0.68 ± 0.08 and 0.51 ± 0.18, respectively, showing significant bias (F = 29.88, P < 0.05). Under conditions of snow and high wind speed ( ≥ 3.4 m/s), PM2.5 concentration decreased to the valley point. The valley value of I/O ratio only occurred after several days of high windy weather. Moreover, the PM2.5 concentration level of indoor air showed a delayed 1-2 days after the haze weakened or disappeared.

Conclusion: Mass concentration of indoor PM2.5 increased significantly with increases of outdoor concentration. Haze and setting off fireworks/firecracker could lead to a serious decline of indoor air quality (IAQ), and the improvement of IAQ was lagging behind the outdoor changes.