Lower Marine Boundary Layer Research Articles

Multi-process atmospheric particle number size distribution over the Bohai Sea: Insights from cruise and onboard unmanned aerial vehicle observations

A better understanding of the particle number size distribution aids to better assess the direct and indirect effects of particles on air quality and climate. In this study, particle number concentrations (PNCs) and size distribution along with chemical components, gaseous pollutants, and meteorological parameters were measured during a cruise campaign over the Bohai Sea. Four distinct periods were identified, showing significant impacts from continental outflows and ocean emissions on the particle number size distributions. Under the strong terrestrial pollution transport period, an obvious increase of PNCs below 1 μm was observed. The significant correlation between black carbon (BC) and PNCs in the size range of 0.35–1.0 μm suggested aging of aerosols during the long-range transport. An invaded dust pollution significantly reduced PNCs below 0.5 μm, while tripled the number concentration of particles larger than 0.5 μm. The ocean moisture facilitated the mixing between dust and secondary aerosols. An intense sea fog event observed during the cruise slightly increased PNCs smaller than 1 μm while decreasing larger ones. The correlations between PNCs below 0.5 μm and BC as well as SO2 demonstrated the impact of shipping activities on the formation of fresh particles. Aerosol vertical profiles in the marine boundary layer were measured by an unmanned aerial vehicle platform. The vertical patterns of particle number size distribution were largely governed by the origins of transported air masses and profiles of wind fields. Observations showed that the particle distribution in the lower marine boundary layer was typically inhomogeneous and rapidly evolving with time, highlighting the importance of strengthening the vertical observations of atmospheric parameters over the ocean.

Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 1: Mean structure and diurnal cycle

Abstract. Atmospheric subsidence over the subtropical southeast Pacific (SEP) leads to a low-level anticyclonic circulation, a cool sea surface and a cloud-topped marine boundary layer (MBL). Observations in this region from a major field campaign during October and November 2008, the VOCALS Regional Experiment, provide ample data to characterize the lower atmospheric features over the SEP. The observations are also useful to test the ability of an area-limited, high-resolution atmospheric model to simulate the SEP conditions. Observations and model-results (where appropriate) improve the characterization of the mean state (Part 1) and variability (Part 2) of the lower troposphere including circulation, MBL characteristics and the upsidence wave. Along 20° S the MBL is generally deeper offshore (1600 m at 85° W) but there is also considerable variability. MBL depth and variability decrease towards the coast and maximum inversion strength is detected between 74–76° W. Weather Research and Forecasting (WRF) simulations underestimate MBL height the most near the coast but improve offshore. Southeasterly trades prevail within the MBL although the wind speed decreases toward the coast. Above the MBL along the coast of Chile, flow is northerly, has a maximum at 3 km, and extends westward to ~74° W, apparently due to the mechanical blocking exerted by the Andes upon the westerly flow aloft. Mean MBL features along northern Chile (18–25° S) are remarkably similar (e.g., MBL depth just below 1 km) in spite of different SST. Observed diurnal cycles of the temperature at the coast and further offshore exhibit a number of conspicuous features that are consistent with the southwestward propagation of an upsidence wave initiated during late evening along the south Peru coast. Furthermore, the passage of the vertical motion results in either constructive or deconstructive interference with the radiatively-forced diurnal cycle of MBL depth. Interference is clearly seen in the soundings at Iquique which are driven by a strong upsidence wave contrary to the radiation-driven cycle, leading to a diurnal cycle opposite of the other sites. Because WRF simulations have a lower MBL height, the speed of the simulated gravity wave is slower than observations and accounts for most of the discrepancy between observed and simulated phase speeds.

Chemical composition of aerosol, sea fog, and rainwater in the marine boundary layer of the northwestern North Pacific and its marginal seas

