On the Influence of Air Mass Origin on Low-Cloud Properties in the Southeast Atlantic

Abstract. Atmospheric particulate matter (PM) remains poorly understood due to the lack of comprehensive measurements at high time resolution for tracking its dynamic features and the lack of long-term observation for tracking its seasonal variability. Here, we present highly time-resolved and seasonal compositions and characteristics of non-refractory components in PM with a diameter less than 1 μm (NR-PM1) at a suburban site in Hong Kong. The measurements were made with an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) at the Hong Kong University of Science and Technology (HKUST) Air Quality Research Supersite for 4 months, with one in each season of the year. The average NR-PM1 concentration of ~ 15 μg m−3 is higher than those AMS measurements made in South Korea and Japan, but lower than those in North China, the Yangtze River Delta and the nearby Pearl River Delta. The seasonal dependence of the total NR-PM1 monthly averaged concentrations was small, but that of the fractions of the species in NR-PM1 was significant. Site characteristic plays an important role in the relative fractions of species in NR-PM1 and our results are generally consistent with measurements at other non-urban sites in this regard. Detailed analyses were conducted on the AMS data in the aspects of (1) species concentrations, (2) size distributions, (3) degree of oxygenation of organics, and (4) positive matrix factorization (PMF)-resolved organic factors in a seasonal context, as well as with air mass origin from back-trajectory analysis. Sulfate had the highest fraction in NR-PM1 (> 40%), and the surrogates of secondary organic species – semi-volatile oxygenated organic aerosol (SVOOA) and low-volatility oxygenated organic aerosol (LVOOA) – prevailed (~ 80%) in the organic portion of NR-PM1. Local contributions to the organic portion of NR-PM1 at this suburban site was strongly dependent on season. The hydrocarbon-like organic aerosol (HOA) factor related to local traffic emissions contributed > 10% to organic aerosols in spring and summer but only 6–7% in autumn and winter. The cooking organic aerosol (COA) factor contributed > 10% to organic aerosols in winter. With the aid of highly time-resolved data, diurnal patterns of the degree of oxygenation of organic aerosols were used to determine the sources and formation processes of the least understood organic portion of PM. The oxygen-to-carbon atomic ratio (O : C) and average carbon oxidation state OS C) showed little variation in autumn and winter, when the long-range transport of oxidized organics dominated, whereas they peaked in the afternoon in spring and summer, when locally produced secondary organic aerosol prevailed. Air mass origin, in contrast, had a strong influence on both NR-PM1 concentrations and the fractions of species in NR-PM1. The findings of the current study provide a better understanding of the role of air mass origin in the seasonal characteristics of the PM composition and the relative importance of local vs. transported organic aerosols in this region.

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Influences of Recent Particle Formation on Southern Ocean Aerosol Variability and Low Cloud Properties

Journal of Geophysical Research Atmospheres ◽

10.1029/2020jd033529 ◽

2021 ◽

Author(s):

Isabel L. McCoy ◽

Christopher S. Bretherton ◽

Robert Wood ◽

Cynthia H. Twohy ◽

Andrew Gettelman ◽

...

Keyword(s):

Southern Ocean ◽

Particle Formation ◽

Cloud Properties ◽

Low Cloud

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Processes contributing to cloud dissipation and formation events on the North Slope of Alaska

Atmospheric Chemistry and Physics ◽

10.5194/acp-21-4149-2021 ◽

2021 ◽

Vol 21 (5) ◽

pp. 4149-4167

Author(s):

Joseph Sedlar ◽

Adele Igel ◽

Hagen Telg

Keyword(s):

