scholarly journals Effect of relative humidity on light scattering by mineral dust aerosol as measured in the marine boundary layer over the tropical Atlantic Ocean

1998 ◽  
Vol 103 (D23) ◽  
pp. 31113-31121 ◽  
Author(s):  
X. Li-Jones ◽  
H. B. Maring ◽  
J. M. Prospero
Keyword(s):  
Boundary Layer ◽  
Light Scattering ◽  
Relative Humidity ◽  
Atlantic Ocean ◽  
Marine Boundary Layer ◽  
Mineral Dust ◽  
Dust Aerosol ◽  
Tropical Atlantic ◽  
Tropical Atlantic Ocean

scholarly journals Chemistry, transport and dry deposition of trace gases in the boundary layer over the tropical Atlantic Ocean and the Guyanas during the GABRIEL field campaign

2007 ◽  
Vol 7 (14) ◽  
pp. 3933-3956 ◽  
Author(s):  
A. Stickler ◽  
H. Fischer ◽  
H. Bozem ◽  
C. Gurk ◽  
C. Schiller ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atlantic Ocean ◽  
Dry Deposition ◽  
Deposition Velocity ◽  
Ozone Production ◽  
Organic Peroxides ◽  
Tropical Atlantic ◽  
Model Calculations ◽  
Tropical Atlantic Ocean ◽  
Positive Tendency

Abstract. We present a comparison of different Lagrangian and chemical box model calculations with measurement data obtained during the GABRIEL campaign over the tropical Atlantic Ocean and the Amazon rainforest in the Guyanas, October 2005. Lagrangian modelling of boundary layer (BL) air constrained by measurements is used to derive a horizontal gradient (≈5.6 pmol/mol km−1) of CO from the ocean to the rainforest (east to west). This is significantly smaller than that derived from the measurements (16–48 pmol/mol km−1), indicating that photochemical production from organic precursors alone cannot explain the observed strong gradient. It appears that HCHO is overestimated by the Lagrangian and chemical box models, which include dry deposition but not exchange with the free troposphere (FT). The relatively short lifetime of HCHO implies substantial BL-FT exchange. The mixing-in of FT air affected by African and South American biomass burning at an estimated rate of 0.12 h−1 increases the CO and decreases the HCHO mixing ratios, improving agreement with measurements. A mean deposition velocity of 1.35 cm/s for H2O2 over the ocean as well as over the rainforest is deduced assuming BL-FT exchange adequate to the results for CO. The measured increase of the organic peroxides from the ocean to the rainforest (≈0.66 nmol/mol d−1) is significantly overestimated by the Lagrangian model, even when using high values for the deposition velocity and the entrainment rate. Our results point at either heterogeneous loss of organic peroxides and/or their radical precursors, underestimated photodissociation or missing reaction paths of peroxy radicals not forming peroxides in isoprene chemistry. We calculate a mean integrated daytime net ozone production (NOP) in the BL of (0.2±5.9) nmol/mol (ocean) and (2.4±2.1) nmol/mol (rainforest). The NOP strongly correlates with NO and has a positive tendency in the boundary layer over the rainforest.

scholarly journals Global dust optical depth climatology derived from CALIOP and MODIS aerosol retrievals on decadal timescales: regional and interannual variability

2021 ◽  
Vol 21 (17) ◽  
pp. 13369-13395
Author(s):  
Qianqian Song ◽  
Zhibo Zhang ◽  
Hongbin Yu ◽  
Paul Ginoux ◽  
Jerry Shen
Keyword(s):  
Atlantic Ocean ◽  
Optical Depth ◽  
Surface Wind ◽  
Dust Aerosol ◽  
Orthogonal Polarization ◽  
Northwestern Pacific ◽  
Gobi Desert ◽  
Tropical Atlantic ◽  
Shape Information ◽  
Tropical Atlantic Ocean

