scholarly journals Measurements of diurnal variations and eddy covariance (EC) fluxes of glyoxal in the tropical marine boundary layer: description of the Fast LED-CE-DOAS instrument

2014 ◽  
Vol 7 (10) ◽  
pp. 3579-3595 ◽  
Author(s):  
S. Coburn ◽  
I. Ortega ◽  
R. Thalman ◽  
B. Blomquist ◽  
C. W. Fairall ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Eddy Covariance ◽  
Marine Boundary Layer ◽  
Light Emitting Diode ◽  
Chemical Properties ◽  
Diurnal Variations ◽  
Optical Absorption Spectroscopy ◽  
Tropical Ocean ◽  
Mixing Ratios ◽  
Fast Light

Abstract. Here we present first eddy covariance (EC) measurements of fluxes of glyoxal, the smallest α-dicarbonyl product of hydrocarbon oxidation, and a precursor for secondary organic aerosol (SOA). The unique physical and chemical properties of glyoxal – i.e., high solubility in water (effective Henry's law constant, KH = 4.2 × 105 M atm−1) and short atmospheric lifetime (~2 h at solar noon) – make it a unique indicator species for organic carbon oxidation in the marine atmosphere. Previous reports of elevated glyoxal over oceans remain unexplained by atmospheric models. Here we describe a Fast Light-Emitting Diode Cavity-Enhanced Differential Optical Absorption Spectroscopy (Fast LED-CE-DOAS) instrument to measure diurnal variations and EC fluxes of glyoxal and inform about its unknown sources. The fast in situ sensor is described, and first results are presented from a cruise deployment over the eastern tropical Pacific Ocean (20° N to 10° S; 133 to 85° W) as part of the Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOCs (TORERO) field experiment (January to March 2012). The Fast LED-CE-DOAS is a multispectral sensor that selectively and simultaneously measures glyoxal (CHOCHO), nitrogen dioxide (NO2), oxygen dimers (O4), and water vapor (H2O) with ~2 Hz time resolution (Nyquist frequency ~1 Hz) and a precision of ~40 pptv Hz−0.5 for glyoxal. The instrument is demonstrated to be a "white-noise" sensor suitable for EC flux measurements. Fluxes of glyoxal are calculated, along with fluxes of NO2, H2O, and O4, which are used to aid the interpretation of the glyoxal fluxes. Further, highly sensitive and inherently calibrated glyoxal measurements are obtained from temporal averaging of data (e.g., detection limit smaller than 2.5 pptv in an hour). The campaign average mixing ratio in the Southern Hemisphere (SH) is found to be 43 ± 9 pptv glyoxal, which is higher than the Northern Hemisphere (NH) average of 32 ± 6 pptv (error reflects variability over multiple days). The diurnal variation of glyoxal in the marine boundary layer (MBL) is measured for the first time, and mixing ratios vary by ~8 pptv (NH) and ~12 pptv (SH) over the course of 24 h. Consistently, maxima are observed at sunrise (NH: 35 ± 5 pptv; SH: 47 ± 7 pptv), and minima at dusk (NH: 27 ± 5 pptv; SH: 35 ± 8 pptv). In both hemispheres, the daytime flux was directed from the atmosphere into the ocean, indicating that the ocean is a net sink for glyoxal during the day. After sunset the ocean was a source for glyoxal to the atmosphere (positive flux) in the SH; this primary ocean source was operative throughout the night. In the NH, the nighttime flux was positive only shortly after sunset and negative during most of the night. Positive EC fluxes of soluble glyoxal over oceans indicate the presence of an ocean surface organic microlayer (SML) and locate a glyoxal source within the SML. The origin of most atmospheric glyoxal, and possibly other oxygenated hydrocarbons over tropical oceans, remains unexplained and warrants further investigation.

scholarly journals Measurements of diurnal variations and Eddy Covariance (EC) fluxes of glyoxal in the tropical marine boundary layer: description of the Fast LED-CE-DOAS instrument

