Yoko Yokouchi

Measurements of C2-C6hydrocarbons during the Polar Sunrise1992 Experiment: Evidence for Cl atom and Br atom chemistry

The authors report the results of grab samples made in the Arctic winter atmosphere, which were analyzed for the presence of nonmethane hydrocarbons (C{sub 2} - C{sub 6}). The normal removal reactions for such species are due to OH radical reactions, which are expected to be strongly suppressed in winter months. The nonmethane hydrocarbons were observed to correlate well with methane during most of the winter. In April, during low ozone periods, additional depletions of acetylene were observed, which could be correlatied at least in part with Cl concentrations. Not all the variations could be accounted for due to chlorine reactions, and the authors argue that this could be the result of bromine reactions, whose presence would also correlate well with the observed ozone depletions.

Strong emission of methyl chloride from tropical plants

Methyl chloride is the largest natural source of ozone-depleting chlorine compounds, and accounts for about 15 per cent of the present atmospheric chlorine content. This contribution was likely to have been relatively greater in pre-industrial times, when additional anthropogenic sources—such as chlorofluorocarbons—were absent. Although it has been shown that there are large emissions of methyl chloride from coastal lands in the tropics, there remains a substantial shortfall in the overall methyl chloride budget. Here we present observations of large emissions of methyl chloride from some common tropical plants (certain types of ferns and Dipterocarpaceae), ranging from 0.1 to 3.7 µg per gram of dry leaf per hour. On the basis of these preliminary measurements, the methyl chloride flux from Dipterocarpaceae in southeast Asia alone is estimated at 0.91 Tg yr-1, which could explain a large portion of missing methyl chloride sources. With continuing tropical deforestation, natural sources of chlorine compounds may accordingly decrease in the future. Conversely, the abundance of massive ferns in the Carboniferous period may have created an atmosphere rich in methyl chloride.

Photochemical formation of particulate dicarboxylic acids under long-range transport in central Japan

Abstract Long-range transport (LRT) of photochemical air pollution from the coastal area with large emission sources to the inland mountainous region occurs very frequently in the central Japan region on clear summer days. It is caused by local winds and the transport route is almost-fixed geographically. Along this route, behavior of dicarboxylic acids in the airborne aerosols was investigated in the cooperative field observation of meteorology and chemistry. Measurements were made every 3-h at inland sites. The concentrations of dicarboxylic acids as well as NO3− and total organic C increased in the daytime and decreased at night. The maximum are attained when the transported air mass was arrived at the sampling sites. These diurnal variations were similar to that of O3. In the daytime, although the phthalates (di-n-butyl and dioctyl), n-alkanes (C21C32) and pinon aldehyde were abundant in the airborne aerosols, the dicarboxylic acids (C2C10) were the most abundant species and the total concentrations attained from 30 to 50% of the total organic particulate matter. In addition, of the dicarboxylic acids, more than 70% were estimated to be produced by photochemical reactions in the daytime. Thus, it was concluded that most of the dicarboxylic acids were produced by the photochemical oxidation of anthropogenic compounds during LRT.

Characterization of polar organics in airborne particulate matter

Abstract The methanol-extractable highly polar organics in atmospheric aerosol were characterized using GC-MS. Dicarboxylic acids having 2–16 carbon numbers were detected with a total concentration of 172 ng m −3 . Azelaic acid ( C 9 ) was the most abundant diacid and it possibly originated from the ozonolysis of unsaturated carboxylic acids such as oleic acid and linoleic acid, which mainly originate from terrestrial plants. A compound, which was tentatively identified as tetrahydrofuroic acid, contributed to about 10% of the highly polar organics. Other polyfunctional compounds found in the samples included some ketocarboxylic acids and aromatic acids such as phthalic acids, anisic acid and vanillic acid.

