In situ observation of new particle formation (NPF) in the tropical tropopause layer of the 2017 Asian monsoon anticyclone – Part 2: NPF inside ice clouds

Abstract

Abstract. From 27 July to 10 August 2017, the airborne StratoClim mission took place in Kathmandu, Nepal, where eight mission flights were conducted with the M-55 Geophysica up to altitudes of 20 km. New particle formation (NPF) was identified by the abundant presence of nucleation-mode aerosols, with particle diameters dp smaller than 15 nm, which were in-situ-detected by means of condensation nuclei (CN) counter techniques. NPF fields in clear skies as well as in the presence of cloud ice particles (dp > 3 µm) were encountered at upper troposphere–lowermost stratosphere (UTLS) levels and within the Asian monsoon anticyclone (AMA). NPF-generated nucleation-mode particles in elevated concentrations (Nnm) were frequently found together with cloud ice (in number concentrations Nice of up to 3 cm−3) at heights between ∼ 11 and 16 km. From a total measurement time of ∼ 22.5 h above 10 km altitude, in-cloud NPF was in sum detected over ∼ 1.3 h (∼ 50 % of all NPF records throughout StratoClim). Maximum Nnm of up to ∼ 11 000 cm−3 was detected coincidently with intermediate ice particle concentrations Nice of 0.05–0.1 cm−3 at comparatively moderate carbon monoxide (CO) contents of ∼ 90–100 nmol mol−1. Neither under clear-sky nor during in-cloud NPF do the highest Nnm concentrations correlate with the highest CO mixing ratios, suggesting that an elevated pollutant load is not a prerequisite for NPF. Under clear-air conditions, NPF with elevated Nnm (> 8000 cm−3) occurred slightly less often than within clouds. In the presence of cloud ice, NPF with Nnm between 1500–4000 cm−3 was observed about twice as often as under clear-air conditions. NPF was not found when ice water contents exceeded 1000 µmol mol−1 in very cold air (< 195 K) at tropopause levels. This indicates a reduction in NPF once deep convection is prevalent together with the presence of mainly liquid-origin ice particles. Within in situ cirrus near the cold point tropopause, recent NPF or intense events with mixing ration nnm larger than 5000 mg−1 were observed only in about 6 % of the in-cloud NPF data. In determining whether the cloud-internal NPF is attenuated or prevented by the microphysical properties of cloud elements, the integral radius (IR) of the ice cloud population turned out to be indicative. Neither the number of ice particles nor the free distance between the ice particles is clearly related to the NPF rate detected. While the increase in ice particles' mass per time dmdt is proportional to the IR and mainly due to the condensation of water vapour, additional condensation of NPF precursors proceeds at the expense of the NPF rate as the precursor's saturation ratio declines. Numerical simulations show the impact of the IR on the supersaturation of a condensable vapour, such as sulfuric acid, and furthermore illustrate that the IR of the cloud ice determines the effective limitation of NPF rates.

Chemical analysis of the Asian Tropopause Aerosol Layer (ATAL) with emphasis on secondary aerosol particles using aircraft based in situ aerosol mass spectrometry

“…Together with the single particle measurements, we thus have several indications suggesting that the ATAL composition is to a large part controlled by secondary organic and inorganic aerosol formation. Our findings complement concurrent in situ observations of particle microphysical properties by Mahnke et al (2021) and Weigel et al (2021b), demonstrating that new particle formation and subsequent particle growth by condensation and coagulation takes place in the lower ATAL region below the tropopause (see Fig. 3).…”

“…3, which is a comprehended representation of the data from their studies, and which shows the results of particle number concentration larger than 10 nm and larger than 65 nm as a function of altitude and potential temperature. In these publications and in Weigel et al (2021b) it was demonstrated that new particle formation and subsequent particle growth by condensation and coagulation can be frequently observed in the lower ATAL region below the tropopause (between 355 and 370 K). Note however, that COPAS data for particles between 6 and 10 nm in diameter -indicative of such transient new particle formation events -are not included in Fig.…”

A Multimodel Investigation of Asian Summer Monsoon UTLS Transport Over the Western Pacific

“…Information on these parameters is critically needed for improving model representation. The StratoClim campaign obtained the first significant set of airborne in situ and remote‐sensing aerosol data in the ATAL (Höpfner et al., 2019; Mahnke et al., 2021; Weigel, Mahnke, Baumgartner, Dragoneas, et al., 2021; Weigel, Mahnke, Baumgartner, Krämer, et al., 2021) from Kathmandu‐based research flights. These measurements sampled the South Asian region including Nepal, Pakistan, northern India, and the Bay of Bengal.…”

The Asian tropopause aerosol layer within the 2017 monsoon anticyclone: microphysical properties derived from aircraft-borne in situ measurements

“…The TTL, which extends over an altitude range from about 14 to 18 km in this area, acts as a "gateway to the stratosphere" (Fueglistaler et al, 2009), as air from this region can be transported into the lower stratosphere via diabatic ascent (Garny and Randel, 2016). With the transport of climate-relevant natural and anthropogenic trace gases, water vapor, and aerosol particles into the stratosphere, precursor gases also enter the UT-LS, which can lead or contribute to the formation of new particles from the gas phase (new particle formation -NPF) (Brock et al, 1995;Weigel et al, 2011;Williamson et al, 2019). Asia is currently one of the regions with the highest emission of atmospheric sulfur worldwide.…”