A tropical West Pacific OH minimum and implications for stratospheric composition

Abstract

Abstract. Most of the short-lived biogenic and anthropogenic chemical species that are emitted into the atmosphere break down efficiently by reaction with OH and do not reach the stratosphere. Here we show the existence of a pronounced minimum in the tropospheric column of ozone over the West Pacific, the main source region for stratospheric air, and suggest a corresponding minimum of the tropospheric column of OH. This has the potential to amplify the impact of surface emissions on the stratospheric composition compared to the impact when assuming globally uniform OH conditions. Specifically, the role of emissions of biogenic halogenated species for the stratospheric halogen budget and the role of increasing emissions of SO 2 in Southeast Asia or from minor volcanic eruptions for the increasing stratospheric aerosol loading need to be reassessed in light of these findings. This is also important since climate change will further modify OH abundances and emissions of halogenated species. Our study is based on ozone sonde measurements carried out during the TransBrom cruise with the RV Sonne roughly along 140-150 • E in October 2009 and corroborating ozone and OH measurements from satellites, aircraft campaigns and FTIR instruments. Model calculations with the GEOS-Chem Chemistry and Transport Model (CTM) and the ATLAS CTM are used to simulate the tropospheric OH distribution over the West Pacific and the transport pathways to the stratosphere. The potential effect of the OH minimum on species transported into the stratosphere is shown via modeling the transport and chemistry of CH 2 Br 2 and SO 2 .

Tropical tropospheric ozone column retrieval for GOME-2

“…Here, GOME-2 measures minimum tropospheric ozone columns of ∼ 10 DU. These low ozone values are consistent with ozonesonde measurements in the West Pacific, showing ozone concentrations of ∼ 15 ppbv and lower in the tropical troposphere (Kley et al, 1996;Rex et al, 2014). Lidar measurements carried out during the PEM-Tropics B campaign also show very low ozone in the marine boundary layer and upper troposphere in the Pacific (Browell et al, 2001).…”

Impacts of anthropogenic and natural sources on free tropospheric ozone over the Middle East

“…We refer the reader to Jiang et al (2015b) for more details about the application and evaluation of TES O 3 data. An obvious feature is the low O 3 concentrations over the maritime continent and the Amazon, which is consistent with previous studies using measurements from satellite, ozonesonde and aircraft (Rex et al, 2014;Bela et al, 2015). Over the northern middle latitudes, O 3 concentrations are highest in summer.…”

Tropical tropospheric ozone columns from nadir retrievals of GOME-1/ERS-2, SCIAMACHY/Envisat, and GOME-2/MetOp-A (1996–2012)

“…8. The general wave-one pattern is well depicted for all seasons, with higher tropospheric ozone columns over the South Atlantic (∼ 40 DU) and persistently lower values over the Pacific Ocean (∼ 15 DU) (Diab et al, 2003;Rex et al, 2014). In winter, enhanced ozone columns (∼ 40 DU) are noticed over the Atlantic and the northern tropical Pacific Ocean with a secondary maximum over the Indian Ocean.…”

“…These conditions favour the strong advection from east to west by the Walker circulation. For this reason the tropospheric air masses have been in a clean, warm, and humid environment for a long time and loss of odd oxygen, ozone, and ozone precursors like NO x (= NO + NO 2 ; NO x is lost by conversion into HNO 3 followed by washout) proceeded longer than elsewhere in the tropics (Rex et al, 2014).…”