The authors examine the chemistry and related properties of freshly emitted sea spray aerosol. Topics discussed include the seawater organic carbon pool.
Atmospheric black carbon (BC) warms Earth's climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC ...aerosol components that enhance BC absorption, often by a factor of ∼2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E abs ) and mixing state are reported for two California regions. The observed E abs is small—6% on average at 532 nm—and increases weakly with photochemical aging. The E abs is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E abs for BC are possible.
Four North Atlantic Aerosol and Marine Ecosystems Study (NAAMES) field campaigns from winter 2015 through spring 2018 sampled an extensive set of oceanographic and atmospheric parameters during the ...annual phytoplankton bloom cycle. This unique dataset provides four seasons of open-ocean observations of wind speed, sea surface temperature (SST), seawater particle attenuation at 660 nm (c
p,660, a measure of ocean particulate organic carbon), bacterial production rates, and sea-spray aerosol size distributions and number concentrations (N
SSA). The NAAMES measurements show moderate to strong correlations (0.56 < R < 0.70) between N
SSA and local wind speeds in the marine boundary layer on hourly timescales, but this relationship weakens in the campaign averages that represent each season, in part because of the reduction in range of wind speed by multiday averaging. N
SSA correlates weakly with seawater cp,660 (R = 0.36, P << 0.01), but the correlation with cp,660, is improved (R = 0.51, P < 0.05) for periods of low wind speeds. In addition, NAAMES measurements provide observational dependence of SSA mode diameter (d
m) on SST, with d
m increasing to larger sizes at higher SST (R = 0.60, P << 0.01) on hourly timescales. These results imply that climate models using bimodal SSA parameterizations to wind speed rather than a single SSA mode that varies with SST may overestimate SSA number concentrations (hence cloud condensation nuclei) by a factor of 4 to 7 and may underestimate SSA scattering (hence direct radiative effects) by a factor of 2 to 5, in addition to overpredicting variability in SSA scattering from wind speed by a factor of 5.
Oceans cover over two-thirds of the Earth's surface, and the particles emitted to the atmosphere by waves breaking on sea surfaces provide an important contribution to the planetary albedo. During ...the International Chemistry Experiment in the Arctic LOwer Troposphere (ICEALOT) cruise on the R/V Knorr in March and April of 2008, organic mass accounted for 15-47% of the submicron particle mass in the air masses sampled over the North Atlantic and Arctic Oceans. A majority of this organic component (0.1 - 0.4 μ m⁻³) consisted of organic hydroxyl (including polyol and other alcohol) groups characteristic of saccharides, similar to biogenic carbohydrates found in seawater. The large fraction of organic hydroxyl groups measured during ICEALOT in submicron atmospheric aerosol exceeded those measured in most previous campaigns but were similar to particles in marine air masses in the open ocean (Southeast Pacific Ocean) and coastal sites at northern Alaska (Barrow) and northeastern North America (Appledore Island and Chebogue Point). The ocean-derived organic hydroxyl mass concentration during ICEALOT correlated strongly to submicron Na concentration and wind speed. The observed submicron particle ratios of marine organic mass to Na were enriched by factors of ~10²-~10³ over reported sea surface organic to Na ratios, suggesting that the surface-controlled process of film bursting is influenced by the dissolved organic components present in the sea surface microlayer. Both marine organic components and Na increased with increasing number mean diameter of the accumulation mode, suggesting a possible link between organic components in the ocean surface and aerosol-cloud interactions.
Halogen atoms and oxides are highly reactive and can profoundly affect atmospheric composition. Chlorine atoms can decrease the lifetimes of gaseous elemental mercury and hydrocarbons such as the ...greenhouse gas methane. Chlorine atoms also influence cycles that catalytically destroy or produce tropospheric ozone, a greenhouse gas potentially toxic to plant and animal life. Conversion of inorganic chloride into gaseous chlorine atom precursors within the troposphere is generally considered a coastal or marine air phenomenon. Here we report mid-continental observations of the chlorine atom precursor nitryl chloride at a distance of 1,400 km from the nearest coastline. We observe persistent and significant nitryl chloride production relative to the consumption of its nitrogen oxide precursors. Comparison of these findings to model predictions based on aerosol and precipitation composition data from long-term monitoring networks suggests nitryl chloride production in the contiguous USA alone is at a level similar to previous global estimates for coastal and marine regions. We also suggest that a significant fraction of tropospheric chlorine atoms may arise directly from anthropogenic pollutants.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Biogenic sources contribute to cloud condensation nuclei (CCN) in the clean marine atmosphere, but few measurements exist to constrain climate model simulations of their importance. The chemical ...composition of individual atmospheric aerosol particles showed two types of sulfate-containing particles in clean marine air masses in addition to mass-based Estimated Salt particles. Both types of sulfate particles lack combustion tracers and correlate, for some conditions, to atmospheric or seawater dimethyl sulfide (DMS) concentrations, which means their source was largely biogenic. The first type is identified as New Sulfate because their large sulfate mass fraction (63% sulfate) and association with entrainment conditions means they could have formed by nucleation in the free troposphere. The second type is Added Sulfate particles (38% sulfate), because they are preexisting particles onto which additional sulfate condensed. New Sulfate particles accounted for 31% (7 cm
) and 33% (36 cm
) CCN at 0.1% supersaturation in late-autumn and late-spring, respectively, whereas sea spray provided 55% (13 cm
) in late-autumn but only 4% (4 cm
) in late-spring. Our results show a clear seasonal difference in the marine CCN budget, which illustrates how important phytoplankton-produced DMS emissions are for CCN in the North Atlantic.
