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Published November 2014 | Supplemental Material
Journal Article Open

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


This study examines the sources of NO_3^− 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 NO_3^− air-equivalent concentrations in cloud water samples categorized as having been influenced by ship exhaust (2.5 μg m^(−3)), strong marine emissions (2.5 μg m^(−3)) and fires (2.0 μg m^(−3)) were more than twice that in the background cloud water (0.9 μg m^(−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 NO_3^− levels reaching as high as 9.0 μg m^(−3). Nucleation scavenging of chloride depleted sea-salt is a source of cloud water NO^3^−, with the lowest Cl^−:Na^+ ratio (1.5) observed in ship-influenced samples. Surface aerosol measurements show that NO_3^− concentrations peak in the particle diameter range of 1.0–5.6 μm, similar to Na, Cl^− and Si, suggesting that drop activation of crustal particles and sea salt could be an important source of NO_3^− in cloud water. The contrasting behavior of NO_3^− and SO_4^(2−) is emphasized by the NO_3^−:SO_4^(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 NO_3^−:SO_4^(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 HNO_3 uptake by droplets.

Additional Information

© 2014 Elsevier Ltd. Received 18 March 2014. Received in revised form 6 August 2014. Accepted 8 August 2014. Available online 9 August 2014. This work was funded by Office of Naval Research grants N00014-11-1-0783, N00014-10-1-0200, and N00014-10-1-0811, and National Science Foundation grant AGS-1008848. We acknowledge Dean Hegg for providing the cloud water collector. Barbara Ervens acknowledges support from NOAA's Climate Goal.

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