The unique OMI HCHO/NO2 feature during the 2008 Beijing Olympics: Implications for ozone production sensitivity

In preparation of the Beijing Summer Olympic and Paralympics Games, strict emission control measures (ECMs) were imposed between July and September 2008 on motor vehicle traffic and industrial emissions to improve air quality. We estimated changes in the chemical sensitivity of ozone production to these ECMs using Ozone Monitoring Instrument (OMI) column measurements of formaldehyde (HCHO) and nitrogen dioxide (NO2), where their ratio serves as a proxy for the sensitivity. During the ECMs, OMI NO2 significantly decreased, subsequently increasing the HCHO/NO2. For the first half of the ECM time period, the ratios maintained values greater than two indicating that ozone production became primarily NOx-limited. In contrast, ozone production was predominantly volatile organic compound (VOC)-limited or mixed VOC-NOx-limited during the same period in the preceding three years. After the ECMs were lifted, NO2 and HCHO/NO2 returned to their previous values. The 2005–2008 OMI record shows that this transition to a predominantly NOx-limited regime during the ECMs was unique. Meteorological factors likely explain the variability in HCHO/NO2, particularly the transition to a mixed NOx-VOC-limitation in mid-August during the Olympics, where ozone production became sensitive to both NOx and VOCs until the end of the ECMs. The mixed VOC-NOx-limited regime observed during the Paralympics is also unique because previous years show that Beijing in September is predominantly VOC-limited. Beijing’s large-scale tree-planting program was expected to increase levels of biogenic VOCs, but this is not supported by OMI HCHO data. However, MODIS vegetation indices show a small increase in vegetation cover from 2003 leading up to the Games in 2008. After the Games, however, there was a downturn in the indices (2009 and 2010) to levels similar to 2006.