Abstract
Abstract. We use the GEOS-Chem global 3-D chemistry transport model to investigate the relative importance of chemical and physical processes that determine observed variability of hydrogen cyanide (HCN) in the troposphere and lower stratosphere. Consequently, we reconcile ground-based FTIR column measurements of HCN, which show annual and semi-annual variations, with recent space-borne measurements of HCN mixing ratio in the tropical lower stratosphere, which show a large two-year variation. We find that the observed column variability over the ground-based stations is determined by a superposition of HCN from several regional burning sources, with GEOS-Chem reproducing these column data with a positive bias of 5%. GEOS-Chem reproduces the observed HCN mixing ratio from the Microwave Limb Sounder and the Atmospheric Chemistry Experiment satellite instruments with a mean negative bias of 20%, and the observed HCN variability with a mean negative bias of 7%. We show that tropical biomass burning emissions explain most of the observed HCN variations in the upper troposphere and lower stratosphere (UTLS), with the remainder due to atmospheric transport and HCN chemistry. In the mid and upper stratosphere, atmospheric dynamics progressively exerts more influence on HCN variations. The extent of temporal overlap between African and other continental burning seasons is key in establishing the apparent bienniel cycle in the UTLS. Similar analysis of other, shorter-lived trace gases have not observed the transition between annual and bienniel cycles in the UTLS probably because the signal of inter-annual variations from surface emission has been diluted before arriving at the lower stratosphere (LS), due to shorter atmospheric lifetimes.
Highlights
Hydrogen cyanide (HCN) is a tracer of biomass burning (BB; Lobert et al, 1990; Holzinger et al, 1999) and could play a non-negligible role in the nitrogen cycle (Li et al, 2000, 2003)
We find the largest contributions to the global budget are from IND + AUS, northern Africa (NAF), Southern Africa (SAF), South America (SA), and Boreal Asia (BA), all representing more than 10%
We found that the observed annual and semi-annual variations in column HCN at three ground-based stations at low and mid-latitudes are described by the model with a positive bias of 5%
Summary
Hydrogen cyanide (HCN) is a tracer of biomass burning (BB; Lobert et al, 1990; Holzinger et al, 1999) and could play a non-negligible role in the nitrogen cycle (Li et al, 2000, 2003). Recent analysis of satellite HCN measurements from the NASA Aura Microwave Limb Sounder (MLS) (Waters, 2006) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACEFTS) (Boone et al, 2005) show an approximate 2-year cycle of HCN anomalies in the tropical upper troposphere and lower stratosphere (UTLS) (Pumphrey et al, 2008), hypothesized to be due to year-to-year variations in surface burning over Indonesia and Australia. This is supported by a recent model study of year-to-year variations of UTLS CO, another tracer of BB, which showed that variations during 1994–1998 were primarily caused by year-to-year changes in surface burning (Duncan et al, 2007).
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