Studies Of Methyl Bromide And Methyl Chloride In The Shelf Waters Of Eastern China

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) are important atmospheric trace gases, which play significant roles in the global warming and atmospheric chemistry. The ocean plays important and complex roles in the global biogeochemical cycles of these methyl halides. Coastal regions such as continental shelves, estuaries and bays appear to be the sources of atmospheric CH3Br and CH3Cl and responsible for a large part of the oceanic emission of CH3Br and CH3Cl, though their areas only occupy a small part of the world ocean. Therefore, studies on the occurrence of CH3Br and CH3Cl in coastal waters will be helpful to estimate the contribution of oceanic emission to the atmospheric CH3Br and CH3Cl on the global scale, and to predict the influence of oceanic to the global climate, especially in middle-low latitude.In the present dissertation, distributions, saturation anomalies, sea-air fluxes of CH3Br and CH3Cl in the coastal waters of China, i.e. the East China Sea (ECS) and the Yellow Sea (YS), and the Jiaozhou Bay, are studied systematically. The main research works are as follows:1. Distributions, saturation anomalies, sea-air fluxes of CH3Br and CH3Cl are determined in the ECS during Apr-May,2009 and Oct-Nov,2007 and in the waters adjacent to the Yangtze Estuary during Aug,2008. The results show that the average concentrations of CH3Br and CH3Cl in the surface waters of the ECS are 4.75±0.83 and 85.7±18.8 pmol/L in spring; those are 2.74±1.49 and 83.5±11.8 pmol/L in autumn, respectively. In the waters off the Yangtze River Estuary, those are 5.24±0.59 and 90.6±12.8 pmol/L in spring,7.76±1.21 and 86.1±34.6 pmol/L in summer,2.82±1.13 and 87.3±8.7 pmol/L in autumn, respectively. In general, average concentrations of CH3Br in the surface waters in the ECS show obvious seasonal variations, with those in summer higher than those in spring and higher than those in autumn, while seasonal variations of CH3Cl are not distinct. The horizontal distributions of CH3Br and CH3Cl in the ECS are obviously influenced by the Yangtze River and the Kuroshio waters in spring and autumn, illustrating a decreasing trend from inshore to offshore sites, without being strongly biased by temporal change. The vertical distributions along different transects and at different stations of CH3Br and CH3Cl in the ECS are studied. The results show that the distributions are influenced by the coastal currents, the Taiwan Warm Current and the Kuroshio. The vertical profiles of the two gases differed among stations, with maxima generally appearing in the mixed layer (0-75 m). The surface waters of the ECS in spring, summer and autumn are generally supersaturated for CH3Br and CH3Cl and the saturation anomalies (SA) of CH3Br are 248.3±56.6%,659.4±127.2% and 122.3±106.4%; those of CH3Cl are 27.9±25.9%,91.2±62.4% and 42.6±16.3%, respectively, indicating that the ECS is a net source for the atmospheric CH3Br and CH3Cl in the investigated time. Based on in situ wind speeds and the surface concentrations, Wanninkhof relationship (W92) is employed to estimate the sea-to-air fluxes of CH3Br and CH3Cl. The results show that average CH3Br fluxes in spring, summer and autumn are 59.4±50.5,0.6±0.4 and 31.0±38.0 nmol m-2 d-1; those of CH3Cl are 286.0±324.2,3.7±3.4 and 463.0±385.8 nmol m-2 d-1, respectively. Therefore, the and CH3Cl fluxes display obvious seasonal variations, which are not agreement with the concentration variations, with the lowest in summer due to lower wind speeds.2. Distributions and fluxes of CH3Br and CH3Cl are determined during 5 surveys on the western part of the Southern Yellow Sea (WPSYS) in Jul, Aug,2008, Mar, May,2009 and Nov,2007. The surface CH3Br and CH3Cl concentrations show apparently seasonal variations, with the highest observed in summer and the lowest in autumn (CH3Br) or early spring (CH3Cl). The average concentrations of CH3Br in the surface waters of 5 surveys above are 11.85±5.51,5.79±2.18, 4.24±1.26,5.86±1.07 and 3.93±1.30 pmol/L; those of CH3C1 are 243.9±104.1, 135.0±50.1,81.6±31.8,86.1±22.1 and 104.5±23.2 pmol/L, respectively. The two gases show different distributions in the WPSYS of 5 surveys, and the bloom of Enteromorpha Prolifera affects strongly the distributions of CH3Br and CH3Cl in Jul,2008. Results of the vertical profiles of CH3Br and CH3Cl at 35°N transect in the Southern Yellow Sea (SYS) show that the distribution are influenced by SYS Cool Water and anthropogenic activity, with maxima generally appearing