资源环境科技发展态势分析平台

Despite the recently reported beginning of a recovery in global stratospheric ozone (O-3), an unexpected O-3 decline in the tropical mid-stratosphere (around 30-35 km altitude) was observed in satellite measurements during the first decade of the 21st century. We use SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) measurements for the period 2004-2012 to confirm the significant O-3 decline. The SCIAMACHY observations show that the decrease in O-3 is accompanied by an increase in NO2.

To reveal the causes of these observed O-3 and NO2 changes, we performed simulations with the TOMCAT 3-D chemistry-transport model (CTM) using different chemical and dynamical forcings. For the 2004-2012 time period, the TOMCAT simulations reproduce the SCIAMACHY-observed O-3 decrease and NO2 increase in the tropical mid-stratosphere. The simulations suggest that the positive changes in NO2 (around 7% decade(-1)) are due to similar positive changes in reactive odd nitrogen (NOy), which are a result of a longer residence time of the source gas N2O and increased production via N2O+ O(D-1). The model simulations show a negative change of 10% decade(-1) in N2O that is most likely due to variations in the deep branch of the Brewer-Dobson Circulation (BDC). Interestingly, modelled annual mean "age of air" (AoA) does not show any significant changes in transport in the tropical mid-stratosphere during 2004-2012.

However, further analysis of model results demonstrates significant seasonal variations. During the autumn months (September-October) there are positive AoA changes that imply transport slowdown and a longer residence time of N2O allowing for more conversion to NOy, which enhances O-3 loss. During winter months (January-February) there are negative AoA changes, indicating faster N2O transport and less NOy production. Although the variations in AoA over a year result in a statistically insignificant linear change, non-linearities in the chemistry-transport interactions lead to a statistically significant negative N2O change.