DEDUCING DROPLET CONCENTRATION AND SUPERSATURATION IN MARINE BOUNDARY-LAYER CLOUDS FROM SURFACE AEROSOL MEASUREMENTS

Air parcels in the marine boundary layer (MEL) are mixed up through no nprecipitating clouds at the top of the MBL many times (on average) be fore they can be removed by precipitation scavenging. The equivalent d ry size of the particles (cloud condensation nuclei, CCN) upon which d roplets are formed increases because of liquid phase oxidation of solu ble trace gases during the cloud processing. The observed separation o f the submicron size distribution into an interstitial mode and cloud droplet residue mode makes it possible to infer the effective MBL clou d supersaturation and cloud droplet concentrations from surface measur ements of the aerosol size distribution during periods when nonprecipi tating MBL clouds are present in the back trajectory and the MBL is we ll mixed. The effect of particle composition on the accuracy of the in ferred cloud supersaturations is evaluated. A large database of hundre ds of size distributions taken on an Atlantic and a Pacific cruise and an airship flight off the Oregon coast are used to calculate the rang e of effective MBL cloud supersaturations and droplet concentrations e ncountered during these expeditions. The inferred droplet concentratio ns on the Pacific cruise were mostly in the 25 to 150 cm(-3) range, wh ereas they were mostly in the 50 to 500 cm(-3) range for the Atlantic cruise. The inferred effective supersaturation in the tropical MBL clo uds was typically in the 0.15% to 0.25% range. Recent work of rang and Munkelwitz [1994] would indicate that particles consisting of mixture s of ammonium sulfate and sulfuric acid would not have recrystalized i n the differential mobility analyzer (DMA) within the range of relativ e humidities (45% to 60%) at which the DMA was operated. At these humi dities the hydrated size can be as much as 20% greater than the dry si ze. Corrections for the hydrated size within the DMA at the time of me asurement are included and are also used to correct previous measureme nts of the relationship between dry size and critical supersaturation made using the Naval Research Laboratory (NRL) DMA and NRL thermal gra dient CCN counter.