Even though the Arctic is remote, aerosol properties observed there are
strongly influenced by anthropogenic emissions from outside the Arctic. This
is particularly true for the so-called Arctic haze ...season (January through
April). In summer (June through September), when atmospheric transport
patterns change, and precipitation is more frequent, local Arctic sources,
i.e., natural sources of aerosols and precursors, play an important role.
Over the last few decades, significant reductions in anthropogenic emissions
have taken place. At the same time a large body of literature shows evidence
that the Arctic is undergoing fundamental environmental changes due to
climate forcing, leading to enhanced emissions by natural processes that may
impact aerosol properties. In this study, we analyze 9 aerosol chemical species and 4 particle
optical properties from 10 Arctic observatories (Alert, Kevo, Pallas,
Summit, Thule, Tiksi, Barrow/Utqiaġvik, Villum, and Gruvebadet and Zeppelin
Observatory – both at Ny-Ålesund Research Station) to understand changes
in anthropogenic and natural aerosol contributions. Variables include
equivalent black carbon, particulate sulfate, nitrate, ammonium,
methanesulfonic acid, sodium, iron, calcium and potassium, as well as
scattering and absorption coefficients, single scattering albedo and
scattering Ångström exponent. First, annual cycles are investigated, which despite anthropogenic emission
reductions still show the Arctic haze phenomenon. Second, long-term trends
are studied using the Mann–Kendall Theil–Sen slope method. We find in total
41 significant trends over full station records, i.e., spanning more than a
decade, compared to 26 significant decadal trends. The majority of
significantly declining trends is from anthropogenic tracers and occurred
during the haze period, driven by emission changes between 1990 and 2000.
For the summer period, no uniform picture of trends has emerged. Twenty-six
percent of trends, i.e., 19 out of 73, are significant, and of those 5 are
positive and 14 are negative. Negative trends include not only anthropogenic
tracers such as equivalent black carbon at Kevo, but also natural indicators
such as methanesulfonic acid and non-sea-salt calcium at Alert. Positive
trends are observed for sulfate at Gruvebadet. No clear evidence of a significant change in the natural aerosol
contribution can be observed yet. However, testing the sensitivity of the
Mann–Kendall Theil–Sen method, we find that monotonic changes of around 5 % yr−1 in an aerosol property are needed to detect a significant
trend within one decade. This highlights that long-term efforts well beyond
a decade are needed to capture smaller changes. It is particularly important
to understand the ongoing natural changes in the Arctic, where interannual
variability can be high, such as with forest fire emissions and their
influence on the aerosol population. To investigate the climate-change-induced influence on the aerosol
population and the resulting climate feedback, long-term observations of
tracers more specific to natural sources are needed, as well as of particle
microphysical properties such as size distributions, which can be used to
identify changes in particle populations which are not well captured by
mass-oriented methods such as bulk chemical composition.
Between April 2002 and June 2017, the Global Monitoring Laboratory (GML) of the National Oceanic and Atmospheric Administration (NOAA) made continuous measurements of a suite of in situ aerosol ...optical properties at a long-term monitoring site near Trinidad Head (THD), California. In addition to aerosol optical properties, between 2002–2006 a scanning humidograph system was operated, and inorganic ion and total aerosol mass concentrations were obtained from filter measurements. A combined analysis of these datasets demonstrates consistent patterns in aerosol climatology and highlights changes in sources throughout the year. THD is predictably dominated by sea salt aerosols; however, marine biogenic aerosols are the largest contributor to PM1 in the warmer months. Additionally, a persistent combustion source appears in the winter, likely a result of wintertime home heating. While the influences of local anthropogenic sources from vehicular and marine traffic are visible in the optical aerosol data, their influence is largely dictated by the wind direction at the site. Comparison of the THD aerosol climatology to that reported for other marine sites shows that the location is representative of clean marine measurements, even with the periodic influence of anthropogenic sources.