Samples of aerosol, sea fog, and rainwater were collected during a research cruise in the northwestern North Pacific, the Sea of Okhotsk, and the Sea of Japan in the summer of 1998. High concentrations of NO3−, nss‐SO42− and NH4+ in aerosol over the Sea of Japan suggest that anthropogenic substances were transported to this region. Although the Sea of Okhotsk was covered with a clean marine air mass, the concentration of nss‐SO42− was comparatively high in the aerosol samples. This nss‐SO42− is probably of marine biogenic origin. The pH values of fogwater samples were measured to be <3.0 over the Sea of Okhotsk and the Sea of Japan. The concentrations of NO3− and NH4+ in the fogwater collected over the Sea of Japan were higher than those in the other regions, suggesting that the sea fog scavenged anthropogenic substances. The concentration of nss‐SO42− in fogwater over the Sea of Okhotsk was equivalent to that over the Sea of Japan, probably because nss‐SO42− and SO2 of marine biogenic origin were scavenged by the sea fog over the Sea of Okhotsk. The pH values of rainwater samples ranged from 6.1 to 7.2 during the cruise, and acidification of the rain was not significant. The concentrations of nss‐Ca2+ in the rainwater were higher than those of the fogwater. This suggests that the rain‐scavenged continental CaCO3 may have existed above the lower marine boundary layer, where sea fog appeared. Comparisons of the composition of aerosol and fogwater indicated that coarse particles, such as sea salts predominantly act as condensation nuclei of sea fog droplets rather than fine particles such as (NH4)2SO4.

Aerosol physical properties and processes in the lower marine boundary layer: a comparison of shipboard sub-micron data from ACE-1 and ACE-2

Aerosol physical properties and processes in the lower marine boundary layer: a comparison of shipboard sub-micron data from ACE-1 and ACE-2

Aerosol physical properties and processes in the lower marine boundary layer: a comparison of shipboard sub-micron data from ACE-1 and ACE-2

The goals of the IGAC Aerosol Characterization Experiments (ACE) are to determine and understand the properties and controlling processes of the aerosol in a globally representative range of natural and anthropogenically perturbed environments. ACE-1 was conducted in the remote marine atmosphere south of Australia while ACE-2 was conducted in the anthropogenically modified atmosphere of the Eastern North Atlantic. In-situ shipboard measurements from the RV Discoverer(ACE-1) and the RV Professor Vodyanitskiy(ACE-2), combined with calculated back trajectories can be used to define the physical properties of the sub-micron aerosol in marine boundary layer (MBL) air masses from the remote Southern Ocean, Western Europe, the Iberian coast, the Mediterranean and the background Atlantic Ocean. The differences in these aerosol properties, combined with dimethylsulfide, sulfur dioxide and meteorological measurements provide a means to assess processes that affect the aerosol distribution. The background sub-micron aerosol measured over the Atlantic Ocean during ACE-2 was more abundant (number and volume) and appeared to be more aged than that measured over the Southern Ocean during ACE-1. Based on seawater DMS measurements and wind speed, the oceanic source of non-sea-salt sulfur and sea-salt to the background marine atmosphere during ACE-1 and ACE-2 was similar. However, the synoptic meteorological pattern was quite different during ACE-1 and ACE-2. The frequent frontal passages during ACE-1 resulted in the mixing of nucleation mode particles into the marine boundary layer from the free troposphere and relatively short aerosol residence times. In the more stable meteorological setting of ACE-2, a significant nucleation mode aerosol was observed in the MBL only for a half day period associated with a weak frontal system. As a result of the longer MBL aerosol residence times, the average background ACE-2 accumulation mode aerosol had a larger diameter and higher number concentration than during ACE-1. The sub-micron aerosol number size distributions in the air masses that passed over Western Europe, the Mediterranean, and coastal Portugal were distinctly different from each other and the background aerosol. The differences can be attributed to the age of the air mass and the degree of cloud processing.

Processes controlling the distribution of aerosol particles in the lower marine boundary layer during the First Aerosol Characterization Experiment (ACE 1)

