Residential coal combustion as a source of primary sulfate in Xi'an, China

Sulfate is a major component of atmospheric fine particulate matter in the troposphere. Globally, observed sulfate concentrations are generally high in summer and low in winter. Current air quality models fail to reproduce high winter sulfate concentrations observed during substantial pollution episodes in northern China. From ambient measurement data, it was reported that contrary to current belief, sulfate, that is normally considered as a secondary species, was actually largely primary. The present study presents direct evidence of primary sulfate emissions from residential coal combustion (RCC) by testing particulate matter (PM) emissions from briquettes and chunk coal burned in a household stove. Sulfate accounted for 16.6 +/- 7.7% and 29.2 +/- 8.7% of PM2.5 mass emissions from this stove burning coal briquettes and chunk coal, respectively. Using the 2014 emissions inventory, 1215 tons of PM10 sulfate were estimated to have been emitted from RCC in Xi'an, a megacity that continually experiences severe PM pollution in northern China. The RCC contribution was also estimated using a constrained positive matrix factorization in which RCC accounted for averages of 26% and 32% of ambient PM2.5 mass during the heating season at urban and rural sites in Xi'an, respectively. The winter contributions were higher than that during non-heating season. PM2.5 emissions from RCC during heating season were 1.5-4.0 times those during non-heating season for the urban sites, and 2.5 to 6.8 times those for the rural sites. Thus, severe PM pollution in rural areas resulted from intense household coal combustion for space heating. Primary sulfate from RCC was responsible for a large fraction of ambient sulfate during heating season, on average contributing 38.9% and 49.2% of ambient sulfate mass in PM2.5 at urban sites and rural sites, respectively. These results highlight the fact that RCC is an important primary source of atmospheric sulfate during the heating season, and provides information that will improve air quality models and support development of more effective abatement strategies for atmospheric particles.