2014 ◽  
Vol 7 (6) ◽  
pp. 6245-6285 ◽  
Author(s):  
S. Coburn ◽  
I. Ortega ◽  
R. Thalman ◽  
B. Blomquist ◽  
C. W. Fairall ◽  
...  
Keyword(s):  
Eddy Covariance ◽  
Marine Boundary Layer ◽  
Light Emitting Diode ◽  
Chemical Properties ◽  
Diurnal Variations ◽  
Optical Absorption Spectroscopy ◽  
Tropical Ocean ◽  
Mixing Ratios ◽  
Flux Measurements ◽  
Fast Light

Abstract. Here we present first Eddy Covariance (EC) measurements of fluxes of glyoxal, the smallest α-dicarbonyl product of hydrocarbon oxidation, and a precursor for secondary organic aerosol (SOA). The unique physical and chemical properties of glyoxal, i.e., high solubility in water (Henry's Law constant, KH = 4.2 × 105 M atm−1) and short atmospheric lifetime (~2 h at solar noon) make it a unique indicator species for organic carbon oxidation in the marine atmosphere. Previous reports of elevated glyoxal over oceans remain unexplained by atmospheric models. Here we describe a Fast Light Emitting Diode Cavity Enhanced Differential Optical Absorption Spectroscopy (Fast LED-CE-DOAS) instrument to measure diurnal variations and EC fluxes of glyoxal, and inform about its unknown sources. The fast in situ sensor is described, and first results are presented from a cruise deployment over the Eastern tropical Pacific Ocean (20° N to 10° S; 133° W to 85° W) as part of the Tropical Ocean Troposphere Exchange of Reactive Halogens and OVOC (TORERO) field experiment (January to March 2012). The Fast LED-CE-DOAS is a multispectral sensor that selectively and simultaneously measures glyoxal (CHOCHO), nitrogen dioxide (NO2), oxygen dimers (O4) and water vapor (H2O) with ~2 Hz time resolution, and a precision of ~40 pptv Hz−0.5 for glyoxal. The instrument is demonstrated to be a "white-noise" sensor suitable for EC flux measurements; further, highly sensitive and inherently calibrated glyoxal measurements are obtained from temporal averaging of data (~2 pptv detection limit over 1 h). The campaign averaged mixing ratio in the Southern Hemisphere (SH) is found to be 43 ± 9 pptv glyoxal, and is higher than in the Northern Hemisphere (NH: 32 ± 6 pptv; error reflects variability over multiple days). The diurnal variation of glyoxal in the MBL is measured for the first time, and mixing ratios vary by ~8 ppt (NH) and ~12 pptv (SH) over the course of 24 h. Consistently, maxima are observed at sunrise (NH: 35 ± 5 pptv; SH: 47 ± 7 pptv) and minima at dusk (NH: 27 ± 5 pptv; SH: 35 ± 8 pptv). Ours are the first EC flux measurements of glyoxal. In both hemispheres, the daytime flux was directed from the atmosphere into the ocean, indicating that the ocean is a net sink for glyoxal during the day. After sunset the ocean was a source for glyoxal to the atmosphere (positive flux) in the SH; this primary ocean source was operative throughout the night. In the NH, the nighttime flux was positive only shortly after sunset, and negative during most of the night. Positive EC fluxes of soluble glyoxal over oceans indicate the presence of an ocean surface organic microlayer (SML), and locate a glyoxal source within the SML. The origin of atmospheric glyoxal, and possibly other oxygenated hydrocarbons over tropical oceans warrants further investigation.

scholarly journals In situ measurements of molecular iodine in the marine boundary layer: the link to macroalgae and the implications for O<sub>3</sub>, IO, OIO and NO<sub>x</sub>

2010 ◽  
Vol 10 (1) ◽  
pp. 361-390
Author(s):  
R.-J. Huang ◽  
K. Seitz ◽  
J. Buxmann ◽  
D. Poehler ◽  
K. E. Hornsby ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Single Point ◽  
Molecular Iodine ◽  
In Situ Measurements ◽  
Gas Chromatography Mass Spectrometry ◽  
Optical Absorption Spectroscopy ◽  
Mixing Ratios ◽  
Concentration Levels

