Marine Primary and Secondary Organic Aerosols and Their Effect on Indirect Radiative Forcing
The number concentration, size distribution and chemical composition of natural aerosols (both from terrestrial and marine sources) were suggested to be one of the major uncertainties in model-predicted indirect radiative forcing. Due to their massive source regions underlying an atmosphere with low aerosol concentration, marine aerosols derived from sea spray and ocean emitted biogenic volatile organic compounds (BVOCs) are extremely important for the Earth's radiative budget, regional air quality and biogeochemical cycling of elements. Here we constrain the chemical composition of ocean-derived aerosol through laboratory experiments simulating the bubble-bursting process and experiments characterizing BVOC emission rates. In addition, we evaluate the sensitivity of cloud droplet number concentration due to changes in assumed emissions using the NCAR Community Atmosphere Model (CAM5.0), coupled with the PNNL Modal Aerosol Model.
Bubble bursting experiments: The factors that determine the organic enrichment of sea spray aerosol were quantifyied by conducting laboratory experiments measuring the size and cloud condensation nuclei (CCN) distributions of aerosols generated from a bubble-bursting generator using artificial sea-water. Before each experiment the generator (rectangular prism constructed from Plexiglas) was cleaned and filled with 3.5 L ultrapure water (18.2 MW) and sea salt to yield a concentration of 35‰, typical of natural sea water. Zero air was forced through the porous medium to form the bubbles. Particles formed from the bursting bubbles were ejected into the headspace above the solution (~1.5 L) and sampled continuously by a size-resolved CCN system. Air removed from the headspace was replaced with filtered room air. Bubble size was controlled by varying the pore size of the medium and the bubble size distribution is measured using optical imaging. A known amount of a model organic compound (chosen to simulate dissolved organic carbon in the ocean as well as surface slicks) was added to the water. Organic aerosol enrichment and aerosol number emissions and CCN number emissions at 0.2% and 1% supersaturation were measured as a function of bubble size (~ 0.2-2 mm), organic compound concentration (mass fractions 10-7–10-2 relative to sea salt). Organic contribution to marine primary aerosol emissions from the bubble-bursting mechanism was modeled in terms of the aerosol number emissions and the hyroscopicity parameter - κ. Our results show that decreasing the bubble diameter from 1000 to 200 µm decreased the κ-value of the emitted aerosols from ~1.2 to ~0.6. A κ-value of 0.6 suggests that the aerosol is composed to ~50% organic by volume, which is orders of magnitude greater than the amount added to the artificial sea water. An organic film thickness >~1µm at the surface also decreased the κ-value to <0.6, reaching κ near 0 for films >50 mm. This implyies that despite the fact that a large number of particles are being emitted above the solution, a 50µm thick surface layer over the water reduceed the number emission of aerosols capable of serving as CCN (at 0.2% super saturation) to near 0.
BVOC emissions: Biogenic trace gas emission rates from the ocean were quantifyied using laboratory grown phytoplankton monocultures and Neuse River Estuary samples. The samples were grown and maintained in 9.5L Pyrex bottles at climate controlled room. BVOCs accumulated in the water and headspace above the water were measured by passing the sample through a gas chromatography/mass spectrometry (Varian 220 GC-MS) system equipped with a sample pre-concentrator (CDS 8000). Inside the preconcentrator the compounds were trapped on a sorbent material, heated, and backflushed into the GC-MS that contains a CP-PoraBOND Q Fused Silica column. A split/splitess injection was used, with an initial splittess mode for 0.75 minutes, followed by a split ratio of 100:1 for 2.25 minutes, and then kept at a 20:1 split ratio for the remainder of the analysis. The column oven was be maintained at 50oC for 2 minutes, followed by an increase of 6oC/min until 250oC (total of 35.33 minutes). The preconcentrartor/GC-MS system gave at least 1000 times magnification of the sample concentrations, allowing detection of low ppt levels of over 50 different hydrocarbons. Diatoms Thalassiosira weissflogii and Thalassiosira pseudonana, prymnesiophyte Pleurochrysis carterae and dinoflagellates Karina brevis and Procentrum minimum were subjected to differential light regimes to assess the photoproduction of different BVOCs. Our results show that diatoms had the highest isoprene production rate of 2.8 µmol isoprene (g Chl-a)-1 h-1 with ranges between 1.4 and 3.6 µmol isoprene (g Chl-a)-1 h-1 at light levels between 90 and 900 µE m-2s-1, respectively. The prymnesiophyte and dinoflagellate species had an average isoprene production rate of 1.3±0.4 µmol isoprene (g Chl-a)-1 h-1 with a similar light dependency as diatoms. Three monoterpenes detected from each species were a-pinene, camphene, and D-limonene. Diatoms had the highest average a-pinene and D-limonene production rates of 0.034 µmol a-pinene (g Chl-a)-1 h-1 and 0.015 µmol D-limonene (g Chl-a)-1 h-1, while the prymnesiophyte species had the highest average camphene production of 0.021 µmol camphene (g Chl-a)-1 h-1. Production rates of D-limonene and camphene did not show a well-defined light dependency, but both isoprene and a-pinene show an increase in terpene production with increasing light intensities for all phytoplankton species. Remotely-sensed Chl-a concentration in conjunction with laboratory measurements of BVOCs was used to generate ocean biogenic trace gas emission inventories.
Model Results: The effects of marine BVOCs and organic carbon aerosol emissions on microphysical properties of clouds were explored by conducting 10 year CAM5.0 model simulations at a grid resolution 1.9°×2.5° with 30 vertical layers. Model-predicted relationship between ocean physical and biological systems and the abundance of CCN in remote marine atmosphere was compared to data from the A-Train satellites (MODIS, CALIPSO, AMSR-E). Model simulations show that on average, primary and secondary organic aerosol emissions from the ocean can yield up to 5% increase in droplet number concentration of global maritime shallow clouds. Changes associated with cloud properties increase short wave forcing by up to -0.25 Wm-2. By using different emission scenarios, and droplet activation parameterizations, this study suggests that addition of marine primary aerosols and biologically generated reactive gases makes an important difference in radiative forcing assessments.