Ship impacts on the marine atmosphere: insights into the contribution of shipping emissions to the properties of marine aerosol and clouds

Abstract. We report properties of marine aerosol and clouds measured in the shipping lanes between Monterey Bay and San Francisco off the coast of Central California. Using a suite of aerosol instrumentation onboard the CIRPAS Twin Otter aircraft, these measurements represent a unique set of data contrasting the properties of clean and ship-impacted marine air masses in dry aerosol and cloud droplet residuals. Below-cloud aerosol exhibited average mass and number concentrations of 2 μg m⁻³ and 510 cm⁻³, respectively, which are consistent with previous studies performed off the coast of California. Enhancements in vanadium and cloud droplet number concentrations are observed concurrently with a decrease in cloud water pH, suggesting that periods of high aerosol loading are primarily linked to increased ship influence. Mass spectra from a compact time-of-flight Aerodyne aerosol mass spectrometer reveal an enhancement in the fraction of organic at m/z 42 (f₄₂) and 99 (f₉₉) in ship-impacted clouds. These ions are well correlated to each other (R²>0.64) both in and out of cloud and constitute 14% (f₄₄) and 3% (f₉₉) of organic mass during periods of enhanced sulfate. High-resolution mass spectral analysis of these masses from ship measurements suggests that the ions responsible for this variation were oxidized, possibly due to cloud processing. We propose that the organic fractions of these ions be used as a metric for determining the extent to which cloud-processed ship emissions impact the marine atmosphere where (f₄₂ > 0.15; f₉₉ > 0.04) would imply heavy influence from shipping emissions, (0.05 < f₄₂ < 0.15; 0.01 < f₉₉ < 0.04) would imply moderate, but persistent, influences from ships, and (f₄₂ < 0.05; f₉₉ < 0.01) would imply clean, non-ship-influenced air.