Abstract. "Single-point" in situ measurements of molecular iodine (I2) were carried out in the coastal marine boundary layer (MBL) using diffusion denuders in combination with a gas chromatography-mass spectrometry (GC-MS) method. Comparison measurements were taken at Mace Head and Mweenish Bay, on the West Coast of Ireland. The observed mixing ratios of I2 at Mweenish Bay are much higher than that at Mace Head, indicating the emissions of I2 are correlated with the local algal biomass density and algae species. The concentration levels of I2 were found to correlate inversely with tidal height and correlate positively with the concentration levels of O3 in the surrounding air. However, the released I2 can also lead to O3 destruction via the reaction of O3 with iodine atoms that are formed by the photolysis of I2 during the day and via the reaction of I2 with NOx at night. IO and OIO were measured by long-path differential optical absorption spectroscopy (LP-DOAS). The results show that the concentrations of both daytime and nighttime IO are correlated with the mixing ratios of I2. OIO was observed not only during the day but also, for the first time at both Mace Head and Mweenish Bay, at night. In addition, I2 was measured simultaneously by the LP-DOAS technique and compared with the "single-point" in situ measurement. The results suggest that the local algae sources dominate the inorganic iodine chemistry at Mace Head and Mweenish Bay.

scholarly journals Iodine monoxide in the Western Pacific marine boundary layer

2013 ◽  
Vol 13 (6) ◽  
pp. 3363-3378 ◽  
Author(s):  
K. Großmann ◽  
U. Frieß ◽  
E. Peters ◽  
F. Wittrock ◽  
J. Lampel ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Western Pacific ◽  
Optical Absorption Spectroscopy ◽  
Mixing Ratios ◽  
Iodine Monoxide ◽  
German Research ◽  
Inorganic Iodine ◽  
The Tropics ◽  
The Western Pacific

Abstract. A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH3I, CH2I2, CH2ClI, CH2BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved if an additional sea-air flux of inorganic iodine (e.g., I2) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.

scholarly journals Iodine monoxide in the Western Pacific marine boundary layer

2012 ◽  
Vol 12 (10) ◽  
pp. 27475-27519 ◽  
Author(s):  
K. Großmann ◽  
U. Frieß ◽  
E. Peters ◽  
F. Wittrock ◽  
J. Lampel ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Western Pacific ◽  
Optical Absorption Spectroscopy ◽  
Mixing Ratios ◽  
Iodine Monoxide ◽  
German Research ◽  
Inorganic Iodine ◽  
The Tropics ◽  
The Western Pacific

Abstract. A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne, which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH3I, CH2I2, CH2ClI, CH2BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved, if an additional sea-air flux of inorganic iodine (e.g. I2) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.

scholarly journals Atmospheric bromoform at Mace Head, Ireland: seasonality and evidence for a peatland source

2005 ◽  
Vol 5 (11) ◽  
pp. 2927-2934 ◽  
Author(s):  
L. J. Carpenter ◽  
D. J. Wevill ◽  
S. O'Doherty ◽  
G. Spain ◽  
P. G. Simmonds
Keyword(s):  
Boundary Layer ◽  
Wind Direction ◽  
Marine Boundary Layer ◽  
Land Breeze ◽  
Peat Bogs ◽  
Mixing Ratios ◽  
The North ◽  
Prevailing Wind Direction ◽  
Atmospheric Observations ◽  
Summer Minimum

