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

This study examines the sources of NO₃^- 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 NO₃^- air-equivalent concentrations in cloud water samples categorized as having been influenced by ship exhaust (2.5μgm^-3), strong marine emissions (2.5μgm^-3) and fires (2.0μgm^-3) were more than twice that in the background cloud water (0.9μgm^-3). 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 NO₃^- levels reaching as high as 9.0μgm^-3. Nucleation scavenging of chloride depleted sea-salt is a source of cloud water NO₃^-, with the lowest Cl^-:Na⁺ ratio (1.5) observed in ship-influenced samples. Surface aerosol measurements show that NO₃^- concentrations peak in the particle diameter range of 1.0-5.6μm, similar to Na, Cl^- and Si, suggesting that drop activation of crustal particles and sea salt could be an important source of NO₃^- in cloud water. The contrasting behavior of NO₃^- and SO₄^2- is emphasized by the NO₃^-:SO₄^2- 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 NO₃^-:SO₄^2- 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.