The Indian Ocean Experiment (INDOEX) was an international, multiplatform field campaign to measure long-range transport of air pollution from South and Southeast Asia toward the Indian Ocean during ...the dry monsoon season in January to March 1999. Surprisingly high pollution levels were observed over the entire northern Indian Ocean toward the Intertropical Convergence Zone at about 6°S. We show that agricultural burning and especially biofuel use enhance carbon monoxide concentrations. Fossil fuel combustion and biomass burning cause a high aerosol loading. The growing pollution in this region gives rise to extensive air quality degradation with local, regional, and global implications, including a reduction of the oxidizing power of the atmosphere.
The physical, chemical and radiative properties of aerosols are investigated over the tropical Indian Ocean during the first field phase (FFP) of the international Indian Ocean Experiment. The FFP ...was conducted during February 20 to March 31, 1998. The results shown here are from the Kaashidhoo Climate Observatory (KCO), a new surface observatory established on the tiny island of Kaashidhoo (4.965°N, 73.466°E) in the Republic of Maldives. From simultaneous measurements of aerosol physical, chemical, and radiative properties and the vertical structure from lidar, we have developed an aerosol model which, in conjunction with a Monte Carlo radiative transfer model, successfully explains (within a few percent) the observed solar radiative fluxes at the surface and at the top of the atmosphere. This agreement demonstrates the fundamental importance of measuring aerosol physical and chemical properties for modeling radiative fluxes. KCO, during the northeast monsoon period considered here, is downwind of the Indian subcontinent and undergoes variations in the aerosol visible optical depth τν from ∼0.1 to 0.4, with a monthly mean of ∼0.2. Lidar data suggest that the aerosol is confined largely to the first 3 kms. Sulfate and ammonium contribute ∼29% to τν; sea‐salt and nitrate contributes ∼17%; mineral dust contributes ∼15%; and the inferred soot, organics, and fly ash contribute 11%, 20%, and 8% respectively. We estimate that anthropogenic sources may contribute as much as 65% to the observed τν. We consider both an externally and an internally mixed aerosol model with very little difference between the two in the computed radiative forcing. The observed scattering coefficients are in the upper range of those reported for other oceanic regions, the single‐scattering albedos are as low as 0.9, and the Angstrom wavelength exponents of ∼1.2 indicate the abundance of submicron aerosols. In summary, the data and the model confirm the large impact of anthropogenic sources. The surface global fluxes (for overhead Sun) decrease by as much as 50 to 80 W m−2 owing to the presence of the aerosols, and the top of the atmosphere fluxes increase by as much as 15 W m−2, thus indicating that anthropogenic aerosols are having a large impact on the tropical Indian Ocean.
Emissions of reactive chlorine‐containing compounds from nine discrete classes of biomass burning were estimated on a 1° latitude by 1° longitude grid based on a biomass burning inventory for carbon ...emissions. Variations on approaches incorporating both emission ratios relative to CO and CO2 and the chlorine content of biomass burning fuels were used to estimate fluxes and associated uncertainties. Estimated, global emissions are 640 Gg Cl yr−1 for CH3Cl; 49 Gg Cl yr−1 for CH2Cl2; 1.8 Gg Cl yr−1 for CHCl3; 13 Gg Cl yr−1 for CH3CCl3; and 6350 Gg Cl yr−1 for the sum of volatile‐inorganic and particulate chlorine. Biomass burning appears to be the single largest source of atmospheric CH3Cl and a significant source of CH2Cl2; contributions of CHCl3 and CH3CCl3 are less than 2% of known sources.
Emission inventories for major reactive tropospheric Cl species (particulate Cl, HCl, ClNO2, CH3Cl, CHCl3, CH3CCl3, C2Cl4, C2HCl3, CH2Cl2, and CHClF2) were integrated across source types (terrestrial ...biogenic and oceanic emissions, sea‐salt production and dechlorination, biomass burning, industrial emissions, fossil‐fuel combustion, and incineration). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model calculations suggest that conventional acid‐displacement reactions involving S(IV) + O3, (IV) + O3 H2O2, and H2SO4 and HNO3 scavenging account for minor fractions of sea‐salt dechlorination globally. Other important chemical pathways involving sea‐salt aerosol apparently produce most volatile chlorine in the troposphere. The combined emissions of CH3Cl from known sources account for about half of the modeled sink, suggesting fluxes from known sources were underestimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH3Cl emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl3 sink. Although poorly constrained, significant fractions of tropospheric CH2Cl2 (25%), C2HCl3 (10%), and C2Cl4 (5%) are emitted from the surface ocean; the combined contributions of C2Cl4 and C2HCl3 from all natural sources may be substantially higher than the estimated oceanic flux.