Abstract. In situ atmospheric observations of bromoform (CHBr3) made over a 2.5 year period at Mace Head, Ireland from May 2001- Dec 2003, including during the NAMBLEX (North Atlantic Marine Boundary Layer Experiment) campaign, show broad maxima from spring until autumn and winter minima, with mixing ratios of 5.3+1.0 pptv (mid March - mid October) and 1.8+0.8 pptv (December-February). This indicates that, unlike CHCl3, which has a summer minimum and winter maximum at Mace Head, local biological sources of CHBr3 have a greater influence on the atmospheric data than photochemical decay during long-range transport. The emission sources are predominantly macroalgal, but we find evidence for a small terrestrial flux from peatland ecosystems, which so far has not been accounted for in the CHBr3 budget. Sharp increases in CHCl3 and CHBr3 concentrations and decreases in O3 concentrations occurred at night when the wind direction switched from an ocean- to a land-based sector (land breeze) and the wind speed dropped to below 5 ms-1. These observations infer a shallow atmospheric boundary layer with increased O3 deposition and concentration of local emissions of both CHCl3 and CHBr3. The ratio of ΔCHCl3/ΔCHBr3 varied strongly according to the prevailing wind direction; from 0.60+0.15 in south-easterly (100-170° and northerly (340-20°) air to 2.5+0.4 in north-easterly (40-70°) air. Of these land-sectors, the south-easterly air masses are likely to be strongly influenced by macroalgal beds along the coast and the emission ratios probably reflect those from seaweeds in addition to land sources. The north-easterly airmasses however had an immediate fetch inland, which locally is comprised of coastal peatland ecosystems (peat bogs and coastal conifer plantations), previously identified as being strong sources of atmospheric CHCl3 under these conditions. Although we cannot entirely rule out other local land or coastal sources, our observations also suggest peatland ecosystem emissions of CHBr3. We use correlations between CHCl3 and CHBr3 during the north-easterly land breeze events in conjunction with previous estimates of local wetland CHCl3 release to tentatively deduce a global wetland CHBr3 source of 20.4(0.4-948) Gg yr-1, which is approximately 7% of the total global source.

scholarly journals A broadband cavity ringdown spectrometer for in-situ measurements of atmospheric trace gases

2005 ◽  
Vol 5 (3) ◽  
pp. 3491-3532 ◽  
Author(s):  
M. Bitter ◽  
S. M. Ball ◽  
I. M. Povey ◽  
R. L. Jones
Keyword(s):  
Boundary Layer ◽  
Water Vapour ◽  
Marine Boundary Layer ◽  
Thermal Dissociation ◽  
Optical Absorption Spectroscopy ◽  
Integration Time ◽  
Research Station ◽  
Absorption Bands ◽  
Optical Extinction ◽  
Cavity Ringdown

Abstract. This paper describes a broadband cavity ringdown spectrometer and its deployment during the 2002 North Atlantic Marine Boundary Layer Experiment (NAMBLEX) to measure ambient concentrations of NO3, N2O5, I2 and OIO at the Mace Head Atmospheric Research Station, Co. Galway, Ireland. The effective absorption path lengths accessible with the spectrometer generally exceeded 10 km, enabling sensitive localised ''point'' measurements of atmospheric absorbers to be made adjacent to the other instruments monitoring chemically related species at the same site. For the majority of observations, the spectrometer was used in an open path configuration thereby avoiding surface losses of reactive species. A subset of observations targeted the N2O5 molecule by detecting the additional NO3 formed by the thermal dissociation of N2O5. In all cases the concentrations of the atmospheric absorbers were retrieved by fitting the differential structure in the broadband cavity ringdown spectra using a methodology adapted from long path differential optical absorption spectroscopy. The uncertainty of the retrieval depends crucially on the correct treatment and fitting of the absorption bands due to water vapour, a topic that is discussed in the context of analysing broadband cavity ringdown spectra. The quality of the measurements and the retrieval method are illustrated with representative spectra acquired during NAMBLEX in spectral regions around 660 nm (NO3 and N2O5) and 570 nm (I2 and OIO). Typical detection limits were 1 pptv for NO3 in an integration time of 100 s, 4 pptv for OIO and 20 pptv for I2 in an integration time of 10 min. Additionally, the concentrations of atmospheric water vapour and the aerosol optical extinction were retrieved in both spectral regions. A companion paper in this issue presents the time series of the measurements and discusses their significance for understanding the variability of short lived nitrogen and iodine compounds in the marine boundary layer.

scholarly journals Constraining the concentration of the hydroxyl radical in a stratocumulus-topped marine boundary layer from sea-to-air eddy covariance flux measurements of dimethylsulfide

