Wind speed dependent size-resolved parameterization for the organic mass fraction of sea spray aerosol

For oceans to be a significant source of primary organic aerosol (POA), sea spray aerosol (SSA) must be highly enriched with organics relative to the bulk seawater. We propose that organic enrichment at the air-sea interface, chemical composition of seawater, and the aerosol size are three main parameters controlling the organic mass fraction of sea spray aerosol (OM<sub>SSA</sub>). To test this hypothesis, we developed a new marine POA emission function based on a conceptual relationship between the organic enrichment at the air-sea interface and surface wind speed. The resulting parameterization is explored using aerosol chemical composition and surface wind speed from Atlantic and Pacific coastal stations, and satellite-derived ocean concentrations of chlorophyll-<i>a</i>, dissolved organic carbon, and particulate organic carbon. Of all the parameters examined, a multi-variable logistic regression revealed that the combination of 10 m wind speed and surface chlorophyll-<i>a</i> concentration ([Chl-<i>a</i>]) are the most consistent predictors of OM<sub>SSA</sub>. This relationship, combined with the published aerosol size dependence of OM<sub>SSA</sub>, resulted in a new parameterization for the organic mass fraction of SSA. Global emissions of marine POA are investigated here by applying this newly-developed relationship to existing sea spray emission functions, satellite-derived [Chl-<i>a</i>], and modeled 10 m winds. Analysis of model simulations shows that global annual submicron marine organic emission associated with sea spray is estimated to be from 2.8 to 5.6 Tg C yr<sup>−1</sup>. This study provides additional evidence that marine primary organic aerosols are a globally significant source of organics in the atmosphere

Model evaluation of marine primary organic aerosol emission schemes

In this study, several marine primary organic aerosol (POA) emission schemes have been evaluated using the GEOS-Chem chemical transport model in order to provide guidance for their implementation in air quality and climate models. These emission schemes, based on varying dependencies of chlorophyll <i>a</i> concentration ([chl <i>a</i>]) and 10 m wind speed (<i>U</i><sub>10</sub>), have large differences in their magnitude, spatial distribution, and seasonality. Model comparison with weekly and monthly mean values of the organic aerosol mass concentration at two coastal sites shows that the source function exclusively related to [chl <i>a</i>] does a better job replicating surface observations. Sensitivity simulations in which the negative <i>U</i><sub>10</sub> and positive [chl <i>a</i>] dependence of the organic mass fraction of sea spray aerosol are enhanced show improved prediction of the seasonality of the marine POA concentrations. A top-down estimate of submicron marine POA emissions based on the parameterization that compares best to the observed weekly and monthly mean values of marine organic aerosol surface concentrations has a global average emission rate of 6.3 Tg yr<sup>−1</sup>. Evaluation of existing marine POA source functions against a case study during which marine POA contributed the major fraction of submicron aerosol mass shows that none of the existing parameterizations are able to reproduce the hourly-averaged observations. Our calculations suggest that in order to capture episodic events and short-term variability in submicron marine POA concentration over the ocean, new source functions need to be developed that are grounded in the physical processes unique to the organic fraction of sea spray aerosol

Global relevance of marine organic aerosol as ice nucleating particles

Ice nucleating particles (INPs) increase the temperature at which supercooled droplets start to freeze. They are therefore of particular interest in mixed-phase cloud temperature regimes, where supercooled liquid droplets can persist for extended periods of time in the absence of INPs. When INPs are introduced to such an environment, the cloud can quickly glaciate following ice multiplication processes and the Wegener–Bergeron–Findeisen (WBF) process. The WBF process can also cause the ice to grow to precipitation size and precipitate out. All of these processes alter the radiative properties. Despite their potential influence on climate, the ice nucleation ability and importance of different aerosol species is still not well understood and is a field of active research. In this study, we use the aerosol–climate model ECHAM6-HAM2 to examine the global relevance of marine organic aerosol (MOA), which has drawn much interest in recent years as a potentially important INPs in remote marine regions. We address the uncertainties in emissions and ice nucleation activity of MOA with a range of reasonable set-ups and find a wide range of resulting MOA burdens. The relative importance of MOA as an INP compared to dust is investigated and found to depend strongly on the type of ice nucleation parameterisation scheme chosen. On the zonal mean, freezing due to MOA leads to relative increases in the cloud ice occurrence and in-cloud number concentration close to the surface in the polar regions during summer. Slight but consistent decreases in the in-cloud ice crystal effective radius can also be observed over the same regions during all seasons. Regardless, MOA was not found to affect the radiative balance significantly on the global scale, due to its relatively weak ice activity and a low sensitivity of cloud ice properties to heterogeneous ice nucleation in our model

On the representativeness of coastal aerosol studies to open ocean studies: Mace Head – a case study

