Long-term Variations in Ozone Levels in the Troposphere and Lower Stratosphere over Beijing: Observations and Model Simulations

Abstract

<p><strong>Abstract.</strong> Tropospheric ozone is both a major pollutant and a short-lived greenhouse gas and has therefore attracted much concern in recent years. The ozone profile in the troposphere and lower stratosphere over Beijing has been observed since 2002 by ozonesondes developed by the Institute of Atmospheric Physics. Increasing concentrations of tropospheric ozone from 2002 to 2010 measured by these balloon-based observations have been reported previously. As more observations are now available, we used these data to analyze the long-term variability of ozone over Beijing during the whole period from 2002 to 2018. The ozonesondes measured increasing concentrations of ozone from 2002 to 2012 in both the troposphere and lower stratosphere. There was a sudden decrease in observed ozone between 2011 and 2012. After this decrease, the increasing trend in ozone concentrations slowed down, especially in the mid-troposphere, where the positive trend became neutral. We used the Chemical Lagrangian Model of the Stratosphere (CLaMS) to determine the influence of the transport of ozone from the stratosphere to the troposphere on the observed ozone profiles. CLaMS showed a weak increase in the contribution of stratospheric ozone before the decrease in 2011–2012 and a much more pronounced decrease after this time. Because there is no tropospheric chemistry in CLaMS, the sudden decrease simulated by CLaMS indicates that a smaller downward transport of ozone from the stratosphere after 2012 may explain a significant part of the observed decrease in ozone in the mid-troposphere and lower stratosphere. However, the influence of stratospheric ozone in the lower troposphere is negligible in CLaMS and the hiatus in the positive trend after 2012 can be attributed to a reduction in ozone precursors as a result of stronger pollution control measures in Beijing.</p>

Ozone pollution in the North China Plain spreading into the late-winter haze season

“…The model underestimate of the springtime trend may reflect an increase in background ozone, which is particularly important in that season ( 50 ). Ozonesonde measurements over the 2013 to 2018 period in Beijing ( 51 ) show an increase in free tropospheric ozone in spring but not in other seasons ( SI Appendix , Fig. S11 ), which may be due to rising anthropogenic emissions in South and Southeast Asia ( 52 ).…”

“…This process has been previously reported to occur in oil/gas fields in the United States in winter ( 12 ), and here we see that it operates in urban China as well. Increasing background ozone in spring ( 51 , 52 , 57 ) could also contribute to the increase in surface ozone pollution.…”