We observed 51 positions in the OH 1667 MHz main line transitions in the translucent high latitude cloud MBM40. We detected OH emission in 8 out of 8 positions in the molecular core of the cloud and 24 out of 43 in the surrounding, lower extinction envelope and periphery of the cloud. Using a linear relationship between the integrated OH line intensity and E(B-V), we estimate the mass in the core, the envelope, and the periphery of the cloud to be 4, 8, and 5 M ⊙ . As much as a third of the total cloud mass may be found in the periphery (E(B-V) < 0.12 mag) and about a half in the envelope (0.12 ≤ E(B-V) ≤ 0.17 mag). If these results are applicable to other translucent clouds, then the OH 1667 MHz line is an excellent tracer of gas in very low extinction regions and high-sensitivity mapping of the envelopes of translucent molecular clouds may reveal the presence of significant quantities of molecular mass.
Despite the importance of tidal ecosystems in the global carbon budget, the relationships between environmental drivers and carbon dynamics in these wetlands remain poorly understood. This limited understanding results from the challenges associated with in situ flux studies and their correlation with satellite imagery which can be affected by periodic tidal flooding. Carbon dioxide eddy covariance (EC) towers are installed in only a few wetlands worldwide, and the longest eddy-covariance record from Georgia (GA) wetlands contains only two continuous years of observations. The goals of the present study were to evaluate the performance of existing MODIS Gross Primary Production (GPP) products (MOD17A2) against EC derived GPP and develop a tide-robust Normalized Difference Moisture Index (NDMI) based model to predict GPP within a Spartina alterniflora salt marsh on Sapelo Island, GA. These EC tower-based observations represent a basis to associate CO2 fluxes with canopy reflectance and thus provide the means to use satellite-based reflectance data for broader scale investigations. We demonstrate that Light Use Efficiency (LUE)-based MOD17A2 does not accurately reflect tidal wetland GPP compared to a simple empirical vegetation index-based model where tidal influence was accounted for. The NDMI-based GPP model was capable of predicting changes in wetland CO2 fluxes and explained 46% of the variation in flux-estimated GPP within the training data, and a root mean square error of 6.96 g C m−2 in the validation data. Our investigation is the first to create a MODIS-based wetland GPP estimation procedure that demonstrates the importance of filtering tidal observations from satellite surface reflectance data.
We present high-resolution (1. 3 × 1. 6) observations of the CH 2 Π 1/2 (F = 1-1) emission line at 3335 MHz in two high-latitude translucent clouds, MBM 3 and 40. At the assumed cloud distances, the angular resolution corresponds to ∼0.05 pc, nearly an order of magnitude better than previous studies. Comparisons of the CH emission with previously obtained CO(1-0) data are difficult to interpret: the CO and CH line emission correlates in MBM 40 but not in MBM 3. In both clouds, there is a spatial offset in the peak emission, and perhaps in velocity for MBM 40. The difference in emission characteristics for the two tracers are noticeable in these two nearby clouds because of the high spatial resolution. Since both CH and CO are deemed to be reliable tracers of H 2 , our results indicate that more care should be taken when using one of these tracers to determine the mass of a nearby molecular cloud.
We made CO(1-0) observations of 103 lines of sight in the core and envelope of the highlatitude cloud MBM 40 to determine how the CO-H 2 conversion factor (X CO ) varies throughout the cloud. Calibrating X CO with CH data at similar resolution (1 for CO, 1.5 for CH) yields values of X CO ranging from 0.6 × 10 20 to 3.3 × 10 20 cm −2 [K km s −1 ] −1 with an average of 1.3 × 10 20 cm −2 [K km s −1 ] −1 . Given that the cloud has a peak reddening of 0.24 mag, it should be classed as a diffuse rather than a translucent molecular cloud. The mass obtained from the CO data and our values of X CO is 9.6 M for the core, 12 M for the envelope, and 10 M for the periphery of the cloud. A third of the molecular mass of the cloud is found in a region with E(B-V) < 0.12 mag. With these mass estimates, we determine that the cloud is not gravitationally bound.
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