||Geochemical study on low molecular weight dicarboxylic acids and related compounds in the marine aerosols from the Pacific Ocean
Hoque, Mir Md. Mozammal
To better understand the sources and atmospheric processing of organic aerosols inthe remote marine atmosphere, aerosol (TSP) samples were collected over the western NorthPacific (13°14’N-53°37’N and 140°46’E-179°54’W) during May-July 2010, central Pacific(1°59’N-35°N and 171°54’E-90°58’W) during September-October 1999, and North-SouthPacific (3°05’N-34°02’N, 6°59’S-25°46’S and 144°52’E-173°55’W) during February-April1994 and were studied for dicarboxylic acids (C2-C11), ω-oxoacids, pyruvic acid and α-dicarbonyls as well as organic and elemental carbon and water-soluble organic carbon. Overthe western North Pacific, diacids are most abundant followed by ω-oxoacids and α-dicarbonyls. Although the molecular compositions of diacids are generally characterized bythe predominance of oxalic (C2) acid, I found a depletion of C2 in four samples, which werecollected in the high latitudes (48°N-54°N) of the western North Pacific where succinic (C4)acid is dominant. I consider that photochemical degradation of unsaturated fatty acids emittedfrom the ocean surface produced C4 over the high latitudinal western North Pacific, whereChlorophyll-a maximized during the cruise. Moreover, seven samples collected in mid andhigh latitudes of the western North Pacific show predominance of malonic (C3) acid over C4,suggesting photochemical degradation of C4 to C3. Spatial distributions of diacids, ω-oxoacids and α-dicarbonyls together with total carbon were characterized by their higherabundances in the coastal western North Pacific followed by high and low latitudinal regions,signifying that continental aerosols are transported long distances to the remote marineatmosphere. However, in the central Pacific aerosols collected between Japan to Mexico, Ifound a predominance of oxalic (C2) acid followed by malonic (C3) and succinic (C4) acid,where oxalic acid comprises 74% of total diacids. Abundances of oxalic acid showed 3 timeshigher values in the upwelling zone of the central Pacific than those of the non-upwellingzone. Positive relation (r2=0.79) between oxalic acid and oxidation products (ωC2+Gly+Pyr)4of biological volatile organic compounds (BVOCs) suggests that secondary production ofoxalic acid occurs in aqueous aerosol phase via the oxidation of marine derived BVOCs inthe upwelling central Pacific. Atmospheric abundances of short chain (C2-C4) diacids duringthe North-South Pacific cruise, showed 4 times higher values in the North Pacific than in theSouth Pacific. During this cruise, abundances of C2 in the western North Pacific illustrated 5to 10 time higher values than in the central North and South Pacific, respectively. However,the aerosol samples that were collected in the western North Pacific, demonstrated thatglyoxylic (ωC2) acid and methylglyoxal (MeGly) were dominant with C2. I found a strongcorrelation between C2 and ωC2 (r2=0.89) and C2 and MeGly (r2=0.96) in the same samplesthat were collected in the western North Pacific but their correlations are significantly weakfor the rest of the samples. These results together with 7-day back-ward air masses trajectoryindicate that ωC2 and MeGly are both photochemical oxidation products of aromatichydrocarbons in polluted-continental air masses, which are then serve as precursor of C2during long-range atmospheric transport.
Hokkaido University（北海道大学）. 博士(環境科学)