Abstract. We derived two observation-based global monthly mean dust aerosol optical depth (DAOD) climatological datasets from 2007 to 2019 with a 2∘ (latitude) × 5∘ (longitude) spatial resolution, one based on Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and the other on Moderate Resolution Imaging Spectroradiometer (MODIS) observations. In addition, the CALIOP climatological dataset also includes dust vertical extinction profiles. Dust is distinguished from non-dust aerosols based on particle shape information (e.g., lidar depolarization ratio) for CALIOP and on dust size and absorption information (e.g., fine-mode fraction, Ångström exponent, and single-scattering albedo) for MODIS, respectively. The two datasets compare reasonably well with the results reported in previous studies and the collocated Aerosol Robotic Network (AERONET) coarse-mode AOD. Based on these two datasets, we carried out a comprehensive comparative study of the spatial and temporal climatology of dust. On a multi-year average basis, the global (60∘ S–60∘ N) annual mean DAOD is 0.032 and 0.067 according to CALIOP and MODIS retrievals, respectively. In most dust-active regions, CALIOP DAOD generally correlates well (correlation coefficient R>0.6) with the MODIS DAOD, although the CALIOP value is significantly smaller. The CALIOP DAOD is 18 %, 34 %, 54 %, and 31 % smaller than MODIS DAOD over the Sahara, the tropical Atlantic Ocean, the Caribbean Sea, and the Arabian Sea, respectively. Applying a regional specific lidar ratio (LR) of 58 sr instead of the 44 sr used in the CALIOP operational retrieval reduces the difference from 18 % to 8 % over the Sahara and from 34 % to 12 % over the tropical Atlantic Ocean. However, over eastern Asia and the northwestern Pacific Ocean (NWP), the two datasets show weak correlation. Despite these discrepancies, CALIOP and MODIS show similar seasonal and interannual variations in regional DAOD. For dust aerosol over the NWP, both CALIOP and MODIS show a declining trend of DAOD at a rate of about 2 % yr−1. This decreasing trend is consistent with the observed declining trend of DAOD in the southern Gobi Desert at a rate of 3 % yr−1 and 5 % yr−1 according to CALIOP and MODIS, respectively. The decreasing trend of DAOD in the southern Gobi Desert is in turn found to be significantly correlated with increasing vegetation and decreasing surface wind speed in the area.

scholarly journals Chemistry, transport and dry deposition of trace gases in the boundary layer over the tropical Atlantic Ocean and the Guyanas during the GABRIEL field campaign

2007 ◽  
Vol 7 (2) ◽  
pp. 4781-4855 ◽  
Author(s):  
A. Stickler ◽  
H. Fischer ◽  
H. Bozem ◽  
C. Gurk ◽  
C. Schiller ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Steady State ◽  
Atlantic Ocean ◽  
Dry Deposition ◽  
Trace Gases ◽  
Deposition Velocity ◽  
Organic Peroxides ◽  
Tropical Atlantic ◽  
Tropical Atlantic Ocean ◽  
Positive Tendency

Abstract. We present a comparison of different Lagrangian and steady state box model runs with measurement data obtained during the GABRIEL campaign over the tropical Atlantic Ocean and the rainforest in the Guyanas, October 2005. Lagrangian modelling of boundary layer (BL) CO constrained by measurements of reactive trace gases and radiation is used to derive a horizontal gradient (≈5.6 pmol/mol km−1) of this compound from the ocean to the rainforest (east to west). This is significantly smaller than that derived from the measurements (16–48 pmol/mol km−1), indicating that photochemical production from organic precursors alone cannot explain the observed strong gradient. It appears that HCHO is overestimated by the Lagrangian and "steady state" models, which include dry deposition but not exchange with the free troposphere (FT). The relatively short lifetime of HCHO (50–100 min) implies substantial BL-FT exchange. The mixing-in of FT air affected by African and South American biomass burning at an estimated rate of 0.12 h−1 increases the CO and lowers the HCHO mixing ratios, leading to a better agreement with measurements. A 24 h mean deposition velocity of 1.35 cm/s for H2O2 over the ocean as well as over the rainforest is deduced assuming BL-FT exchange adequate to the results for CO. The measured increase of the organic peroxides from the ocean to the rainforest (≈0.66 nmol/mol d−1) is significantly overestimated by the Lagrangian model, even when using high values for the deposition velocity and the entrainment rate. Our results point at either heterogeneous loss of organic peroxides and/or their radical precursors or a missing reaction path of peroxy radicals not forming peroxides in isoprene chemistry. We calculate a mean integrated daytime net ozone production (NOP) in the BL of (0.2±5.9) nmol/mol (ocean) and (2.4±2.1) nmol/mol (rainforest). The NOP strongly correlates with NO and shows a positive tendency in the boundary layer over the rainforest.

scholarly journals Mechanisms Controlling the Annual Cycle of Precipitation in the Tropical Atlantic Sector in an Atmospheric GCM*

2004 ◽  
Vol 17 (24) ◽  
pp. 4708-4723 ◽  
Author(s):  
M. Biasutti ◽  
D. S. Battisti ◽  
E. S. Sarachik
Keyword(s):  
Boundary Layer ◽  
Atlantic Ocean ◽  
Annual Cycle ◽  
Convective Available Potential Energy ◽  
Net Energy ◽  
Tropical Atlantic ◽  
Atlantic Sector ◽  
Tropical Atlantic Ocean ◽  
Low Level ◽  
The Impact

