03 Jan 2023
03 Jan 2023
Status: this preprint is currently under review for the journal ACP.

New Particle Formation in the Tropical Free Troposphere during CAMP2Ex: Statistics and Impact of Emission Sources, Convective activity, and Synoptic Condition

Qian Xiao1, Jiaoshi Zhang1, Yang Wang2, Luke D. Ziemba3, Ewan Crosbie3,4, Edward L. Winstead3, Claire E. Robinson3, Joshua P. DiGangi3, Glenn S. Diskin3, Jeffrey S. Reid5, K. Sebastian Schmidt6, Armin Sorooshian7,8, Miguel Ricardo A. Hilario8, Sarah Woods9, Paul Lawson9, Snorre A. Stamnes3, and Jian Wang1 Qian Xiao et al.
  • 1Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA
  • 2Department of Chemical, Environmental and Materials Engineering, University of Miami, Coral Gables, FL 33124, USA
  • 3NASA Langley Research Center, Hampton, VA 23666, USA
  • 4Science Systems and Applications, Inc., Hampton, VA 23666, USA
  • 5Marine Meteorology Division, Naval Research Laboratory, Monterey, CA, USA
  • 6Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA
  • 7Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, 85721, USA
  • 8Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ 85721, USA
  • 9Stratton Park Engineering Company (SPEC), Boulder, CO 80301, USA

Abstract. Nucleation in the free troposphere (FT) and subsequent growth of new particles represents a globally important source of cloud condensation nuclei (CCN). Whereas new particle formation (NPF) has been shown to occur frequently in the upper troposphere over tropical oceans, there have been few studies of NPF at lower altitudes over the tropical marine environment. In addition, the impact of anthropogenic emissions and biomass burning on NPF over the tropics remains poorly understood. In this study, we examine NPF in the lower and mid troposphere (3–8.5 km) over ocean and coastal regions of the Sulu Sea and Northern Subtropical Pacific Ocean in Southeast Asia using airborne measurements during the recent Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex). CAMP2Ex took place from 25 August through 5 October 2019, including both late southwest monsoon and monsoon transition. Recent NPF events, as evidenced by elevated concentrations of newly formed particles (i.e., particles of diameters between 3 and 10 nm), were observed during 4 % of the total flight time (5 out of 128 hours). The frequency of NPF increases with altitude, reaching 49 % above an altitude of 8 km. NPF was mostly observed at altitudes above 3 km and coincided with elevated relative humidity (RH), suggesting that NPF is closely associated with convective cloud outflow in conditions of low temperature and reduced pre-existing particle concentrations. Air masses are categorized into background, biomass burning-influenced, and urban-influenced air based on in-situ CO, CH4 and O3 measurements. NPF in background air was mostly observed above 6 km, typically accompanied by the lowest surface area among all air mass types. NPF occurred above the 0 ºC level at 5.5–7 km in air masses influenced by convectively detrained biomass burning and/or urban emissions and was enhanced by 1) scavenged primary particles; 2) elevated precursor concentrations and 3) enhanced irradiance due to cloud reflections. However, NPF was suppressed in aged urban influenced air masses where the reactive precursors were mostly consumed while existing particle surface area remained relatively high due to longer aerosol lifetimes in the free troposphere. The results highlight the role of convective clouds that efficiently scavenge existing aerosol particles, inject reactive precursors into free troposphere, and enhance UV irradiance, all of which facilitate NPF. This study also illustrates the competing influences of different variables and complex interactions between anthropogenic emissions, transport, convective clouds, and meteorology, which lead to NPF under a variety of conditions and at different altitudes in tropical marine environment.

Qian Xiao et al.

Status: open (until 14 Feb 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-822', Anonymous Referee #1, 24 Jan 2023 reply

Qian Xiao et al.


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Short summary
Using recent airborne measurements, we show that the influences of anthropogenic emissions, transport, convective clouds, and meteorology lead to new particle formation (NPF) under a variety of conditions and at different altitudes in tropical marine environment. NPF is enhanced by fresh urban emissions in convective outflow but is suppressed in air masses influenced by aged urban emissions where reactive precursors are mostly consumed while particle surface area remains relatively high.