Sources of nitrate in stratocumulus cloud water: Airborne measurements during the 2011 E-PEACE and 2013 NiCE studies

Abstract

This study examines the sources of NO₃ˉ in stratocumulus clouds over the eastern Pacific Ocean off the California coast using airborne and surface measurement data from the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE; 2011) and Nucleation in California Experiment (NiCE; 2013). Average NO₃ˉ air-equivalent concentrations in cloud water samples categorized as having been influenced by ship exhaust (2.5 μg m ˉ³), strong marine emissions (2.5 μg m ˉ³) and fires (2.0 μg m ˉ³) were more than twice that in the background cloud water (0.9 μg m ˉ³). During periods when biomass burning plumes resided above cloud top, 16 of 29 cloud water samples were impacted due to instability in the entrainment interface layer with NO₃ˉ levels reaching as high as 9.0 μg m ˉ³. Nucleation scavenging of chloride depleted sea-salt is a source of cloud water NO₃ˉ , with the lowest Cl ˉ:Na⁺ ratio (1.5) observed in shipinfluenced samples. Surface aerosol measurements show that NO₃ˉ concentrations peak in the particle diameter range of 1.0-5.6 μm, similar to Na, Cl ˉ and Si, suggesting that drop activation of crustal particles and sea salt could be an important source of NO₃ˉ in cloud water. The contrasting behavior of NO₃ˉ

and SO ²₄ˉ is emphasized by the NO₃ˉ :SO ²₄ˉ mass concentration ratio which is highest in cloud water (by more than a factor of two) followed by above cloud aerosol, droplet residual particles, and below cloud aerosol. Trends of a decreasing NO₃ˉ :SO ²₄ˉ ratio with altitude in clouds are confirmed by parcel model studies due to the higher rate of in-cloud sulfate formation as compared to HNO₃ uptake by droplets.