2009 ◽  
Vol 9 (23) ◽  
pp. 9225-9236 ◽  
Author(s):  
M. Yang ◽  
B. W. Blomquist ◽  
B. J. Huebert
Keyword(s):  
Boundary Layer ◽  
Hydroxyl Radical ◽  
Eddy Covariance ◽  
Diurnal Cycle ◽  
Marine Boundary Layer ◽  
High Reactivity ◽  
Simple Method ◽  
Bromine Oxide ◽  
Dms Oxidation ◽  
Strong Inversion

Abstract. The hydroxyl radical (OH) is an important oxidant in the troposphere due to its high reactivity and relative abundance. Measuring the concentration of OH in situ, however, is technically challenging. Here we present a simple method of estimating an OH-equivalent oxidant concentration ("effective OH") in the marine boundary layer (MBL) from the mass balance of dimethylsulfide (DMS). We use shipboard eddy covariance measurements of the sea-to-air DMS flux from the Vamos Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in October and November of 2008. The persistent stratocumulus cloud-cover off the west coast of South America and the associated strong inversion between MBL and the free troposphere (FT) greatly simplify the dynamics in this region and make our budget estimate possible. From the observed diurnal cycle in DMS concentration, the nighttime entrainment velocity at the inversion is estimated to be 4 mm s−1. We calculate 1.4(±0.2)×106 OH molecules cm−3 from the DMS budget, which represents a monthly effective concentration and is well within the range of previous estimates. Furthermore, when linearly proportioned according to the intensity of solar flux, the resultant diel OH profile, together with DMS surface and entrainment fluxes, enables us to accurately replicate the observed diurnal cycle in DMS (correlation coefficient over 0.9). The nitrate radical (NO3) is found to have little contribution to DMS oxidation during VOCALS-REx. An upper limit estimate of 1 pptv of bromine oxide radical (BrO) would account for 30% of DMS oxidation and lower the OH concentration to 1.0)×106 OH molecules cm−3. Our effective OH estimate includes the oxidation of DMS by such radicals.

scholarly journals Parameterizing radiative transfer to convert MAX-DOAS dSCDs into near-surface box averaged mixing ratios and vertical profiles

2012 ◽  
Vol 5 (5) ◽  
pp. 7641-7673 ◽  
Author(s):  
R. Sinreich ◽  
A. Merten ◽  
L. Molina ◽  
R. Volkamer
Keyword(s):  
Boundary Layer ◽  
Radiative Transfer ◽  
Mexico City ◽  
Vertical Gradient ◽  
Trace Gas ◽  
Optical Absorption Spectroscopy ◽  
Transfer Model ◽  
Model Calculations ◽  
Near Surface ◽  
Mixing Ratios

Abstract. We present a novel parameterization method to convert Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) differential Slant Column Densities (dSCDs) into near-surface box averaged volume mixing ratios. The approach is applicable inside the planetary boundary layer under conditions with significant aerosol load, does not require a-priori assumptions about the trace gas vertical distribution and builds on the increased sensitivity of MAX-DOAS near the instrument altitude. It parameterizes radiative transfer model calculations and significantly reduces the computational effort. The biggest benefit of this method is that the retrieval of an aerosol profile, which usually is necessary for deriving a trace gas concentration from MAX-DOAS dSCDs, is not needed. The method is applied to NO2 MAX-DOAS dSCDs recorded during the Mexico City Metropolitan Area 2006 (MCMA-2006) measurement campaign. The retrieved volume mixing ratios of two elevation angles (1° and 3°) are compared to volume mixing ratios measured by two long-path (LP)-DOAS instruments located at the same site. Measurements are found to agree well during times when vertical mixing is expected to be strong. However, inhomogeneities in the air mass above Mexico City can be detected by exploiting the different horizontal and vertical dimensions probed by MAX-DOAS measurements at different elevation angles, and by LP-DOAS. In particular, a vertical gradient in NO2 close to the ground can be observed in the afternoon, and is attributed to reduced mixing coupled with near surface emission. The existence of a vertical gradient in the lower 250 m during parts of the day shows the general challenge of sampling the boundary layer in a representative way and emphasizes the need of vertically resolved measurements.

scholarly journals Diurnal Course of Carbon Dioxide Mixing Ratios in the Urban Boundary Layer in Response to Surface Emissions

