Ciaran Monahan

On the occurrence of open ocean particle production and growth events

[1] We present new results from the Mace Head coastal station illustrating that open ocean new particle production and growth events occur frequently during periods of high oceanic productivity over the N.E. Atlantic. For the first time, we report events during which a recently-formed nucleation mode (—15 nm diameter) is detected and is observed to grow into an Aitken mode (~50 nm diameter) over periods up to 48 hours. A growth rate of 0.8 nm hour -1 is estimated in a typical case study, pointing to a source region ~700 km off-shore. The duration of the growth also suggests the particle production events are occurring over large spatial scales. Analysis of seven years of data show occurrence of extended growth events (lasting at least 24 hours) from March to September, with a peak occurrence in May. The events suggest that secondary marine boundary layer aerosol formation contributes to the marine aerosol population.

Coastal iodine emissions. 1. Release of I2 by Laminaria digitata in chamber experiments

Tidally exposed macroalgae emit large amounts of I(2) and iodocarbons that produce hotspots of iodine chemistry and intense particle nucleation events in the coastal marine boundary layer. Current emission rates are poorly characterized, however, with reported emission rates varying by 3 orders of magnitude. In this study, I(2) emissions from 25 Laminaria digitata samples were investigated in a simulation chamber using incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). The chamber design allowed gradual extraction of seawater to simulate tidal emersion of algae. Samples were exposed to air with or without O(3) and to varying irradiances. Emission of I(2) occurred in four distinct stages: (1) moderate emissions from partially submerged samples; (2) a strong release by fully emerged samples; (3) slowing or stopping of I(2) release; and (4) later pulses of I(2) evident in some samples. Emission rates were highly variable and ranged from 7 to 616 pmol min(-1) gFW(-1) in ozone-free air, with a median value of 55 pmol min(-1) gFW(-1) for 20 samples.

On the contribution of organics to the North East Atlantic aerosol number concentration

k-means statistical-cluster analysis of submicron aerosol size distributions is combined with coincident humidity tandem differential mobility analyser data, leading to five unique aerosol categories for hygroscopic growth factors (HGFs): low sea-salt background marine, high sea-salt background marine, coastal nucleation, open ocean nucleation and anthropogenically influenced scenarios. When considering only marine conditions, and generic aerosol species associated with this environment (e.g. non-sea-salt sulfate, sea-salt, partly soluble organic matter and water insoluble organic matter), the two-year annual average contribution to aerosol number concentration from the different generic species was made up as follows: 46% (30‐54%) of partially modified ammonium sulfate particles; 23% (11‐40%) of partially modified sea-salt; and the remaining 31% (25‐35%) contribution attributed to two distinct organic species as evidenced by different, but low, HGFs. The analysis reveals that on annual timescales, 30% of the submicron marine aerosol number concentration is sourced from predominantly organic aerosol while 60% of the anthropogenic aerosol number is predominantly organic. Coastal nucleation events show the highest contribution of the lowest HGF mode (1.19), although this contribution is more likely to be influenced by inorganic iodine oxides. While organic mass internally mixed with inorganic salts will lower the activation potential of these mixed aerosol types, thereby potentially reducing the concentration of cloud condensation nuclei (CCN), pure organic water soluble particles are still likely to be activated into cloud droplets, thereby increasing the concentration of CCN. A combination of dynamics and aerosol concentrations will determine which effect will prevail under given conditions.