A unique opportunity arose during the MAP project to compare open ocean aerosol measurements with those undertaken at the Mace Head Global Atmosphere Watch Station, a station used for decades for aerosol process research and long-term monitoring. The objective of the present study is to demonstrate that the key aerosol features and processes observed at Mace Head are characteristic of the open ocean, while acknowledging and allowing for spatial and temporal gradients. Measurements were conducted for a 5-week period at Mace Head and offshore, on the Research Vessel Celtic Explorer, in generally similar marine air masses, albeit not in connected-flow scenarios. The results of the study indicate, in terms of aerosol number size distribution, higher nucleation mode particle concentrations at Mace Head than offshore, pointing to a strong coastal source of new particles that is not representative of the open ocean. The Aitken mode exhibited a large degree of similarity, with no systematic differences between Mace Head and the open ocean, while the accumulation mode showed averagely 35% higher concentrations at Mace Head. The higher accumulation mode concentration can be attributed equally to cloud processing and to a coastal enhancement in concentration. Chemical analysis showed similar or even higher offshore concentrations for dominant species, such as nss-SO<sub>4</sub><sup>-2</sup>, WSOC, WIOC and MSA. Sea salt concentration differences determined a 40% higher supermicron mass at Mace Head, although this difference can be attributed to a higher wind speed at Mace Head during the comparison period. Moreover, the relative chemical composition as a function of size illustrated remarkable similarity. While differences to varying degrees were observed between offshore and coastal measurements, no convincing evidence was found of local coastal effects, apart from nucleation mode aerosol, thus confirming the integrity of previously reported marine aerosol characterisation studies at Mace Head

Is chlorophyll-athe best surrogate for organic matter enrichment in submicron primary marine aerosol?

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Top-down and bottom-up aerosol–cloud closure: towards understanding sources of uncertainty in deriving cloud shortwave radiative flux

Top-down and bottom-up aerosol–cloud shortwave radiative flux closures were conducted at the Mace Head Atmospheric Research Station in Galway, Ireland, in August 2015. This study is part of the BACCHUS (Impact of Biogenic versus Anthropogenic emissions on Clouds and Climate: towards a Holistic UnderStanding) European collaborative project, with the goal of understanding key processes affecting aerosol–cloud shortwave radiative flux closures to improve future climate predictions and develop sustainable policies for Europe. Instrument platforms include ground-based unmanned aerial vehicles (UAVs)1 and satellite measurements of aerosols, clouds and meteorological variables. The ground-based and airborne measurements of aerosol size distributions and cloud condensation nuclei (CCN) concentration were used to initiate a 1-D microphysical aerosol–cloud parcel model (ACPM). UAVs were equipped for a specific science mission, with an optical particle counter for aerosol distribution profiles, a cloud sensor to measure cloud extinction or a five-hole probe for 3-D wind vectors. UAV cloud measurements are rare and have only become possible in recent years through the miniaturization of instrumentation. These are the first UAV measurements at Mace Head. ACPM simulations are compared to in situ cloud extinction measurements from UAVs to quantify closure in terms of cloud shortwave radiative flux. Two out of seven cases exhibit sub-adiabatic vertical temperature profiles within the cloud, which suggests that entrainment processes affect cloud microphysical properties and lead to an overestimate of simulated cloud shortwave radiative flux. Including an entrainment parameterization and explicitly calculating the entrainment fraction in the ACPM simulations both improved cloud-top radiative closure. Entrainment reduced the difference between simulated and observation-derived cloud-top shortwave radiative flux (δRF) by between 25 and 60 W m−2. After accounting for entrainment, satellite-derived cloud droplet number concentrations (CDNCs) were within 30 % of simulated CDNC. In cases with a well-mixed boundary layer, δRF is no greater than 20 W m−2 after accounting for cloud-top entrainment and up to 50 W m−2 when entrainment is not taken into account. In cases with a decoupled boundary layer, cloud microphysical properties are inconsistent with ground-based aerosol measurements, as expected, and δRF is as high as 88 W m−2, even high (> 30 W m−2) after accounting for cloud-top entrainment. This work demonstrates the need to take in situ measurements of aerosol properties for cases where the boundary layer is decoupled as well as consider cloud-top entrainment to accurately model stratocumulus cloud radiative flux

Light-absorbing carbon in Europe – Measurement and modelling, with a focus on residential wood combustion emissions