Abstract A set of AGCM experiments is used to study the annual cycle of precipitation in the region surrounding the tropical Atlantic Ocean. The experiments are designed to reveal the relative importance of insolation over land and the (uncoupled) SST on the annual cycle of precipitation over the tropical Atlantic Ocean, Africa, and the tropical Americas. SST variations impact the position of the maritime ITCZ by forcing the hydrostatic adjustment of the atmospheric boundary layer and changes in surface pressure and low-level convergence. The condensation heating released in the ITCZ contributes substantially to the surface circulation and the maintenance of the SST-induced ITCZ anomalies. The remote influence of SST is felt in equatorial coastal areas and the Sahel. The circulation driven by condensation heating in the maritime ITCZ extends to the coastal regions, thus communicating the SST signal onshore. Conversely, the Sahel responds to variations in SST through boundary layer processes that do not involve the maritime ITCZ. The atmospheric response to changes in subtropical SST is advected inland and forces changes in sea level pressure and low-level convergence across a large part of tropical Africa. The impact of local insolation on continental precipitation can be explained by balancing net energy input at the top of the atmospheric column with the export of energy by the divergent circulation that accompanies convection. Increased insolation reduces the stability of the atmosphere in the main continental convection centers, but not in monsoon regions. Insolation over land impacts the intensity of the maritime ITCZ via its influence on precipitation in Africa and South America. Reduced land precipitation induces the cooling of the Atlantic upper troposphere and the enhancement of convective available potential energy in the maritime ITCZ.

scholarly journals Black carbon concentrations and sources in the marine boundary layer of the tropical Atlantic Ocean using four methodologies

2013 ◽  
Vol 13 (11) ◽  
pp. 29785-29810
Author(s):  
K. Pohl ◽  
M. Cantwell ◽  
P. Herckes ◽  
R. Lohmann
Keyword(s):  
Atlantic Ocean ◽  
Black Carbon ◽  
Biomass Burning ◽  
Marine Boundary Layer ◽  
Optical Transmittance ◽  
Atmospheric Particulate Matter ◽  
Tropical Atlantic ◽  
Tropical Atlantic Ocean ◽  
Pyrene Fluorescence ◽  
The Caribbean

Abstract. Black carbon (BC) is the highly carbonaceous byproduct of biomass burning and fossil fuel combustion with a composition ranging from thermally stable soot to less recalcitrant charcoal. Atmospheric particulate matter samples across the tropical Atlantic Ocean were quantified for BC using four different methods: chemothermal oxidation at 375 °C (CTO-375), pyrene fluorescence loss, thermal optical transmittance, and optical transmission attenuation. The highest BC concentrations were detected in the Caribbean Sea and off the African coast, with a regional average of 0.6 μg m−3 for both. The lowest average concentrations were measured off the coast of South America at 0.2 to 0.3 μg m−3. The thermally-based CTO-375 method generally detected lower BC concentrations than the other three methods. The ratio of soot-like BC, as defined by the CTO-375 method, relative to the broader BC combustion continuum, as defined by the pyrene fluorescence loss, was <1 for all regions except for the Caribbean, supporting that charcoal was an important fraction of the aerosol BC. Regions impacted by biomass burning emissions should utilize multiple methods to better apportion the BC concentrations and sources.

scholarly journals Black carbon concentrations and sources in the marine boundary layer of the tropical Atlantic Ocean using four methodologies

2014 ◽  
Vol 14 (14) ◽  
pp. 7431-7443 ◽  
Author(s):  
K. Pohl ◽  
M. Cantwell ◽  
P. Herckes ◽  
R. Lohmann
Keyword(s):  
Boundary Layer ◽  
South America ◽  
Atlantic Ocean ◽  
Black Carbon ◽  
Marine Boundary Layer ◽  
Caribbean Sea ◽  
Tropical Atlantic ◽  
Carbonaceous Aerosols ◽  
The Caribbean ◽  
Carbon Concentrations

Abstract. Combustion-derived aerosols in the marine boundary layer have been poorly studied, especially in remote environments such as the open Atlantic Ocean. The tropical Atlantic has the potential to contain a high concentration of aerosols, such as black carbon, due to the African emission plume of biomass and agricultural burning products. Atmospheric particulate matter samples across the tropical Atlantic boundary layer were collected in the summer of 2010 during the southern hemispheric dry season when open fire events were frequent in Africa and South America. The highest black carbon concentrations were detected in the Caribbean Sea and within the African plume, with a regional average of 0.6 μg m−3 for both. The lowest average concentrations were measured off the coast of South America at 0.2 to 0.3 μg m−3. Samples were quantified for black carbon using multiple methods to provide insights into the form and stability of the carbonaceous aerosols (i.e., thermally unstable organic carbon, soot like, and charcoal like). Soot-like aerosols composed up to 45% of the carbonaceous aerosols in the Caribbean Sea to as little as 4% within the African plume. Charcoal-like aerosols composed up to 29% of the carbonaceous aerosols over the oligotrophic Sargasso Sea, suggesting that non-soot-like particles could be present in significant concentrations in remote environments. To better apportion concentrations and forms of black carbon, multiple detection methods should be used, particularly in regions impacted by biomass burning emissions.

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