Aerosol non‐sea‐salt sulfate in the remote marine boundary layer under clear‐sky and normal cloudiness conditions: Ocean‐derived biogenic alkalinity enhances sea‐salt sulfate production by ozone oxidation

Abstract

Observations of remote marine boundary layer (RMBL) non‐sea‐salt sulfate (NSS) aerosols indicate a substantial NSS fraction may be found in coarse (supermicron diameter) sea‐salt aerosols. Aqueous phase mechanisms account for a portion of this coarse aerosol NSS. Aerosol samples were collected at Baring Head, New Zealand, during clear‐sky and normal cloudiness conditions to assess the contribution that the O3 oxidation mechanism may make to coarse NSS. Southern Ocean derived clear‐sky coarse aerosols had >8 nmol NSS m−3; the normal cloudiness coarse aerosols contained <3 nmol NSS m−3. Data were obtained during high wind speeds (U ∼ 11 m s−1), minimizing coarse aerosol lifetimes and thus cloud interaction. Both clear‐sky and normal cloudiness coarse aerosols contained Ca excesses that contributed >200 and 35–40 times, respectively, more alkalinity than did bulk seawater (equivalents basis). Consequently, aqueous phase, O3 oxidation in sea‐salt aerosols was markedly enhanced and may account for essentially all observed coarse aerosol NSS. Satellite data indicate the Southern Ocean upwind of Baring Head contains ∼3 times the annual primary productivity (with associated biogenic Ca enhancement) of open ocean surface waters. The large and variable upwind biogenic Ca source plus the high wind speeds encountered at Baring Head make this site atypical of many RMBL regions. More typical RMBL conditions, found during western Pacific Ocean shipboard measurements, indicate that ∼1 to ∼2.5 times additional alkalinity (beyond that from bulk seawater) is likely to be found in sea‐salt aerosols due to seawater biogenic sources. Coarse aerosols, laden with NSS, experience large dry deposition rates which results in rapid recycling of ocean‐derived sulfur. This process has been overlooked or understated in RMBL sulfur budget analyses and models. One major consequence is that new particle production from biogenically derived oceanic sulfur is severely limited in the RMBL since ubiquitous O3 oxidation in sea‐salt aerosols is energetically favored over homogeneous nucleation.