in the mixed layer (0-30 m). The CH3Br SA of 5 surveys are 940.4±496.0,432.4±205.3, 98.6±52.1,257.3±76.4 and 163.4±75.6%; those of CH3Cl are 352.4±189.8, 169.4±100.0,-19.7±28.9,7.2±25.8 and 48.2±29.5%, respectively. Therefore, the SA of CH3Br and CH3Cl show obvious seasonal variations, which are similar to the concentrations. The SYS are supersaturated for CH3Br in 5 surveys and also for CH3Cl except for the survey on Mar,2009, indicating that the WPSYS is a net source of atmospheric CH3Br during the studied time and of CH3Cl except for Mar,2009. Study on the flux in the WPSYS demonstrates that there exists the seasonal cycle of the fluxes of CH3Br and CH3Cl. The fluxes of CH3Br of 5 surveys in the WPSYS are 108.9±82.1,0.4±0.4,35.1±29.2,77.1±109.7 and 29.2±31.2 nmol m-2 d-1; those of CH3Cl are 1738.1±1002,6.6±7.3,-272.5±506.2, 104.0±311.1 and 307.9±245.3 nmol m-2 d-1, respectively. The fluxes in summer are lowest due to lower wind speeds, similar to the ECS.3. Distributions and fluxes of CH3Br and CH3Cl are determined in the Northern Yellow Sea (NYS) in May,2009 and Oct,2007. The results show that the average concentrations of CH3Br and CH3Cl in the surface waters are 5.06±0.57 and 92.2±6.92 pmol/L in spring; those are 6.22±3.21 and 83.6±19.0 pmol/L in autumn, respectively. Therefore, the CH3Br and CH3Cl concentrations in the NYS also varied with the season, but the range of variation are not large. The saturation anomalies of CH3Br and CH3Cl are 172.7±36.9% and 4.4±10.1% in spring; those are 344.5±227.0% and 27.7±28.8% in autumn, respectively. Therefore, the seasonal variations of the SA are observed in the NYS for the two gases, with the SA in autumn higher than those in spring. The NYS are supersaturated for the two gases, indicating that the NYS is a net source of atmospheric CH3Br and CH3Cl during the investigated time. The fluxes of CH3Br and CH3Cl in the NYS are 56.4±24.5,35.0±207.6 nmol m-2 d-1 in spring and 66.8±86.9,231.6±356.7 nmol m-2 d-1 in autumn, respectively.4. Distributions and fluxes of CH3Br and CH3Cl are determined during 3 surveys on the Jiaozhou Bay in Jun, Aug,2007, Jan,2008 and its adjacent areas in Oct,2007 and Nov,2008. The surface CH3Br and CH3Cl concentrations in the Jiaozhou Bay show apparently seasonal variations, with the concentrations in summer higher than those in spring and the lowest in winter. The average concentrations of CH3Br in the surface waters of 3 surveys above are 10.99±4.38,11.87±5.74 and 6.22±1.95 pmol/L; those of CH3Cl are 249.6±60.8,382.9±145.4 and 145.5±43.0 pmol/L, respectively. The average concentrations of CH3Br in the waters adjacent to the Bay are 5.86±1.07 pmol/L in Oct,2007 and 4.16±1.40 pmol/L in Nov,2008; those of CH3Cl are 151.6±24.8 and 144.4±48.6 pmol/L, respectively. The two gases show similar distributions in the Jiaozhou Bay and display a decrease from the eastern to western coast, from the northern to southern part of the Bay, influenced strongly by the input of the polluted river waters and the sewage waters along the eastern coast. The CH3Br SA of 5 surveys in the Bay and its adjacent areas are 707.1±322.1%,949.7±460.0%,140.8±72.0%,449.1±107.2% and 170.9±88.5%; those of CH3C1 are 290.4±95.1%,668.3±296.9%,31.7±40.7%, 169.7±42.6% and 94.2±63.7%, respectively. Therefore, the Jiaozhou Bay is supersaturated and is a net source for atmospheric CH3Br and CH3Cl. Based on long-term wind speeds and the surface concentrations, Wanninkhof relationship (W92) is employed to calculate the sea-to-air fluxes of CH3Br and CH3Cl in the Jiaozhou Bay. The results show that average CH3Br fluxes of 5 surveys are 73.0±33.2,84.1±40.6,26.8±14.4,55.3±13.3 and 27.4±14.3 nmol m-2 d-1; those of CH3Cl are 1253.5±410.7,2312.8±1022.4,225.5±291.5,864.5±218.1 and 659.8±447.5 nmol m-2 d-1, respectively. Therefore, sea-to-air fluxes of CH3Br and CH3Cl in the Jiaozhou Bay show obvious seasonal variations, with the highest occurring in summer and the lowest in winter, which are agreement with the concentration variations.5. Based on the investigated area and the average fluxes of CH3Br and CH3Cl, the annual CH3Br and CH3C1 emission from the ECS and the Yellow Sea (YS) are estimated to be 2.0 and 8.1 Gg/a. Although the ECS and YS occupies only 0.3% of the total ocean in area, the contributions of the ECS and YS to the net global sea-to-air CH3Br and CH3Cl fluxes are estimated to be about 3% and 1%, which means that shelf waters contribute significant amount to the total oceanic CH3Br and CH3Cl fluxes compared with open waters, especially in middle-low latitude shelf waters.