NO_x emission trends for China, 1995 - 2004: The view from the ground and the view from space

A rapid increase of NO₂ columns over China has been observed by satellite instruments in recent years. We present a 10-a regional trend of NO_x emissions in China from 1995 to 2004 using a bottom-up methodology and compare the emission trends with the NO₂ column trends observed from GOME and SCIAMACHY, the two spaceborne instruments. We use a dynamic methodology to reflect the dramatic change in China's NO_x emissions caused by energy growth and technology renewal. We use a scenario analysis approach to identify the possible sources of uncertainties in the current bottom-up inventory, in comparison with the satellite observation data. Our best estimates for China's NO_x emissions are 10.9 Tg in 1995 and 18.6 Tg in 2004, increasing by 70% during the period considered. NO_x emissions and satellite-based NO₂ Columns show broad agreement in temporal evolution and spatial distribution. Both the emission inventory data and the satellite observations indicate a continuous and accelerating growth rate between 1996 and 2004 over east central China. However, the growth rate from the emission inventory is lower than that from the satellite observations. From 1996 to 2004, NO_x emissions over the region increased by 61% according to the inventory, while a 95% increase in the NO₂ columns measured by satellite was observed during the same period. We found good agreement during summertime but a large discrepancy during wintertime. The consistency between the summertime trends suggests that the bias cannot be due to systematic error of activity data or emission factors. The reasons for the discrepancy cannot yet be fully identified, but possible explanations include an underestimation in seasonal emission variations, variability of meteorology, NO_x injection height, and the increasing trend of sulfate aerosols.