Coastal iodine emissions: part 2. Chamber experiments of particle formation from Laminaria digitata-derived and laboratory-generated I2

Laboratory studies into particle formation from Laminaria digitata macroalgae were undertaken to elucidate aerosol formation for a range of I(2) (0.3-76 ppb(v)) and O(3) (<3-96 ppb(v)) mixing ratios and light levels (E(PAR) = 15, 100, and 235 μmol photons m(-2) s(-1)). No clear pattern was observed for I(2) or aerosol parameters as a function of light levels. Aerosol mass fluxes and particle number concentrations, were, however, correlated with I(2) mixing ratios for low O(3) mixing ratios of <3 ppb(v) (R(2) = 0.7 and 0.83, respectively for low light levels, and R(2) = 0.95 and 0.98, respectively for medium light levels). Additional experiments into particle production as a function of laboratory-generated I(2), over a mixing ratio range of 1-8 ppb(v), were conducted under moderate O(3) mixing ratios (∼24 ppb(v)) where a clear, 100-fold or greater, increase in the aerosol number concentrations and mass fluxes was observed compared to the low O(3) experiments. A linear relationship between particle concentration and I(2) was found, in reasonable agreement with previous studies. Scaling the laboratory relationship to aerosol concentrations typical of the coastal boundary layer suggests a I(2) mixing ratio range of 6-93 ppt(v) can account for the observed particle production events. Aerosol number concentration produced from I(2) is more than a factor of 10 higher than that produced from CH(2)I(2) for the same mixing ratios.

Simulating Marine New Particle Formation and Growth Using the M7 Modal Aerosol Dynamics Modal

A modal atmospheric aerosol model (M7) is evaluated in terms of predicting marine new particle formation and growth. Simulations were carried out for three different nucleation schemes involving (1) kinetic self-nucleation of OIO (2) nucleation via OIO activation by H2SO4 and (3) nucleation via OIO activation by H2SO4 plus condensation of a low-volatility organic vapour. Peak OIO and H2SO4 vapour concentrations were both limited to molecules cm-3 at noontime while the peak organic vapour concentration was limited to molecules cm-3. All simulations produced significant concentrations of new particles in the Aitken mode. From a base case particle concentration of 222 cm-3 at radii >15 nm, increases in concentrations to 366 cm-3 were predicted from the OIO-OIO case, 722 cm-3 for the OIO-H2SO4 case, and 1584 cm-3 for the OIO-H2SO4 case with additional condensing organic vapours. The results indicate that open ocean new particle production is feasible for clean conditions; however, new particle production becomes most significant when an additional condensable organic vapour is available to grow the newly formed particles to larger sizes. Comparison to sectional model for a typical case study demonstrated good agreement and the validity of using the modal model.

Modelling marine aerosol precursor vapours & impact on aerosol population

Regional climate model REMOTE is employed to assess the impact of two new sources of marine aerosol precursor vapours (DMS and volatile organo-iodine vapours) on the aerosol population. Microphysical model M7 is applied within REMOTE to simulate gas to particle conversion processes. Results indicate new ultra-fine particle production of the order of 103 particles cm−3 in the North East Atlantic, predominantly due to the nucleation of iodinecontaining vapours. The consideration of DMS vapours in the model leads to an increase in sulphur gases (factor of 1.5-2) which is of sufficient magnitude to enhance the growth of accumulation mode particles over the model domain, the resulting effect on the radiative properties of the atmosphere of potential global significance.

A dual behavior of primary marine organics

High-time resolution measurements of primary marine organic sea-spray physico-chemical properties reveal submicron organic marine aerosol plume concentrations peaking at 3.8 μg m−3, moreover, it shows an apparent dichotomous behavior of marine organics in terms of water uptake: specifically sea-spray aerosol enriched in organic matter possesses a low Hydroscopic Growth Factor (HGF∼1.25) while simultaneously having a cloud condensation nucleus/condensation nuclei (CCN/CN) activation efficiency of between 83% at 0.25% supersaturation and 100% at 0.75%. Simultaneous retrieval of Cloud Droplet Number Concentration (CDNC) during primary organic aerosol plumes reveals CDNC concentrations of 350 cm−3 for organic mass concentrations 3-4 μg m−3. It is demonstrated that the retrieved high CDNCs under clean marine conditions can only be explained by organic sea-spray and corroborates the high CCN activation efficiency associated with primary organics. It is postulated that marine hydrogels are responsible for this d...