Pergamon J. Aemsolki. Vol. 29, Suppl. I. pp. SIOll-SlO12, 1998

0 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

0021~8502/98 $19.00+ 0.00

NUCLEATION IN THE REMOTE TROPOSPHERE

A. CLARKE, F. EISELE*, V. KAPUSTIN, K. MOORE, L. MAULDIN* Department of Oceanography, University of Hawaii, Honolulu, Hawaii

*National Center for Atmospheric Research, Boulder CO.80303

KEYWORDS

Nucleation, Remote Troposphere, Boundary layer, Clouds

The origin of condensation nuclei (CN) in the remote marine boundary layer (MBL) has been an objective of numerous research programs for the past two decades. Recent (1995 1996) experiments ACE- 1 (Aerosol Characterization Expt., NSF) and PEMT (Pacific Exploratory Mission-Tropics, NASA) employed a combination of aerosol and gas phase instrumentation directed at this issue. New particle production was observed a) for midlatitude regions of cloud outflow at 2-3km alt.; b) at 6-8 km alt. in the clean well scavenged marine air in the Intertropical Convergence Zone (ITCZ); c) in aged and scavenged air above the South Pacific Convergence Zone (SPCZ) influenced by a distant continental combustion and d) in MBL recently scavenged by nearby convective cloud elements. Nucleation was seen in regions where sulfuric acid concentrations were near 1x10’ molecules cme3, when surface areas were near or below 5 urn’ cme3 and when relative humidities (RI-I) were relatively higher than adjacent regions.

A PEMT flight from Hawaii to Tahiti (Figure 1) went over the ITCZ and the SPCZ. These are separated by a zone of subsidence with light easterly winds and the SPCZ tends to be bounded by more polluted air in westerly flow to the south and cleaner easterly air to the north. Marked changes in aerosol size along the flight track reveal regions of nucleation aloft above the ITCZ and SPCZ, larger aerosol at intermediate aititudes and even larger bimodal distributions in the MBL with a cloud processed intermode minimum. Above Tahiti the intermediate mode at about 40nm was found to have a soot core and appears associated with biomass burning in Africa. Highest concentrations of total particles were usually found aloft in what appeared to be cloud scavenged air with low mass concentrations. A vertical descent near the ITCZ shows ultrafine CN (UFCN, Dp>3nm) and DCN (3nm<Dp<l2nm) concentra-

Figure 1 NH.96 Figure 2

s1012 Abstracts of the 5th International Aerosol Conference 1998

tions are highest aloft (Fig. 2) and decrease toward the surface where the aerosol appears well mixed below 2km and no ultrafine are present in the MBL. Variable and intermediate RI-I values above indicate cloud pumped air that is also associated with high small nuclei, DCN, counts. The gradient from alot? toward the surface is also linked to changes in the size distribution. The dry subsiding free troposphere (FT) air near 3km has a broad monomodal number distribution centered near 0.05um. Above this altitude recently formed nuclei near 0.02um are evident in the regions of high DCN and moderate RH indicative cloud outflow (Clarke, 1993, Hegg et al., 1990). The two bimodal distributions in the upper and lower MBL show lower particle concentrations and a shift to larger diameters near O.O6um, consistent with a monomodal aerosol subsided into MBL followed by cloud processing and heteroge- neous growth.

On PEMT we observed nucleation in the MBL in westerly flow off the west coast of Panama and in a cloudy region with average RH along the leg of 95%. Figure 3a shows altitude UFCN, CN/UFCN ratio (Dp>l2nm/Dp>3nm) for the boundary layer leg and the nucleation event of interest but the descent profile showed no evidence of enhanced nuclei above 3km. Elevated UFCN occur near the descent and particularly near 20.0 hrs. This event immediately follows a region of high sulfuric acid and low total surface area(Fig. 3b). The peak in UFCN is followed by an increase in surface area dominated by the growing new nuclei (Dp<0,25um)(Fig. 2). Fig.3c is a contour plot of the size distribution and concentra- tion that indicates the regions of new aerosol forma- tion. CumIemr. “ml

65 Nucleation predicted from classical binary 36

nucleation theory and the critical sulfuric acid concen- 27 tration (Kerminen and Wexler, 1994) is inconsistent ‘r with observations in the MBL but was consistent for higher altitudes aloft over the SPCZ and ITCZ (Fig. 1) “8?:; 3 and for high RH near cloud outflow, suggesting other considerations (eg. ammonia, ions) might be involved in the MBL nucleation. These cases indicate nucle- ation appears only to occur in the cleanest regions of O0 emme lwm “WI, m-3 the remote atmosphere but at any altitude. In spite of 10 rare MBL observations(eg. Covert et al., 1993), the more common occurrence of nucleation aloft fol- OS lowed by subsidence into the MBL suggests that this o. mechanism will be more important in supplying new aerosol to the MBL than in-situ MBL formation.

Figure 3

REFERENCES

Clarke, A.D., (1993) Atmospheric nuclei in the Pacific midtroposphere: their nature, concen tration, and evolution, J. GeophyxRes., 98, 20,633-20,647.

Covert, D.S., V.N. Kapustin, P.K. Quinn and T.S. Bates, (1993) New particle formation in the marine boundary layer, J. Geophys. Res., 97,20,581-20,589.

Hegg, D.A., L.F. Radke and P.V. Hobbs, (1990) Particle production associated with marine clouds, J. Geophys. Res, 95, 13,917-13,926.

Kerminen, V. and A. Wexler, (1994) Particle formation due to SO, oxidation and high relative humidity in the remote marine boundary layer, J. Geophys. Res, 99, 25,607.