Ship-based detection of glyoxal over the remote tropical Pacific Ocean

摘要

We present the first detection of glyoxal (CHOCHO) over the remote tropicalPacific Ocean in the Marine Boundary Layer (MBL). The measurements wereconducted by means of the University of Colorado Ship Multi-AxisDifferential Optical Absorption Spectroscopy (CU SMAX-DOAS) instrumentaboard the research vessel Ronald H. Brown. The research vessel was on acruise in the framework of the VAMOS Ocean-Cloud-Atmosphere-Land Study –Regional Experiment (VOCALS-REx) and the Tropical Atmosphere Ocean (TAO)projects lasting from October 2008 through January 2009 (74 days at sea).The CU SMAX-DOAS instrument features a motion compensation system tocharacterize the pitch and roll of the ship and to compensate for shipmovements in real time. We found elevated mixing ratios of up to 140 pptCHOCHO located inside the MBL up to 3000 km from the continental coast overbiologically active upwelling regions of the tropical Eastern Pacific Ocean.This is surprising since CHOCHO is very short lived (atmospheric life time~2 h) and highly water soluble (Henry's Law constant H =4.2 × 105 M/atm). This CHOCHO cannot be explained by transport of it or itsprecursors from continental sources. Rather, the open ocean must be a sourcefor CHOCHO to the atmosphere. Dissolved Organic Matter (DOM) photochemistryin surface waters is a source for Volatile Organic Compounds (VOCs) to theatmosphere, e.g. acetaldehyde. The extension of this mechanism to verysoluble gases, like CHOCHO, is not straightforward since the air-sea flux isdirected from the atmosphere into the ocean. For CHOCHO, the dissolvedconcentrations would need to be extremely high in order to explain ourgas-phase observations by this mechanism (40–70 μM CHOCHO, compared to~0.01 μM acetaldehyde and 60–70 μM DOM). Further, whilethere is as yet no direct measurement of VOCs in our study area,measurements of the CHOCHO precursors isoprene, and/or acetylene overphytoplankton bloom areas in other parts of the oceans are too low (by afactor of 10–100) to explain the observed CHOCHO amounts. We conclude thatour CHOCHO data cannot be explained by currently understood processes. Yet,it supports first global source estimates of 20 Tg/year CHOCHO from theoceans, which likely is a significant source of secondary organic aerosol(SOA). This chemistry is currently not considered by atmospheric models.