The goals of the International Global Atmospheric Chemistry (IGAC) Program's First Aerosol Characterization Experiment (ACE 1) are to determine and understand the properties and controlling factors of the aerosol in the remote marine atmosphere that are relevant to radiative forcing and climate. A key question in terms of this goal and the overall biogeochemical sulfur cycle is what factors control the formation, growth, and evolution of particles in the marine boundary layer (MBL). To address this question, simultaneous measurements of dimethylsulfide (DMS), sulfur dioxide (SO2), the aerosol chemical mass size distribution, and the aerosol number size distribution from 5 to 10,000 nm diameter were made on the National Oceanic and Atmospheric Administration (NOAA) ship Discoverer. From these data we conclude that the background MBL aerosol during ACE 1 often was composed of four distinct modes: an ultrafine (UF) mode (Dp = 5–20 nm), an Aitken mode (Dp = 20–80 nm), an accumulation mode (Dp = 80–300 nm), and a coarse mode (Dp > 300 nm). The presence of UF mode particles in the MBL could be explained by convective mixing between the free troposphere (FT) and the MBL associated with cloud pumping and subsidence following cold frontal passages. There was no evidence of major new particle production in the MBL. Oceanic emissions of DMS appeared to contribute to the growth of Aitken and accumulation mode particles. Coarse mode particles were comprised primarily of sea salt. Although these particles result from turbulence at the air‐sea interface, the instantaneous wind speed accounted for only one third of the variance in the coarse mode number concentration in this region.

孤立した山岳の風下にできる筋状雲の生成メカニズムに関する数値実験

Cloud streets are frequently observed when a cold air mass advects over a warm sea surface in the winter season; thick and long cloud streets can be sometimes seen in the lee of some islands or mountains near a coast. According to the conventional theory, the vertical shear and static instability play an important role in the formation of the thin cloud streets. For the thick cloud streets in the lee of the isolated mountain, however, another mechanism seems to be important in addition to the conventional theory. The effects of the two factors, strong static instability and topographically induced mechanical disturbance, are investigated by use of the CSU RANTS (Regional Atmospheric Modeling System) with high horizontal and vertical resolution. In the calculation, uniform stratification and wind velocity (low Froude number) are assumed at the inflow boundary. To clarify the importance of both the strong static instability and the topographically induced mechanical disturbance on the formation and maintenance of cloud streets, three kinds of numerical experiments with different sea-surface temperature were carried out, including a numerical experiment without a mountain. Cloud streets were successfully simulated in the lee of an isolated mountain near a coast, with the addition of a large sensible heat flux at the sea surface. Well developed cloud streets occur in the simulation between a pair of convective rolls below a height of 1 km over the sea. The following five results were obtained: 1) For the formation of the pair of convective rolls, both strong static instability and a topographically induced mechanical disturbance are required at the same time. 2) Strong sensible heat flux from the sea surface is the main source of the convective rolls (the buoyancy caused by the condensation process in the cloud is negligibly small). 3) The pair of convective rolls contain two sub-rolls. One is the outer roll, which has a large radius but a weak circulation, and the other is the inner roll, which has a small radius but a strong circulation. The former gathers a large amount of moisture by convergence in the lower marine boundary layer, and the latter transfers the convergent moisture to the upper boundary layer by the strong vertical motion between the rolls. 4) The pair of inner rolls form the line-shaped cloud streets, and keep them on a narrow line along the center-line of the domain. 5) Cloud streets can be simulated both by non-hydrostatic and by hydrostatic models, which implies that vertical inertia is not always important for cloud-street formation. The horizontal scale of the topography does not seem to be restricted to within the small scale, where non-hydrostatic effects are important.

Atmospheric measurements of carbonyl sulfide, dimethyl sulfide,and carbon disulfide using the electron capture sulfur detector

Measurements of atmospheric dimethyl sulfide (DMS), carbonyl sulfide (COS), and carbon disulfide (CS2) were conducted over the Atlantic Ocean on board the NASA Electra aircraft during the Chemical Instrumentation Test and Evaluation (CITE 3) project using the electron capture sulfur detector (ECD‐S). The system employed cryogenic preconcentration of air samples, gas chromatographic separation, catalytic fluorination, and electron capture detection. Samples collected for DMS analysis were scrubbed of oxidants with NaOH impregnated glass fiber filters prior to preconcentration. The detection limits (DL) of the system for COS, DMS, and CS2 were 5, 5, and 2 ppt, respectively. COS concentrations ranged from 404 to 603 ppt with a mean of 489 ppt for measurements over the North Atlantic Ocean (31°N to 41°N), and from 395 to 437 ppt with a mean of 419 ppt for measurements over the Tropical Atlantic Ocean (11°S to 2°N). DMS concentrations in the lower marine boundary layer, below 600‐m altitude, ranged from below DL to 150 ppt from flights over the North Atlantic, and from 9 to 104 ppt over the Tropical Atlantic. CS2 concentrations ranged from below DL to 29 ppt over the North Atlantic. Almost all CS2 measurements over the Tropical Atlantic were below DL.