We use GEOS-Chem chemical transport model simulations of sulfate–ammonium aerosol data from the NASA ARCTAS and NOAA ARCPAC aircraft campaigns in the North American Arctic in April 2008, together ...with longer-term data from surface sites, to better understand aerosol sources in the Arctic in winter–spring and the implications for aerosol acidity. Arctic pollution is dominated by transport from mid-latitudes, and we test the relevant ammonia and sulfur dioxide emission inventories in the model by comparison with wet deposition flux data over the source continents. We find that a complicated mix of natural and anthropogenic sources with different vertical signatures is responsible for sulfate concentrations in the Arctic. East Asian pollution influence is weak in winter but becomes important in spring through transport in the free troposphere. European influence is important at all altitudes but never dominant. West Asia (non-Arctic Russia and Kazakhstan) is the largest contributor to Arctic sulfate in surface air in winter, reflecting a southward extension of the Arctic front over that region. Ammonium in Arctic spring mostly originates from anthropogenic sources in East Asia and Europe, with added contribution from boreal fires, resulting in a more neutralized aerosol in the free troposphere than at the surface. The ARCTAS and ARCPAC data indicate a median aerosol neutralization fraction NH
4
+/(2SO
4
2− + NO
3
−) of 0.5 mol mol
−1 below 2 km and 0.7 mol mol
−1 above. We find that East Asian and European aerosol transported to the Arctic is mostly neutralized, whereas West Asian and North American aerosol is highly acidic. Growth of sulfur emissions in West Asia may be responsible for the observed increase in aerosol acidity at Barrow over the past decade. As global sulfur emissions decline over the next decades, increasing aerosol neutralization in the Arctic is expected, potentially accelerating Arctic warming through indirect radiative forcing and feedbacks.
► We interpret Arctic sulfate–ammonium aerosol data using a chemical transport model. ► A mix of natural and anthropogenic sources contribute to Arctic sulfate burdens. ► West Asia is the largest source of sulfate to Arctic surface air in winter. ► Arctic ammonium is dominated by agricultural and open burning emissions. ► Spring aerosol is acidic throughout the troposphere and most acidic at the surface.
A shallow cumulus cloud transition from a sugar to flower type of organization occurred under a layer of mineral dust on 2 February 2020, during the multinational Atlantic Tradewind Ocean‐Atmosphere ...Mesoscale Interaction Campaign (ATOMIC) and the Elucidating the Role of Clouds‐Circulation Coupling in Climate (EUREC4A) campaigns. Lagrangian large eddy simulations following an airmass trajectory along the tradewinds are used to explore radiative impacts of the diel cycle and mineral dust on the sugar‐to‐flower (S2F) cloud transition. The large‐scale meteorological forcing is derived from the European Center for Medium‐Range Weather Forecasts Reanalysis Fifth Generation and based on aerosol measurements from the U.S. Ronald H. Brown Research Vessel and the French ATR‐42 Research Aircraft during the field campaigns. A 12‐hr delay in the diel cycle accelerates the S2F transition at night, leading to more cloud liquid water and precipitation. The aggregated clouds generate more and stronger cold pools, which alter the original mechanism responsible for the organization. Although there is still mesoscale moisture convergence in the cloud layer, the near‐surface divergence associated with cold pools transports the subcloud moisture to the drier surrounding regions. New convection forms along the cold‐pool edges, generating new flower clouds. The modulation of the surface radiative budget by free‐tropospheric mineral dust poses a less dramatic effect on the S2F transition. Mineral dust releases longwave radiation, reducing the cloud amount at night, and absorbs shortwave radiation during the day, cooling the boundary‐layer temperature and increasing the overall cloud amount. Cloud‐top radiative heating because of more clouds strengthens the mesoscale organization, enlarging the aggregate areas, and increasing the cloud amount further.
Plain Language Summary
During a joint field study called ATOMIC and EUREC4A, a transition between two cloud systems took place during the day on 2 February 2020. Very small and shallow clouds called “sugar” transitioned into deeper and wider cloud aggregates called “flowers.” A dense mineral‐dust layer was also observed above the tradewind cumulus cloud field, likely modulating the radiation interacting with the clouds. High‐resolution simulations are applied to help understand the same cloud transition as if it had taken place at night, and to explore the impacts of mineral dust on the transition. A 12‐hr delay in the daily cycle such that the transition occurs at night affects the cloud transition more significantly than when the transition occurs during the day under a layer of mineral dust. The cloud transition that occurs at night produces more clouds and rain. The mineral dust blocks the solar radiation and cools the air beneath during the day, but does not change the cloud and rain amount as much.
Key Points
The transition from sugar to flowers occurs more rapidly at night, producing more cloud and rain, with stronger organization and cold pools
Precipitation and cold pools weaken the mechanism of cloud aggregation as they transport moisture to drier regions to form new convection
Mineral dust above the clouds modulates radiative fluxes below leading to weaker mesoscale organization at night but stronger during the day
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