Abstract
Abstract. Airborne in-situ observations of ClO in the tropics were made during the TROCCINOX (Aracatuba, Brazil, February 2005) and SCOUT-O3 (Darwin, Australia, November/December 2005) field campaigns. While during most flights significant amounts of ClO (≈10–20 parts per trillion, ppt) were present only in aged stratospheric air, instances of enhanced ClO mixing ratios of up to 40 ppt – significantly exceeding those expected from gas phase chemistry – were observed in air masses of a more tropospheric character. Most of these observations are associated with low temperatures or with the presence of cirrus clouds (often both), suggesting that cirrus ice particles and/or liquid aerosol at low temperatures may promote significant heterogeneous chlorine activation in the tropical upper troposphere lower stratosphere (UTLS). In two case studies, particularly high levels of ClO observed were reproduced by chemistry simulations only under the assumption that significant denoxification had occurred in the observed air. However, to reproduce the ClO observations in these simulations, O3 mixing ratios higher than observed had to be assumed, and at least for one of these flights, a significant denoxification is in contrast to the observed NO levels, suggesting that the coupling of chlorine and nitrogen compounds in the tropical UTLS may not be completely understood.
Highlights
Ozone trends in the tropopause region play an important role in the radiative forcing of the Earth’s climate system (Ramaswamy et al, 2001)
During most flights, elevated Chlorine monoxide (ClO) mixing ratios (≈10–20 ppt) are found only in air masses with O3 mixing ratios higher than ≈300 ppb, i.e. containing any significant fraction of aged stratospheric air. This is consistent with the expected dependence of ClO on photochemical processing and available Cly: mixing ratios found in stratospheric air did not significantly exceed typical noontime ClO levels expected from the known gas phase chemistry, which were estimated by the Chemical Lagrangian Model of the Stratosphere (CLaMS) ST simulations for gas-phase only and are represented by the black line in Fig. 3, with the dark grey shading representing uncertainties due to time of day and temperature
If heterogeneous chemistry is included in the CLaMS ST simulations does the range of simulated noontime ClO mixing ratios – marked by the blue shaded area in Fig. 3 – correspond reasonably well to the high values that were sometimes observed, suggesting that heterogeneous reactions caused substantial production of active chlorine in the tropical tropopause layer (TTL)
Summary
Ozone trends in the tropopause region play an important role in the radiative forcing of the Earth’s climate system (Ramaswamy et al, 2001). The Cly content inside high altitude cirrus clouds coupled to local deep convection is expected to be low and significant amounts of Cly that can be activated by heterogeneous reactions are thought to be present only at the interface between the cloud and stratospheric air at the top of the cloud (Solomon et al, 1997). The amount of inorganic chlorine in the tropical upper troposphere and lower stratosphere (UTLS) resulting from the decomposition of very short lived species (VSLS) could be quite significant (e.g. WMO, 2006; Laube et al, 2008; Mebarki et al, 2010) This as well as the possibility of convective transport of inorganic chlorine (HCl, ClO, sea salt) from the marine boundary layer into the UTLS will be discussed in more detail in Sect. Case studies for incidences of unusually high ClO are presented, where observations are compared to model simulations of heterogeneous activation and chemical deactivation of active chlorine (Sect. 4)
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