R. Gabriel

Origin of carbonaceous aerosols over the tropical Indian Ocean: Biomass burning or fossil fuels?

We present an analysis of the carbon, potassium and sulfate content of the extensive aerosol haze layer observed over the tropical Indian Ocean during the Indian Ocean Experiment (INDOEX). The black carbon (BC) content of the haze is as high as 17% of the total fine particle mass (the sum of carbonaceous and soluble ionic aerosol components) which results in significant solar absorption. The ratio of black carbon to organic carbon (OC) (over the Arabian Sea and equatorial Indian Ocean) was a factor of 5 to 10 times larger than expected for biomass burning. This ratio was closer to values measured downwind of industrialized regions in Japan and Western Europe. These results indicate that fossil fuel combustion is the major source of carbonaceous aerosols, including black carbon during the events considered. If the data set analyzed here is representative of the entire INDOEX study then fossil fuel emissions from South Asia must have similarly contributed to aerosols over the whole study region. The INDOEX ratios are substantially different from those reported for some source regions of South Asia, thus raising the possibility that changes in composition of carbonaceous aerosol may occur during transport.

Carbonaceous aerosols over the Indian Ocean during the Indian Ocean Experiment (INDOEX): Chemical characterization, optical properties, and probable sources

components) of fine-mode particles in these layers was 15.3 ± 7.9 m gm � 3 . The major components were particulate organic matter (POM, 35%), SO4� (34%), black carbon (BC, 14%), and NH4 + (11%). The main difference between the composition of the marine boundary layer (MBL, 0 to � 1.2 km), and the overlying residual continental boundary layer (1.2 to � 3.2 km) was a higher abundance of SO4 2� relative to POM in the MBL, probably due to a faster conversion of SO2 into SO4 2� in the MBL. Our results show that carbon is a major, and sometimes dominant, contributor to the aerosol mass and that its contribution increases with altitude. Low variability was observed in the optical properties of the aerosol in the two layers. Regression analysis of the absorption coefficient at 565 nm on BC mass (BC < 4.0 m gCm � 3 ) yielded a specific absorption cross section of 8.1 ± 0.7 m 2 g � 1 for the whole period. The unusually high fraction of BC and the good correlation between the absorption coefficient and BC suggest that BC was responsible for the strong light absorption observed for the polluted layers during INDOEX. High correlation between BC and total carbon (TC) (r 2 = 0.86) suggest that TC is predominantly of primary origin. Good correlations were also found between the scattering coefficient at 550 nm and the estimated aerosol mass for the fine fraction. These yielded a specific scattering cross section of 4.9 ± 0.4 m 2 g � 1 . The observed BC/TC, BC/OC, SO4� /BC, and K + /BC ratios were fairly constant throughout the period. These ratios suggest that between 60 and 80% of the aerosol in the polluted layers during INDOEX originated from fossil fuel and between 20 and 40% from biofuel combustion. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: carbonaceous aerosols; INDOEX; chemical characterization; optical properties; sources; aerosols

Chemical characterization of pollution layers over the tropical Indian Ocean: Signatures of emissions from biomass and fossil fuel burning

We have performed airborne measurements of atmospheric trace gases and aerosol composition on the National Center for Atmospheric Research C-130 research aircraft over the tropical Indian Ocean during the Indian Ocean Experiment (INDOEX) intensive field phase in February and March 1999. Gases measured included acetone, acetonitrile, sulfur dioxide, and carbon monoxide. The aerosol composition was analyzed for water-soluble ions, and black and organic carbon. South of the Intertropical Convergence Zone (ITCZ), we sampled pristine air originating from the remote southern Indian Ocean. North of the ITCZ, signatures of heavy pollution were evident over large areas of the Indian Ocean. Heavy pollution was present in the marine boundary layer as well as in the free troposphere at altitudes up to almost 4000 m. Outflow from the Indian subcontinent as well as from other source regions (Arabian Sea, Southeast Asia) could be identified by back trajectory calculations using the Hybrid Single Particle Lagrangian Integrated Trajectory model. The highest pollutant concentrations were observed in a free tropospheric pollution layer (“residual layer”), which originated from the Indian continental boundary layer. High mixing ratios of acetonitrile (up to 0.8 ppb) and submicron aerosol potassium (up to 0.6 ppb) indicate an important contribution from biomass or biofuel burning sources. On the other hand, high mixing ratios of sulfiir dioxide (up to 1.5 ppb) and aerosol sulfate (up to 3 ppb) indicate the influence of fossil fuel burning. During most flights the contributions from these two sources were well mixed within the same air mass, suggesting that the sources on the ground are also close to each other. This is consistent with the assumption that biomass is mainly burnt as biofuel for domestic use in populated areas, where fossil fuel is also used. The ratios dX/dCQ (X=acetone, acetonitrile, sulfur dioxide, potassium, or sulfate) measured during the flights indicate that most of the CO in the continental outflow is due to biomass or biofuel burning, whereas the majority of the aerosols results from fossil fuel burning.

Soluble ion chemistry of the atmospheric aerosol and SO2 concentrations over the eastern North Atlantic during ACE‐2

ACE-2, the second Aerosol Characterization Experiment of the International Global Atmospheric Chemistry Project (IGAC), was conducted in the area between Portugal, Tenerife and Madeira from 15 June to 25 July 1997. We determined the concentration of SO2 and the soluble ion composition of the atmospheric aerosol in 113 samples collected by aircraft. Comparison between aircraft and ground-based samples collected from the same or similar airmasses showed good agreement (better than 40%) for the fine fraction of the aerosol, but suggests that, for the coarse fraction, the sampling efficiency of the aircraft inlet is only about 35%. During periods when trajectory analysis suggested no recent contact of the airmass with Europe or North America, SO2 and aerosol ions were at levels comparable to those found over remote ocean regions. The composition of airmasses originating from Europe showed signatures characteristic of the source regions and suggested rapid oxidation of SO2 during transport over the ocean. The first Lagrangian experiment was conducted in an unpolluted airmass and showed the physical and chemical evolution of a marine boundary layer traversing over increasingly warmer ocean waters. The sulfur cycle in this airmass could be explained based on the emission of DMS from the sea surface. In three other Lagrangian experiments, we investigated the evolution of boundary layers with increasing age since having left the European continent. SO2 was removed rapidly with lifetimes on the order of half a day in cloud-topped boundary layers. The production of nss-sulfate ceased after SO2 had declined to background levels, and the aerosol approached a nearly constant composition, with concentrations dominated by physical removal and dilution processes. Aerosol nitrate concentrations suggested that gaseous HNO3 was taken up by seasalt aerosol and subsequently removed by dry deposition.