We simulate the oceanic and atmospheric distribution of methyl iodide (CH3I) with a global 3‐D model driven by assimilated meteorological observations from the Goddard Earth Observing System of the ...NASA Data Assimilation Office and coupled to an oceanic mixed layer model. A global compilation of atmospheric and oceanic observations is used to constrain and evaluate the simulation. Seawater CH3I(aq) in the model is produced photochemically from dissolved organic carbon, and is removed by reaction with Cl− and emission to the atmosphere. The net oceanic emission to the atmosphere is 214 Gg yr−1. Small terrestrial emissions from rice paddies, wetlands, and biomass burning are also included in the model. The model captures 40% of the variance in the observed seawater CH3I(aq) concentrations. Simulated concentrations at midlatitudes in summer are too high, perhaps because of a missing biological sink of CH3I(aq). We define a marine convection index (MCI) as the ratio of upper tropospheric (8–12 km) to lower tropospheric (0–2.5 km) CH3I concentrations averaged over coherent oceanic regions. The MCI in the observations ranges from 0.11 over strongly subsiding regions (southeastern subtropical Pacific) to 0.40 over strongly upwelling regions (western equatorial Pacific). The model reproduces the observed MCI with no significant global bias (offset of only +11%) but accounts for only 15% of its spatial and seasonal variance. The MCI can be used to test marine convection in global models, complementing the use of radon‐222 as a test of continental convection.
Measurements made during the Indian Ocean Experiment (INDOEX) have shown the presence of large aerosol loadings over the region of the northern Indian Ocean and Arabian Sea. In recent years there has ...been significant interannual variability in the magnitude of this aerosol loading during the NE monsoon months of January‐April. Monitoring of the integrated atmospheric column effective aerosol optical properties was initiated in early 1998 and continued in 2000 on the island of Kaashidhoo in the Republic of Maldives. An Aerosol Robotic Network Sun‐sky radiometer at the Kaashidhoo Climate Observatory made spectral measurements of the direct Sun and directional sky radiances which were utilized to infer spectral aerosol optical depths τa, single scattering albedos, asymmetry factors, and aerosol size distributions. Monthly average aerosol optical depths at 500 nm varied by more than a factor of 2 during January through April for the 3 years that were investigated, 1998–2000. Interannual variations in the monthly mean Angstrom wavelength exponent were also observed, resulting from differences in the bimodal aerosol size distributions. Spectral variations in the Angstrom wavelength exponent were observed, especially at high aerosol optical depths when fine mode aerosols dominated over the optical influence of coarse‐mode aerosols. Some differences in spectral single scattering albedo and asymmetry factor were observed for 1999 versus 2000 in the infrared wavelengths, but with relatively little change in the visible wavelengths. The spectral variation in the retrieved single scattering albedo was large, with approximately linear wavelength dependence averaging from 0.91 at 440 nm to 0.83 at 1020 nm for January–March 1999 for observations where τa at 440 nm >0.4.
Extensive undersaturations of carbon tetrachloride (CCl4) in Pacific, Atlantic, and Southern Ocean surface waters indicate that atmospheric CCl4 is consumed in large amounts by the ocean. ...Observations made on 16 research cruises between 1987 and 2010, ranging in latitude from 60° N to 77° S, show that negative saturations extend over most of the surface ocean. Corrected for physical effects associated with radiative heat flux, mixing, and air injection, these anomalies were commonly on the order of −5 to −10 %, with no clear relationship to temperature, productivity, or other gross surface water characteristics other than being more negative in association with upwelling. The atmospheric flux required to sustain these undersaturations is 12.4 (9.4–15.4) Gg yr−1, a loss rate implying a partial atmospheric lifetime with respect to the oceanic loss of 183 (147–241) yr and that ∼ 18 (14–22) % of atmospheric CCl4 is lost to the ocean. Although CCl4 hydrolyzes in seawater, published hydrolysis rates for this gas are too slow to support such large undersaturations, given our current understanding of air–sea gas exchange rates. The even larger undersaturations in intermediate depth waters associated with reduced oxygen levels, observed in this study and by other investigators, strongly suggest that CCl4 is ubiquitously consumed at mid-depth, presumably by microbiota. Although this subsurface sink creates a gradient that drives a downward flux of CCl4, the gradient alone is not sufficient to explain the observed surface undersaturations. Since known chemical losses are likewise insufficient to sustain the observed undersaturations, this suggests a possible biological sink for CCl4 in surface or near-surface waters of the ocean. The total atmospheric lifetime for CCl4, based on these results and the most recent studies of soil uptake and loss in the stratosphere is now 32 (26–43) yr.
