Marine Organic Aerosols and Their Implication to Air Quality

Abstract

The global marine sources of organic carbon (OC) are estimated here using a physically-based parameterization for the emission of marine isoprene and primary organic matter. The emission model developed in this study allowed us, for the first time, to explore the relative contributions of sub- and super-micron organic matter and phytoplankton-produced secondary organic aerosol (SOA) to the total OC fraction of marine aerosol. New laboratory measurements of isoprene production by several abundant phytoplankton species under a range of environmental conditions were scaled up, with the help of satellite products, to infer the total annual mean ocean isoprene emissions of 0.92 Tg C yr-1. The sensitivity studies using different schemes for the euphotic zone depth and ocean phytoplankton speciation produced the upper and the lower range of marine-isoprene emissions of 0.31 to 1.09 Tg C yr-1, respectively. Empirical relationships between emissions of water soluble (WSOM) and water insoluble (WIOM) organic matter (OM) and chlorophyll a concentration were used to estimate the total primary sources of oceanic sub- and super-micron OC of 1.26 and 19.01 Tg C yr-1, respectively. Using a fixed 3% mass yield for the conversion of isoprene to SOA, our emission simulations show minor (less than 0.2%) contribution of ocean produced isoprene to the total marine source of OC. However, our model calculations also indicate that over the tropical waters, marine isoprene-derived SOA could contribute over 40% of the total monthly-averaged sub-micron OC fraction of marine aerosol. The estimated contribution of ocean-isoprene SOA to hourly averaged sub-micron marine OC fluxes is even higher, reaching 100% over the vast regions of the oceans during the midday hours. As it is widely believed that sub-micron OC has the potential to influence the cloud droplet activation of marine aerosols, our findings suggest that marine sources of SOA could play critical role in modulating properties of shallow marine clouds and influencing the climate. The impact of marine isoprene emissions on summertime surface concentrations of isoprene, secondary organic aerosols (SOA), and ozone (O3) in the coastal areas of the continental United States is studied using the U.S. Environmental Protection Agency regional-scale Community Multiscale Air Quality (CMAQ) modeling system. Marine isoprene emission rates are based on the following five parameters: laboratory measurements of isoprene production from different phytoplankton species under a range of light conditions, remotely-sensed chlorophyll-a concentration ([Chl-a]), incoming solar radiation, surface wind speed, and sea-water optical properties. Model simulations show that marine isoprene emissions are sensitive to meteorology and ocean ecosystem productivity, with the highest rates simulated over the Gulf of Mexico. With the isoprene reactions included in this study, the average contribution of marine isoprene to SOA and O3 concentrations is predicted to be small, up to 0.004 µg m-3 for SOA and 0.2 ppb for O3 in coastal urban areas. The light-sensitivity of isoprene production from phytoplankton results in a midday maximum for marine isoprene emissions and a corresponding daytime increase in isoprene and O3 concentrations in coastal locations. While SOA concentrations are consistently higher (up to 0.005 µg m-3) in coastal areas due to marine isoprene emissions, the diurnal trends are dependent on the local micrometeorology. Interannual variability of [Chl-a] concentration is examined in a sensitivity study with values increased and decreased by a factor of five. Our results indicate that marine emissions of isoprene cause minor changes (< 0.5%) to coastal SOA and O3 concentrations, likely due to limitations in current model treatments of isoprene chemistry and a coarse horizontal resolution used in model simulations.