Distribution of methyl iodide, ethyl iodide, bromoform, and dibromomethane over the ocean (east and southeast Asian seas and the western Pacific)

Ambient concentrations of four marine-derived halocarbons (methyl iodide, ethyl iodide, bromoform and dibromomethane) and two man-made halocarbons (trichloroethylene and tetrachloroethylene) were measured during western Pacific cruises and east and southeast Asian cruises. Ethyl iodide was detected in the atmosphere for the first time and was identified as an atmospheric iodine source compound. Bromoform concentrations were positively correlated with those of dibromomethane, and methyl iodide showed variations similar to those of ethyl iodide. However, there was no correlation between the bromocarbons and the iodocarbons. The concentrations of methyl iodide and ethyl iodide changed more markedly, possibly owing to higher rates of photodecomposition of iodocarbons.

Isoprene in the marine boundary layer (southeast Asian Sea, eastern Indian Ocean, and Southern Ocean): Comparison with dimethyl sulfide and bromoform

Sampling for atmospheric isoprene and some other volatile organic compounds was conducted during two cruises in the austral summer, covering the western Pacific, eastern Indian Ocean, Southeast Asian Sea, and Southern Ocean. High isoprene levels were observed in the marine air masses over the southern Indian Ocean (up to 280 parts per trillion by volume (pptv)) and over the Southern Ocean (up to 60 pptv), as well as high levels of dimethyl sulfide and bromoform, both of which are mainly emitted by marine biota. It is highly probable that the high phytoplankton activity in the Southern Ocean during the austral summer was responsible for the high oceanic isoprene levels. The findings suggest a possible significant influence of oceanic isoprene on marine atmospheric chemistry.

Serial gas chromatographic/mass spectrometric measurements of some volatile organic compounds in the Arctic atmosphere during the 1992 Polar Sunrise Experiment

Variations of selected volatile organic compounds (11 halocarbons, 3 hydrocarbons, and acetone) in Arctic air were measured with an automated GC/MS at Alert, Canada, as a part of the 1992 Polar Sunrise Experiment. During the springtime ozone depletion, several volatile organic compounds (VOCs) correlated significantly with ozone. In particular, trichloroethylene had a strong positive correlation (R = 0.90), while bromoform (R = −0.87) and acetone (R = −0.90) were negatively correlated. Isopentane (R = 0.77), n-butane (R = 0.77), and tetrachloroethylene (R = 0.66) were also positively correlated with ozone. These findings suggest that the ozone depletion at Alert, including its small-scale fluctuations, is caused by the advection of air masses in which reactions by Cl and Br atoms rapidly consumed chloroethylenes and alkanes concurrently and destroyed ozone while the air was over the ocean. In winter, however, slightly negative correlations of ozone with trichloroethylene (R = −0.51) and tetrachloroethylene (R = −0.40) were found, which may be caused by the vertical mixing of surface and free tropospheric air.

Seasonal and diurnal variation of isoprene and its reaction products in a semi-rural area

Abstract Isoprene and its reaction products, methyl vinyl ketone (MVK), methacrolein (MAC) and 3-methyl furan (3-MF), in the atmosphere were measured in a semi-rural area in Japan. Measurements were conducted by an automated gas chromatograph/mass spectrometer with a small Tenax TA trap. 1877 data sets were obtained during the period July–December 1991. MVK and MAC showed high intercorrelation ( R =0.95) and with isoprene ( R =0.70 and 0.74, respectively). To elucidate the destructive pathways of isoprene after emission, the relative ratios among the concentrations of isoprene and its products were studied, based on their reactivity with ozone, OH radicals and NO 3 radicals. In summer, the lower ratios of MVK and MAC to isoprene in the daytime were explained by predominant OH reaction as well as high isoprene emission. An increase of MVK/isoprene and MAC/isoprene ratios up to 5 times at night was related with the remaining ozone. Possible explanations for this anticorrelation of ozone and the ratio of products/isoprene are (1) rapid decay of isoprene through the reaction with NO 3 which is formed from the NO 2 + O 3 reaction and (2) vertical transport of air which affects the distribution of isoprene and its products as well as O 3 . In late autumn, the diurnal variation of isoprene was completely different from that in summer, and isoprene was often most abundant in the evening and was anticorrelated with ozone. The ratio of (MAC + MVK) to isoprene was positively correlated with ozone, and its high value of up to 4–6 in daytime could not be explained by OH chemistry alone. These relationships of isoprene and its products with ozone may also be explained by meteorology and/or by NO 3 chemistry.

