Coal combustion has represented a very important atmospheric mercury (Hg) source in the past 5 decades, especially in eastern Asia. Compared with coal-fired power plants (CFPPs) and industrial boilers, Hg emissions from the residential sector have not drawn much attention. In this study, two field campaigns were carried out to quantify Hg emission ratios and Hg speciation discharged into the ambient atmosphere from residential coal combustion (RCC) in Guizhou province, Southwest China. The average Hg emission ratio, based on the mass balance calculation from coal samples collected in 27 families, was estimated to be 99.6%, higher than those found in the majority of the previous studies (52.0-99.8%). Total Hg in the exhausted flue gas from five families in different areas in Guizhou ranged from 5.4 to 18.5 mu g/m(3) and was mainly affected by the Hg contents in the fuel (lump coal or briquette coal). Hg species in the flue gas from RCC were measured on site for the first time, indicating Hg in the exhausted flue gas was dominated by Hg-0 (91.2 +/- 3.8%) rather than Hg2+ (7.6 +/- 3.5%) or Hg(p) (1.2 +/- 1.7%), despite different coal types with different associated Hg contents being used in different families. Such a finding is very different from a previous assumption that Hg(p) is the dominant emitted species. The total Hg emissions from RCC in Guizhou were estimated to be 48.9 Mg (10(6) g) between 1990 and 2016, with annual emission amount of 1.2-2.3 Mg Hg/year. The annual Hg emission amounts from RCC were likely more than double of those emitted from CFPPs in this province in the more recent years, indicating the necessity of better quantifying this source sector and setting stricter emission control measures.