Observed levels and trends of gaseous SO2 and HNO3 at Mt. Waliguan, China: Results from 1997 to 2009

Long-term measurements of SO₂ and HNO₃, particularly those from the background sites, are rarely reported. We present for the first time the long-term measurements of SO₂ and HNO₃ at Waliguan (WLG), the only global baseline station in the back-land of the Eurasian Continent. The concentrations of SO₂ and HNO₃ were observed at WLG from 1997 to 2009. The observed annual mean concentrations of SO₂ and HNO₃ at WLG were 1.28 ± 0.41 and 0.22 ±0.19 μg/m³, respectively. The HNO₃ concentrations were much higher in warmer seasons than in colder seasons, while the SO₂ concentrations showed a nearly reversed seasonal pattern. In most months, the concentration of HNO₃ was significantly correlated with that of SO₂, suggesting that some common factors influence the variations of both gases and the precursors of HNO₃ may partially be from the SO₂-emitting sources. The SO₂ concentration had a very significant (P < 0.0001) decreasing trend (?0.2 μg/(m³·yr)) in 1997–2002, but a significant (P < 0.05) increasing trend (+0.06 μg/(m³·yr)) in 2003–2009. The HNO₃ concentration showed no statistically significant trend during 1997–2009. While the decrease of SO₂ in 1997–2002 agrees with the trend of global SO₂ emissions, the increase in 2003–2009 is not consistent with the decreasing trends in many other regions over the world. Trajectory analysis suggests that the airmasses from the northern Qinghai-Tibetan Plateau and the Takla Makan Desert regions contributed significantly to the increasing trends of SO₂ and HNO₃ at WLG in 2003–2009, with a rate of +0.13 μg/(m³·yr) and +0.007 μg/(m³·yr), respectively.