Correlation slopes of GEM / CO, GEM / CO₂, and GEM / CH₄ and estimated mercury emissions in China, South Asia, the Indochinese Peninsula, and Central Asia derived from observations in northwestern and southwestern China

Abstract

Correlation analyses between atmospheric mercury (Hg) and other trace gases are useful for identification of sources and constraining regional Hg emissions. Emissions of Hg in Asia contribute significantly to the global budget of atmospheric Hg. However, due to the lack of reliable data on the source strength, large uncertainties remain in the emission inventories of Hg in Asia. In the present study, we calculated the correlation slopes of GEM / CO, GEM / CO2, and GEM / CH4 for mainland China, South Asia, the Indochinese Peninsula, and Central Asia using the ground-based observations at three remote sites in northwestern and southwestern China, and applied these values to estimate GEM emissions in the four source regions. The geometric mean GEM / CO correlation slopes for mainland China, South Asia, the Indochinese Peninsula, and Central Asia were 7.3 ± 4.3, 7.8 ± 6.4, 7.8 ± 5.0, and 13.4 ± 9.5 pg m−3 ppb−1, respectively, and values in the same source regions were 33.3 ± 30.4, 27.4 ± 31.0, 23.5 ± 15.3, and 20.5 ± 10.0 pg m−3 ppb−1 for the GEM / CH4 correlation slopes, respectively. The geometric means of GEM / CO2 correlation slopes for mainland China, South Asia, and Central Asia were 240 ± 119, 278 ± 164, 315 ± 289 pg m−3 ppm−1, respectively. These values were the first reported correlation slopes of GEM / CO, GEM / CO2, and GEM / CH4 in four important source regions of Asia, not including the GEM / CO ratios in mainland China. The correlation slopes of GEM / CO, GEM / CO2, and GEM / CH4 in Asia were relatively higher than those observed in Europe, North America, and South Africa, which may highlight GEM emissions from non-ferrous smelting, large-scale and artisanal mercury and gold production, natural sources, and historically deposited mercury (re-emission) in Asia. Using the observed GEM / CO and GEM / CO2 slopes, and the recently reported emission inventories of CO and CO2, the annual GEM emissions in mainland China, South Asia, the Indochinese Peninsula, and Central Asia were estimated to be in the ranges of 1071–1187, 340–470, 125, and 54–90 t, respectively. The estimated quantity of GEM emissions from the GEM / CH4 correlation slopes is significantly larger, which may be due to the larger uncertainties in CH4 emissions in Asia as well as insufficient observations of GEM / CH4 correlation slopes, therefore leading to an overestimate of GEM emissions. Our estimates of GEM emissions in the four Asian regions were significantly higher (3–4 times) than the anthropogenic GEM emissions reported in recent studies. This discrepancy could come from a combination of reasons including underestimates of anthropogenic and natural GEM emissions; large uncertainties related to CO, CO2, and CH4 emission inventories; and inherent limitations of the correlation slope method.