Marine Boundary Air Research Articles

Turbulence in The Marine Boundary Layer and Air Motions Below Stratocumulus Clouds at the ARM Eastern North Atlantic Site

AbstractMarine stratocumulus clouds are intimately coupled to the turbulence in the boundary layer and drizzle is known to be ubiquitous within them. Six years of data collected at the Atmospheric Radiation Measurement (ARM)’s Eastern North Atlantic site are utilized to characterize turbulence in the marine boundary layer and air motions below stratocumulus clouds. Profiles of variance of vertical velocity binned by wind direction (wdir) yielded that the boundary layer measurements are affected by the island when the wdir is between 90° and 310° (measured clockwise from North where air is coming from). Data collected during the marine conditions (wdir<90 or wdir>310) showed that the variance of vertical velocity was higher during the winter months than during the summer months due to higher cloudiness, wind speeds, and surface fluxes. During marine conditions the variance of vertical velocity and cloud fraction exhibited a distinct diurnal cycle with higher values during the nighttime than during the daytime. Detailed analysis of 32 cases of drizzling marine stratocumulus clouds showed that for a similar amount of radiative cooling at the cloud top, within the sub-cloud layer 1) drizzle increasingly falls within downdrafts with increasing rain rates, 2) the strength of the downdrafts increases with increasing rain rates, and 3) the correlation between vertical air motion and rain rate is highest in the middle of the sub-cloud layer. The results presented herein have implications for climatological and model evaluation studies conducted at the ENA site, along with efforts of accurately representing drizzle-turbulence interactions in a range of atmospheric models.

Spatiotemporal distributions of halocarbons in the marine boundary air and surface seawater of the Changjiang estuary and its adjacent East China Sea

Spatiotemporal distributions of volatile halogenated organic compounds (VHOCs) were investigated in the marine boundary air and surface seawater of the Changjiang (Yangtze River) estuary and its adjacent East China Sea in two cruises from March 11, 2015 to March 21, 2015 and from July 9, 2015 to July 20, 2015. Results revealed that the concentrations of released chlorofluorocarbons (CFCs) such as CFC-12, CFC-11, and CFC-114 in China decreased, suggesting that limitations set by the Chinese government on CFCs production and consumption have taken effect. Atmospheric concentrations of CFCs were affected by local industrial sources of emission and transport of terrestrial pollutants from coastal areas to varying degrees. Seasonal variations in atmospheric VHOCs were probably due to seasonal differences in prevalent monsoon and biogenic production. In the study periods, the investigated area was an essential source of atmospheric CH3Br and CH3I but was a net sink of CFC-12, CFC-11, and CH3Cl.

Distribution of natural halocarbons in marine boundary air over the Arctic Ocean

Ongoing environmental changes in the Arctic will affect the exchange of natural volatile organic compounds between the atmosphere and the Arctic Ocean. Among these compounds, natural halocarbons play an important role in atmospheric ozone chemistry. We measured the distribution of five major natural halocarbons (methyl iodide, bromoform, dibromomethane, methyl chloride, and methyl bromide) together with dimethyl sulfide and tetrachloroethylene in the atmosphere over the Arctic Ocean (from the Bering Strait to 79°N) and along the cruise path to and from Japan. Methyl iodide, bromoform, and dibromomethane were most abundant near perennial sea ice in air masses derived from coastal regions and least abundant in the northernmost Arctic, where the air masses had passed over the ice pack, whereas methyl chloride and methyl bromide showed the opposite distribution pattern. Factors controlling those distributions and future prospects for natural halocarbons in the Arctic are discussed.

Global distribution and seasonal concentration change of methyl iodide in the atmosphere

We investigated seasonal variations in atmospheric CH3I at high, middle, and low latitudes in both hemispheres: at Alert (82.5°N, 62.5°W), over the northern and western Pacific Ocean (shipboard measurements); Cape Ochiishi (43.2°N, 145.5°E); Tsukuba (36.0°N, 140.1°E); Happo Ridge (36.7°N, 137.8°E); Hateruma Island (24.1°N, 123.8°E); San Cristobal Island (1.0°S, 89.4°W); Cape Grim (40.4°S, 144.6°E); and Syowa Station in Antarctica (68.5°S, 41.3°E) throughout 2004 and over an extended period of years at some sites. The highest median CH3I concentration was observed at San Cristobal Island, on the equator in the eastern Pacific, although no concentration elevation was found on or near the equator over the western Pacific. The lowest concentration was found in the Arctic and Antarctica, where they showed very similar seasonal variations that were clearly inversely correlated with variations in incident solar radiation. In the marine boundary air at midlatitude, the CH3I concentration was well correlated with surface seawater temperature (SST). Global atmospheric CH3I in wintertime was rather uniformly distributed (around 0.5 ppt) at middle and high latitudes.