The atmospheric concentration of elemental carbon (EC) in Europe during the six-year period 2005–2010 has been simulated with the EMEP MSC-W model. The model bias compared to EC measurements was less than 20% for most of the examined sites. The model results suggest that fossil fuel combustion is the dominant source of EC in most of Europe but that there are important contributions also from residential wood burning during the cold seasons and, during certain episodes, also from open biomass burning (wildfires and agricultural fires). The modelled contributions from open biomass fires to ground level concentrations of EC were small at the sites included in the present study, <3% of the long-term average of EC in PM10. The modelling of this EC source is subject to many uncertainties, and it was likely underestimated for some episodes. EC measurements and modelled EC were also compared to optical measurements of black carbon (BC). The relationships between EC and BC (as given by mass absorption cross section, MAC, values) differed widely between the sites, and the correlation between observed EC and BC is sometimes poor, making it difficult to compare results using the two techniques and limiting the comparability of BC measurements to model EC results. A new bottom-up emission inventory for carbonaceous aerosol from residential wood combustion has been applied. For some countries the new inventory has substantially different EC emissions compared to earlier estimates. For northern Europe the most significant changes are much lower emissions in Norway and higher emissions in neighbouring Sweden and Finland. For Norway and Sweden, comparisons to source-apportionment data from winter campaigns indicate that the new inventory may improve model-calculated EC from wood burning. Finally, three different model setups were tested with variable atmospheric lifetimes of EC in order to evaluate the model sensitivity to the assumptions regarding hygroscopicity and atmospheric ageing of EC. The standard ageing scheme leads to a rapid transformation of the emitted hydrophobic EC to hygroscopic particles, and generates similar results when assuming that all EC is aged at the point of emission. Assuming hydrophobic emissions and no ageing leads to higher EC concentrations. For the more remote sites, the observed EC concentration was in between the modelled EC using standard ageing and the scenario treating EC as hydrophobic. This could indicate too-rapid EC ageing in the model in relatively clean parts of the atmosphere

Aerosol properties associated with air masses arriving into the North East Atlantic during the 2008 Mace Head EUCAARI intensive observing period: an overview

As part of the EUCAARI Intensive Observing Period, a 4-week campaign to measure aerosol physical, chemical and optical properties, atmospheric structure, and cloud microphysics was conducted from mid-May to mid-June, 2008 at the Mace Head Atmospheric Research Station, located at the interface of Western Europe and the N. E. Atlantic and centered on the west Irish coastline. During the campaign, continental air masses comprising both young and aged continental plumes were encountered, along with polar, Arctic and tropical air masses. Polluted-continental aerosol concentrations were of the order of 3000 cm(-3), while background marine air aerosol concentrations were between 400-600 cm(-3). The highest marine air concentrations occurred in polar air masses in which a 15 nm nucleation mode, with concentration of 1100 cm(-3), was observed and attributed to open ocean particle formation. Continental air submicron chemical composition (excluding refractory sea salt) was dominated by organic matter, closely followed by sulphate mass. Although the concentrations and size distribution spectral shape were almost identical for the young and aged continental cases, hygroscopic growth factors (GF) and cloud condensation nuclei (CCN) to total condensation nuclei (CN) concentration ratios were significantly less in the younger pollution plume, indicating a more oxidized organic component to the aged continental plume. The difference in chemical composition and hygroscopic growth factor appear to result in a 40-50% impact on aerosol scattering coefficients and Aerosol Optical Depth, despite almost identical aerosol microphysical properties in both cases, with the higher values been recorded for the more aged case. For the CCN/CN ratio, the highest ratios were seen in the more age plume. In marine air, sulphate mass dominated the sub-micron component, followed by water soluble organic carbon, which, in turn, was dominated by methanesulphonic acid (MSA). Sulphate concentrations were highest in marine tropical air - even higher than in continental air. MSA was present at twice the concentrations of previously-reported concentrations at the same location and the same season. Both continental and marine air exhibited aerosol GFs significantly less than ammonium sulphate aerosol pointing to a significant organic contribution to all air mass aerosol properties

The seaweeds Fucus vesiculosus and Ascophyllum nodosum are significant contributors to coastal iodine emissions

Based on the results of a pilot study in 2007, which found high mixing ratios of molecular iodine (I2) above the intertidal macroalgae (seaweed) beds at Mweenish Bay (Ireland), we extended the study to nine different locations in the vicinity of Mace Head Atmospheric Research Station on the west coast of Ireland during a field campaign in 2009. The mean values of \chem{I_2} mixing ratio found above the macroalgae beds at nine different locations ranged from 104 to 393 ppt, implying a high source strength of I2. Such mixing ratios are sufficient to result in photochemically driven coastal new-particle formation events. Mixing ratios above the Ascophyllum nodosum and Fucus vesiculosus beds increased with exposure time: after 6 h exposure to ambient air the mixing ratios were one order of magnitude higher than those initially present. This contrasts with the emission characteristics of Laminaria digitata, where most I2 was emitted within the first half hour of exposure. Discrete in situ measurements (offline) of I2 emission from ambient air-exposed chamber experiments of L. digitata, A. nodosum and F. vesiculosus substantially supported the field observations. Further online and time-resolved measurements of the I2 emission from O3-exposed macroalgal experiments in a chamber confirmed the distinct I2 emission characteristics of A. nodosum and F. vesiculosus compared to those of L. digitata. The emission rates of A. nodosum and F. vesiculosus were comparable to or even higher than L. digitata after the initial exposure period of ~20–30 min. We suggest that A. nodosum and F. vesiculosus may provide an important source of photolabile iodine in the coastal boundary layer and that their impact on photochemistry and coastal new-particle formation should be reevaluated in light of their longer exposure at low tide and their widespread distribution

Aerosol properties associated with air masses arriving into the North East Atlantic during the 2008 Mace Head EUCAARI intensive observing period: an overview

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