Comment on acp-2022-822

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.