Cloud Formation ◽

North Slope ◽

Near Surface ◽

Air Mass ◽

Low Level ◽

Clear Sky ◽

The North ◽

Synoptic Conditions ◽

North Slope Of Alaska ◽

Low Cloud

Abstract. Clear-sky periods across the high latitudes have profound impacts on the surface energy budget and lower atmospheric stratification; however an understanding of the atmospheric processes leading to low-level cloud dissipation and formation events is limited. A method to identify clear periods at Utqiaġvik (formerly Barrow), Alaska, during a 5-year period (2014–2018) is developed. A suite of remote sensing and in situ measurements from the high-latitude observatory are analyzed; we focus on comparing and contrasting atmospheric properties during low-level (below 2 km) cloud dissipation and formation events to understand the processes controlling clear-sky periods. Vertical profiles of lidar backscatter suggest that aerosol presence across the lower atmosphere is relatively invariant during the periods bookending clear conditions, which suggests that a sparsity of aerosol is not frequently a cause for cloud dissipation on the North Slope of Alaska. Further, meteorological analysis indicates two active processes ongoing that appear to support the formation of low clouds after a clear-sky period: namely, horizontal advection, which was dominant in winter and early spring, and quiescent air mass modification, which was dominant in the summer. During summer, the dominant mode of cloud formation is a low cloud or fog layer developing near the surface. This low cloud formation is driven largely by air mass modification under relatively quiescent synoptic conditions. Near-surface aerosol particles concentrations changed by a factor of 2 around summer formation events. Thermodynamic adjustment and increased aerosol presence under quiescent atmospheric conditions are hypothesized as important mechanisms for fog formation.

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International Satellite Cloud Climatology Project: Extending the Record

Journal of Climate ◽

10.1175/jcli-d-21-0157.1 ◽

2021 ◽

pp. 1-62

Author(s):

William B. Rossow ◽

Kenneth R. Knapp ◽

Alisa H Young

Keyword(s):

Product Design ◽

Cloud Amount ◽

Cloud Microphysics ◽

Temperature Threshold ◽

Ancillary Data ◽

Total Cloud Amount ◽

High Cloud ◽

Enso Events ◽

Cloud Properties ◽

Low Cloud

AbstractISCCP continues to quantify the global distribution and diurnal-to-interannual variations of cloud properties in a revised version. This paper summarizes assessments of the previous version, describes refinements of the analysis and enhanced features of the product design, discusses the few notable changes in the results, and illustrates the long-term variations of global mean cloud properties and differing high cloud changes associated with ENSO. The new product design includes a global, pixel-level product on a 0.1°?grid, all other gridded products at 1.0°-equivalent equal-area, separate-satellite products with ancillary data for regional studies, more detailed, embedded quality information, and all gridded products in netCDF format. All the data products including all input data), expanded documentation, the processing code and an Operations Guide are available online. Notable changes are: (1) a lowered ice-liquid temperature threshold, (2) a treatment of the radiative effects of aerosols and surface temperature inversions, (3) refined specification of the assumed cloud microphysics, and (4) interpolation of the main daytime cloud information overnight. The changes very slightly increase the global monthly mean cloud amount with a little more high and a little less middle and low cloud. Over the whole period, total cloud amount slowly decreases caused by decreases in cumulus/altocumulus; consequently, average cloud top temperature and optical thickness have increased. The diurnal and seasonal cloud variations are very similar to earlier versions. Analysis of the whole record shows that high cloud variations, but not low clouds, exhibit different patterns in different ENSO events.

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Aerosol properties and their influences on low warm clouds during the Two-Column Aerosol Project

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-9515-2019 ◽

2019 ◽

Vol 19 (14) ◽

pp. 9515-9529 ◽

Cited By ~ 4

Author(s):

Jianjun Liu ◽

Zhanqing Li

Keyword(s):

Optical Depth ◽

Aerosol Particles ◽

Large Contribution ◽

Loading Conditions ◽

Liquid Water Path ◽

Air Mass ◽

Aerosol Loading ◽

Cloud Properties ◽

Stable Conditions ◽

Cloud Development

Abstract. Twelve months of measurements collected during the Two-Column Aerosol Project field campaign at Cape Cod, Massachusetts, which started in the summer of 2012, were used to investigate aerosol physical, optical, and chemical properties and their influences on the dependence of cloud development on thermodynamic (i.e., lower tropospheric stability, LTS) conditions. Relationships between aerosol loading and cloud properties under different dominant air-mass conditions and the magnitude of the first indirect effect (FIE), as well as the sensitivity of the FIE to different aerosol compositions, are examined. The seasonal variation in aerosol number concentration (Na) was not consistent with variations in aerosol optical properties (i.e., scattering coefficient, σs, and columnar aerosol optical depth). Organics were found to have a large contribution to small particle sizes. This contribution decreased during the particle growth period. Under low-aerosol-loading conditions, the liquid water path (LWP) and droplet effective radius (DER) significantly increased with increasing LTS, but, under high-aerosol-loading conditions, LWP and DER changed little, indicating that aerosols significantly weakened the dependence of cloud development on LTS. The reduction in LWP and DER from low- to high-aerosol-loading conditions was greater in stable environments, suggesting that clouds under stable conditions are more susceptible to aerosol perturbations than those under more unstable conditions. High aerosol loading weakened the increase in DER as LWP increased and strengthened the increase in cloud optical depth (COD) with increasing LWP, resulting in changes in the interdependence of cloud properties. Under both continental and marine air-mass conditions, high aerosol loading can significantly increase COD and decrease LWP and DER, narrowing their distributions. Magnitudes of the FIE estimated under continental air-mass conditions ranged from 0.07±0.03 to 0.26±0.09 with a mean value of 0.16±0.03 and showed an increasing trend as LWP increased. The calculated FIE values for aerosols with a low fraction of organics are greater than those for aerosols with a high fraction of organics. This implies that clouds over regions dominated by aerosol particles containing mostly inorganics are more susceptible to aerosol perturbations, resulting in larger climate forcing, than clouds over regions dominated by organic aerosol particles.