Although there are many chlorine‐containing trace gases in the atmosphere, only those with atmospheric lifetimes of 2 years or fewer appear to have significant natural sources. The most abundant of ...these gases are methyl chloride, chloroform, dichloromethane, perchloroethylene, and trichloroethylene. Methyl chloride represents about 540 parts per trillion by volume (pptv) Cl, while the others together amount to about 120 pptv Cl. For methyl chloride and chloroform, both oceanic and land‐based natural emissions have been identified. For the other gases, there is evidence of oceanic emissions, but the roles of the soils and land are not known and have not been studied. The global annual emission rates from the oceans are estimated to be 460 Gg Cl/yr for CH3Cl, 320 Gg Cl/yr for CHCl3, 160 Gg Cl/yr for CH2Cl2, and about 20 Gg Cl/yr for each of C2HCl3, and C2Cl4. Land‐based emissions are estimated to be 100 Gg Cl/yr for CH3Cl and 200 Gg Cl/yr for CHCl3. These results suggest that the oceans account for about 12% of the global annual emissions of methyl chloride, although until now oceans were thought to be the major source. For chloroform, natural emissions from the oceans and lands appear to be the major sources. For further research, the complete database compiled for this work is available from the archive, which includes a monthly emissions inventory on a 1° × 1° latitude‐longitude grid for oceanic emissions of methyl chloride.
Sulfur hexafluoride (SF6), an anthropogentically produced compound that is a potent greenhouse gas, has been measured in a number of NOAA GMDL air sampling programs. These include high resolution ...latitudinal profiles over the Atlantic and Pacific oceans, weekly flask samples from seven remote, globally distributed sites, hourly in situ measurements in rural North Carolina, and a series of archived air samples from Niwot Ridge, Colorado. The observed increase in atmospheric mixing ratio is consistent with an overall quadratic growth rate, at 6.9±0.2% yr−1 (0.24±0.01 ppt yr−1) for early 1996. From these data we derive an early 1996 emission rate of 5.9±0.2 Gg SF6 yr−1 and an interhemispheric exchange time of 1.3±0.1 years.
During the Mediterranean Intensive Oxidant Study MINOS in August 2001, 87 air samples were collected at the ground-based station Finokalia (35°19'N, 25°40'E) on the north coast of Crete and ...subsequently analysed by GC-MS. The analysis includes various hydrocarbons, organo-halogens, HCFCs and CFCs. These compounds have a wide variety of sources and sinks and a large range of atmospheric lifetimes. We evaluated the characteristics of the sampling site in terms of proximity to individual sources by plotting the measured variability of these species against lifetime. The resulting linear relationship suggests that the sampling site is representative of intermediate conditions between a remote site and one that is in the vicinity of a wide variety of sources. Our analysis of air mass origin and chemical ratios also shows that several distinct anthropogenic sources influenced the atmospheric composition over Crete. Propane observations are compared to a global model to assess the fossil fuel related emission inventory. Although the model reproduces the general pattern of the propane variations, the model mixing ratios are systematically too low by a factor of 1.5 to 3, probably due to an underestimation of the propane emissions from east European countries in the underlying global database EDGAR. Another important finding was that methyl chloroform, a compound banned under the Montreal protocol, showed significant enhancements from background, which were well correlated with CFC-113. This suggests continued use and emission of methyl chloroform by one or more European countries. We also discuss the observed variations of methyl bromide and suggest that the significant peak observed on 12 August 2001 reflects heavy agricultural use as a soil fumigant in Italy.
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