Contributions of internal emissions to peaks and incremental indoor PM2.5 in rural coal use households.

Indoor air quality is critically important to the human as people spend most time indoors. Indoor PM2.5 is related to the outdoor levels, but more directly influenced by internal sources. Severe household air pollution from solid fuel use has been recognized as one major risk for human health especailly in rural area, however, the issue is significantly overlooked in most national air quality controls and intervention policies. Here, by using low-cost sensors, indoor PM2.5 in rural homes burning coals was monitored for ~4 months and analyzed for its temporal dynamics, distributions, relationship with outdoor PM2.5, and quantitative contributions of internal sources. A bimodal distribution of indoor PM2.5 was identified and the bimodal characteristic was more significant at the finer time resolution. The bimodal distribution maxima were corresponding to the emissions from strong internal sources and the influence of outdoor PM2.5, respectively. Indoor PM2.5 was found to be correlated with the outdoor PM2.5, even though indoor coal combustion for heating was thought to be predominant source of indoor PM2.5. The indoor-outdoor relationship differed significantly between the heating and non-heating seasons. Impacts of typical indoor sources like cooking, heating associated with coal use, and smoking were quantitatively analyzed based on the highly time-resolved PM2.5. Estimated contribution of outdoor PM2.5 to the indoor PM2.5 was ~48% during the non-heating period, but decreased to about 32% during the heating period. The contribution of indoor heating burning coals comprised up to 47% of the indoor PM2.5 during the heating period, while the other indoor sources contributed to ~20%. The study, based on a relatively long-term timely resolved PM2.5 data from a large number of rural households, provided informative results on temporal dynamics of indoor PM2.5 and quantitative contributions of internal sources, promoting scientific understanding on sources and impacts of household air pollution.