Sources of nitrate in stratocumulus cloud water: Airborne measurements during the 2011 E-PEACE and 2013 NiCE studies
Author
Prabhakar, Gouri
Ervens, B.
Wang, Z.
Maudlin, L.C.
Coggon, M.M.
Jonsson, H.H.
Seinfeld, J.H.
Sorooshian, A.
Date
2014Metadata
Show full item recordAbstract
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.
Description
The article of record as published may be found at http://dx.doi.org/10.1016/j.atmosenv.2014.08.019
includes supplementary material