Seasonal and interannual variations of HCN amounts in the upper troposphere and lower stratosphere observed by MIPAS
Atmospheric Chemistry and Physics Discussions
A global HCN dataset covering nearly the complete period June 2002 to April 2012 has been derived from FTIR limb emission spectra measured with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the ENVISAT satellite. HCN is an almost unambiguous tracer of biomass burning with a tropospheric lifetime of 5–6 months and a stratospheric lifetime of about two years. We present a MIPAS HCN climatology with the main focus on biomass burning signatures in the upper troposphere
... pper troposphere and lower stratosphere. HCN observed by MIPAS in the southern tropical and subtropical upper troposphere has an annual cycle peaking in October–November during or shortly after the maximum of the southern hemispheric biomass burning season. Within 1–2 months after the burning season, a considerable portion of the enhanced HCN is transported southward to Antarctic latitudes. The fundamental period in the northern upper troposphere is also an annual cycle, which in the tropics peaks in May after the biomass burning seasons in northern tropical Africa and South Asia, and in the subtropics in July due to trapping of pollutants in the Asian monsoon anticyclone. However, caused by extensive biomass burning in Indonesia and Northern Africa together with northward transport of parts of the southern hemispheric plume, in several years HCN maxima are also found around October/November, which leads to semi-annual cycles in the northern tropics and subtropics. Because of overlap of interannually varying burning activities in different source regions, both southern and northern low-latitude maxima have considerable interannual variations. There is also a temporal shift between enhanced HCN in northern low and mid-to-high latitudes, indicating northward transport of pollutants. Due to additional biomass burning at mid and high latitudes this meridional transport pattern is not as clear as in the Southern Hemisphere. Presumably caused by ocean uptake, upper tropospheric HCN above the tropical oceans decreases to below 200 pptv especially during boreal winter and spring. HCN time series at 10 km altitude indicate a negative trend, which is more distinct in the northern (−1.3 to −2.1% yr<sup>−1</sup>) than in the Southern Hemisphere (−0.1 to −0.7% yr<sup>−1</sup>). The tropical stratospheric tape recorder signal with an apparently biennial period, which has been detected in MLS and ACE-FTS data from mid-2004 to mid-2007, is corroborated by MIPAS HCN data. The tape recorder signal in the whole MIPAS dataset exhibits periodicities of 2 and 4 yr, generated by interannual variations in biomass burning. The strongest positive anomaly in the year 2007 is caused by superposition of enhanced HCN from southern hemispheric and Indonesian biomass burning at the end of the year 2006 and from African sources in spring and the Asian monsoon during summer. The vertical transport time of the anomalies is 1 month or less between 14 and 17 km in the upper troposphere and about 9 months between 17 and 25 km in the lower stratosphere.