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Long-term real-time chemical characterization of submicron aerosols at Montsec (southern Pyrenees, 1570 m a.s.l.)

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-2935-2015 ◽

2015 ◽

Vol 15 (6) ◽

pp. 2935-2951 ◽

Cited By ~ 36

Author(s):

A. Ripoll ◽

M. C. Minguillón ◽

J. Pey ◽

J. L. Jimenez ◽

D. A. Day ◽

...

Keyword(s):

Real Time ◽

Long Range ◽

Chemical Characterization ◽

Western Mediterranean ◽

Continuous Increase ◽

Air Mass ◽

Air Masses ◽

Diurnal Cycles ◽

Western Mediterranean Basin ◽

Air Mass Origin

Abstract. Real-time measurements of inorganic (sulfate, nitrate, ammonium, chloride and black carbon (BC)) and organic submicron aerosols (particles with an aerodynamic diameter of less than 1 μm) from a continental background site (Montsec, MSC, 1570 m a.s.l.) in the western Mediterranean Basin (WMB) were conducted for 10 months (July 2011–April 2012). An aerosol chemical speciation monitor (ACSM) was co-located with other online and offline PM1 measurements. Analyses of the hourly, diurnal, and seasonal variations are presented here, for the first time, for this region. Seasonal trends in PM1 components are attributed to variations in evolution of the planetary boundary layer (PBL) height, air mass origin, and meteorological conditions. In summer, the higher temperature and solar radiation increases convection, enhancing the growth of the PBL and the transport of anthropogenic pollutants towards high altitude sites. Furthermore, the regional recirculation of air masses over the WMB creates a continuous increase in the background concentrations of PM1 components and causes the formation of reservoir layers at relatively high altitudes. The combination of all these atmospheric processes results in a high variability of PM1 components, with poorly defined daily patterns, except for the organic aerosols (OA). OA was mostly composed (up to 90%) of oxygenated organic aerosol (OOA), split in two types: semivolatile (SV-OOA) and low-volatility (LV-OOA), the rest being hydrocarbon-like OA (HOA). The marked diurnal cycles of OA components regardless of the air mass origin indicates that they are not only associated with anthropogenic and long-range-transported secondary OA (SOA) but also with recently produced biogenic SOA. Very different conditions drive the aerosol phenomenology in winter at MSC. The thermal inversions and the lower vertical development of the PBL leave MSC in the free troposphere most of the day, being affected by PBL air masses only after midday, when the mountain breezes transport emissions from the adjacent valleys and plains to the top of the mountain. This results in clear diurnal patterns of both organic and inorganic concentrations. OA was also mainly composed (71%) of OOA, with contributions from HOA (5%) and biomass burning OA (BBOA; 24%). Moreover, in winter sporadic long-range transport from mainland Europe is observed. The results obtained in the present study highlight the importance of SOA formation processes at a remote site such as MSC, especially in summer. Additional research is needed to characterize the sources and processes of SOA formation at remote sites.

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Air-mass origin as a diagnostic of tropospheric transport

Journal of Geophysical Research Atmospheres ◽

10.1002/jgrd.50133 ◽

2013 ◽

Vol 118 (3) ◽

pp. 1459-1470 ◽

Cited By ~ 17

Author(s):

Clara Orbe ◽

Mark Holzer ◽

Lorenzo M. Polvani ◽

Darryn Waugh

Keyword(s):

Air Mass ◽

Air Mass Origin

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