Carbonyl compounds in the lower marine troposphere over the Caribbean Sea and Bahamas

Formaldehyde, acetaldehyde, acetone, and butanone were measured in 59 samples of marine air. Average clean air background concentrations were about 0.55, 0.50, 0.38, and 0.03 ppbv, respectively, in agreement with past measurements. The formaldehyde concentration is also in agreement with that predicted from photooxidation of methane and other locally derived organic matter. Formaldehyde and acetaldehyde showed strong diurnal variations throughout the 12‐day sampling period. Photo‐chemical oxidation of locally derived organic matter, such as nonmethane hydrocarbons and long‐chained lipids, appears to be the major source for both formaldehyde and acetaldehyde in the lower marine boundary layer. Acetone, on the other hand, showed weaker diurnal fluctuations, consistent with its significantly longer lifetime in the troposphere. Sinks for carbonyl compounds in the lower marine boundary layer are less clearly known. Our results suggest that photolysis, reaction with free radicals, and deposition at the sea surface are minor, short‐term sinks during our study. The main sink appears to be vertical mixing, probably followed by photolysis in the upper marine boundary layer and free troposphere.

Radon daughter disequilibria in the lower marine boundary layer

Radon daughters are produced as free ions, but they become attached to aerosol particles at a rate depending on the particle concentration. In the lower marine boundary layer, most of those which do not become attached plate out on the ocean surface. In this paper a simple model is used to examine the influence of several parameters on radon/radon daughter disequilibria in maritime air. The model is compared with experimental data from Cape Grim. The radon daughter to radon ratio, ‘f’, decreases from 0.86 to 0.1 as the particle concentration falls from 1000 to 10 cm-3. Estimates of radon concentration at sea level, based on daughter measurements may therefore be in error by as much as a factor of 10 unless allowance is made for particle concentration. At 100 cm-3, the standard deviation of the distribution of measured ‘f’ values is about 30%, indicating that the particle concentration is not the only factor influencing the loss of radon daughters. The implication is that radon daughters can be used to measure radon concentrations at sea level with an accuracy of about 30%, provided the particle concentration is known. The measurements show that there is a very low proportion of unattached daughers in the air between about 30 and 165 m above sea level. According to the model, this implies that mixing of air up to about 200 m is usually rapid enough to result in plate-out of radon daughters on the ocean on a time scale of less than 100 s.

An analysis of Fresnel zone and diffraction fields near the boresight of practical acoustic radar antennas

Based upon a generalized theory of an analytic model of a practical acoustic radar (echosonde) antenna [S. A. Adekola, J. Acoust. Soc. Am. 60, 230–239 (1976)] whose potential has been exploited to measure atmospheric turbulent velocity fields and temperature fluctuations, we present an analysis of the Fresnel zone and diffraction fields near the boresight of a typical antenna. Invoking the Fresnel zone approximation and employing a tapered parabolic source excitation, the Fresnel zone field at the antenna boresight is evaluated in closed forms in terms of a highly convergent Lommel series function of the form Ω0m (ν,n) that is expedient for practical computations. A generalized diffraction integral equation at the boresight region, which can be used for both circularly symmetric and asymmetric echosonde apertures, is formulated whereby the existing integral, previously derived in the reference above, is split into a sum of zeroth‐order Hankel and Lommel transforms that describe the field in the two principal planes of the aperture. The results clearly demonstrate that circular symmetry still leads to zeroth‐order Hankel transform, while antisymmetry produces an additional Lommel transform. Analytical examples presented include closed form pattern evaluations for some commonly used source excitations, in terms of tabulated Anger–Weber and Struve functions, while illustrative numerical pattern characteristics discussed comprise beam patterns arising from Hamming—cos2 on a pedestal—and doublet‐source distributions. A realization of the echosonde antenna described by the integral equations is discussed. Raw, experimentally measured, beam patterns in the acoustic frequency band 1750–3000 Hz are redrafted, statistically averaged, and presented, using standard error bars. We also discuss a facsimile recording that describes the measurements of turbulence components in the lower marine boundary layer, using an echosonde antenna. An attempt is made towards a rigorous treatment of the echosonde synthesis, based on the synthesis integral equations deduced from the results of the diffraction fields.