Correlations and emission ratios among bromoform, dibromochloromethane, and dibromomethane in the atmosphere

[1] Bromoform (CHBr 3 ), dibromochloromethane (CHBr 2 Cl), and dibromomethane (CH 2 Br 2 ) in the atmosphere were measured at various sites, including tropical islands, the Arctic, and the open Pacific Ocean. Up to 40 ppt of bromoform was observed along the coasts of tropical islands under a sea breeze. Polybromomethane concentrations were highly correlated among the coastal samples, and the ratios CH 2 Br 2 /CHBr 3 and CHBr 2 Cl/ CHBr 3 showed a clear tendency to decrease with increasing CHBr 3 concentration. These findings are consistent with the observations that polybromomethanes are emitted mostly from macroalgae whose growth is highly localized to coastal areas and that CHBr 3 has the shortest lifetime among these three compounds. The relationship between the concentration ratios CHBr 3 /CH 2 Br 2 and CHBr 2 Cl/CH 2 Br 2 suggested a large mixing/ dilution effect on bromomethane ratios in coastal regions and yielded a rough estimate of 9 for the molar emission ratio of CHBr 3 /CH 2 Br 2 and of 0.7 for that of CHBr 2 Cl/CH 2 Br 2 . Using these ratios and an global emission estimate for CH 2 Br 2 (61 Gg/yr (Br)) calculated from its background concentration, the global emission rates of CHBr 3 and CHBr 2 Cl were calculated to be approximately 820(±310) Gg/yr (Br) and 43(±16) Gg/yr (Br), respectively, assuming that the bromomethanes ratios measured in this study are global representative. The estimated CHBr 3 emission is consistent with that estimated in a very recent study by integrating the sea-to-air flux database. Thus the contribution of CHBr 3 and CHBr 2 Cl to inorganic Br in the atmosphere is likely to be more important than previously thought.

Organic and inorganic bromine compounds and their composition in the Arctic troposphere during polar sunrise

Particle and gas phase inorganic bromine, total organic bromine, and several individual organic bromine species were measured in the troposphere during the Polar Sunrise Experiment at Alert, Northwest Territories, Canada, during January 18 to April 21, 1992. The measurements revealed the following: (1) Particle bromide increased gradually from about 10 ng (Br) m−3 during the dark period to >20 ng(Br) m−3 during the light period, with a marked peak of 120 ng(Br) m−3 corresponding to a strong O3 depletion event. (2) Inorganic gaseous bromine (InorgBr) was about 60 ng(Br) m−3 during the dark period and relatively constant. A major peak, up to 280 ng(Br) m−3, before sunrise accompanied a similar peak in the total organic bromine. These episodes originated in the free troposphere over Greenland. After sunrise the peaks in InorgBr corresponded to O3 depletion periods. InorgBr appeared to be the sum of HBr, HOBr, and Br2. (3) Total organic bromine was relatively constant before sunrise at 100 ng(Br) m−3 but more variable afterward, up to 280 ng(Br) m−3. Individual species include CHBr3 with levels of 7–60 ng(Br) m−3. CH2Br2, CH2ClBr, CHClBr2, and CHCl2Br levels were lower at 0.5–7.5 ng(Br) m−3. CHBr3 was the largest contributor to total organic bromine of the five species, on average accounting for 23%, while the other four species amounted to less than 5% on average. CH3Br (not measured) should contribute 44% of total organic bromine assuming a concentration of 40 ng(Br) m−3 (11 parts per trillion by volume). The remaining contribution was probably from ”missing„ species which were episodically dominant after sunrise with concentrations up to 240 ng(Br) m−3 and may include some inorganic species. All the peaks in the organic bromines after sunrise corresponded to the O3 depletion events. (4) CHBr3, CHClBr2, and CHCl2Br were significantly correlated. The ratio CHClBr2/CHBr3 decreased linearly with increasing In(CHBr3), with a steeper decrease after sunrise than before. The decreases suggest different rates of destruction with CHBr3 having a larger rate constant than CHClBr2. A similar relationship existed between the ratio CHCl2Br/CHClBr2 and the In(CHClBr2), but the dark period slope was near zero, indicating a greater difference in rates in the two species in the light period.