2016 ◽  
Vol 55 (3) ◽  
pp. 507-529 ◽  
Author(s):  
B. Crawford ◽  
A. Christen ◽  
I. McKendry
Keyword(s):  
Carbon Dioxide ◽  
Boundary Layer ◽  
Eddy Covariance ◽  
Local Scale ◽  
Urban Boundary Layer ◽  
Model Calculations ◽  
Horizontal Advection ◽  
Mixing Ratios ◽  
Diurnal Course ◽  
Entrainment Fluxes

AbstractObservations of carbon dioxide (CO2) mixing ratios in the urban boundary layer (UBL) are rare, even though there is potential for such measurements to be used to monitor city-scale net CO2 emissions. This work presents a unique dataset of CO2 mixing ratios observed in the UBL above Vancouver, British Columbia, Canada, by means of a tethered balloon system over a continuous 24-h summertime period. Vertical profiles of CO2 mixing ratios are found to vary according to UBL thermal structure and mechanical dynamics (development of convective and nocturnal boundary layers, vertical mixing from mechanical turbulence, horizontal advection from land–sea thermal breezes, and vertical entrainment). A box model is applied to quantify net city-scale surface emissions to the UBL volume using the measured rate of change of UBL CO2 mixing ratios and estimated CO2 advection and entrainment fluxes. The diurnal course of city-scale net emissions predicted by the model is similar to simultaneous local-scale eddy-covariance CO2 flux measurements, although there are relatively large uncertainties in hourly model calculations of horizontal advection and vertical entrainment fluxes due to inputs of regional background CO2 mixing ratios. Daily city-scale emissions totals predicted by the model (20.2 gC m−2 day−1) are 35% larger than those measured simultaneously on an urban local-scale eddy-covariance flux tower and are within 32% of a spatially scaled municipal greenhouse gas inventory. However, these methods are not expected to agree exactly because they represent different spatial source areas and include different CO2 source and sink processes.

scholarly journals Investigation of the Diurnal Variation of Marine Boundary Layer Cloud Microphysical Properties at the Azores

2014 ◽  
Vol 27 (23) ◽  
pp. 8827-8835 ◽  
Author(s):  
Xiquan Dong ◽  
Baike Xi ◽  
Peng Wu
Keyword(s):  
Boundary Layer ◽  
Diurnal Variation ◽  
Marine Boundary Layer ◽  
Surface Wind ◽  
Number Concentration ◽  
Diurnal Variations ◽  
Cloud Layer ◽  
Entire Study ◽  
Microphysical Properties ◽  
Seasonal And Diurnal Variations

Abstract A new method has been developed to retrieve the nighttime marine boundary layer (MBL) cloud microphysical properties, which provides a complete 19-month dataset to investigate the diurnal variation of MBL cloud microphysical properties at the Azores. Compared to the corresponding daytime results presented in the authors' previous study over the Azores region, all nighttime monthly means of cloud liquid water path (LWP) exceed their daytime counterparts with an annual-mean LWP of 140 g m−2, which is ~30.9 g m−2 larger than daytime. Because the MBL clouds are primarily driven by convective instabilities caused by cloud-top longwave (LW) radiative cooling, more MBL clouds are well mixed and coupled with the surface during the night; thus, its cloud layer is deeper and its LWP is higher. During the day, the cloud layer is warmed by the absorption of solar radiation and partially offsets the cloud-top LW cooling, which makes the cloud layer thinner with less LWP. The seasonal and diurnal variations of cloud LWC and optical depth basically follow the variation of LWP. There are, however, no significant day–night differences and diurnal variations in cloud-droplet effective radius (re), number concentration (Nd), and corresponding surface measured cloud condensation nuclei (CCN) number concentration (NCCN) (at supersaturation S = 0.2%). Surface NCCN increases from around sunrise (0300–0600 LT) to late afternoon, which strongly correlates with surface wind speed (r = 0.76) from 0300 to 1900 LT. The trend in hourly-mean Nd is consistent with NCCN variation from 0000 to 0900 LT but not for afternoon and evening with an averaged ratio (Nd/NCCN) of 0.35 during the entire study period.

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