Distribution of methyl chloride, methyl bromide, and methyl iodide in the marine boundary air over the western Pacific and southeastern Indian Ocean.

Methyl chloride (CH3Cl), methyl bromide (CH3Br), and methyl iodide (CH3I) in marine boundary air were measured over the western Pacific and the southeastern Indian Ocean during the period of December 1996–February 1997. The mean concentrations of CH3Cl, CH3Br, and CH3I were 623, 10.3, and 1.1 pptv, respectively, and their highest concentrations were observed in the tropics. The enhancement of CH3Cl concentration in the tropics, particularly near islands, was consistent with the recently reported finding on its high emissions from tropical coastal lands. The enhancement of CH3Br in the tropics was not related to the closeness of the sampling sites to islands, suggesting that land sources were not so important for CH3Br as for CH3Cl. The atmospheric CH3I concentrations showed a little higher concentration in the tropics and in the southern hemisphere (SH) than in the northern hemisphere (NH). This finding suggests that its higher emission from the tropics and SH in austral summer compensates the higher rate of photolytic decay of this compound in the regions.

Air-sea exchange of CO<sub>2</sub> in the central and western equatorial Pacific in 1990

Measurements of CO 2 in marine boundary air and in surface seawater of the central and western Pacific west of 150°W were made during the period from September to December 1990. The meridional section along 150°W showed pCO 2 (sea) maximum over 410 µ atm between the equator and 3°S due to strong equatorial upwelling. In the equatorial Pacific between 150°W and 179°E, pCO 2 (sea) decreased gradually toward the west as a result of biological CO 2 uptake and surface sea temperature increase. Between 179°E and 170°E, the pCO 2 (sea) decreased steeply from 400 µ atm to 350 µ atm along with a decrease of salinity. West of 170°E, where the salinity is low owing to the heavy rainfall, pCO 2 (sea) was nearly equal to pCO 2 (air). The distribution of the atmospheric CO 2 concentration showed a considerable variability (± 3 ppm) in the area north of the Intertropical Convergence Zone due to the regional net source-sink strength of the terrestrial biosphere. The net CO 2 flux from the sea to the atmosphere in the equatorial region of the central and western Pacific (15°S−10°N, 140°E−150°W) was evaluated from the ΔpCO 2 distribution and the several gas transfer coefficients reported so far. It ranged from 0.13 GtC year -1 −0.29 GtC year -1 . This CO 2 outflux is thought to almost disappear during the period of an El Nino event. DOI: 10.1034/j.1600-0889.47.issue4.5.x

Latitudinal distributions of methane in the upper troposphere and marine boundary air over the Pacific in 1990

Measurements of atmospheric methane at 11–13 km were made over the Pacific from 65°N to 65°S by two research flights (INSTAC II, February – March, 1990, and III, October, 1990). Relatively low concentrations were observed in the high latitudes, indicating intrusion of stratospheric methane at 11–13 km. Methane distributions in the tropical and subtropical regions showed north to south gradients and longitudinal gradients from west to central Pacific.During almost the same period of the INSTAC III flight, atmospheric methane in the marine boundary air was measured on boad a research vessel in the Pacific from 35°N to 20°S. In the Northern Hemisphere, methane concentrations in the marine boundary air showed relatively higher values compared with those at 11–13 km. However, the relative methane levels between lower and upper air were reversed between 10°N and the equator, where the Intertropical Convergence Zone (ITCZ) was located. Such positive vertical gradient appearing around the ITCZ was maintained to the mid‐latitudes of the Southern Hemisphere. These results strongly suggests that methane‐rich air in the Northern Hemisphere was actively transported upward at the ITCZ and then meridionally transported through the upper troposphere into the Southern Hemisphere.