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.
Highlights
The process of gas-to-particle conversion, denoted as homogeneous aerosol nucleation and most commonly known as new particle formation (NPF), is a major source of atmospheric aerosols and cloud condensation nuclei, which could promote the cloud formation at intermediate and upper tropospheric altitudes (e.g. Spracklen et al, 2006; Merikanto et al, 2009; Dunne et al, 2016; Gordon et al, 2017)
Considering the quantities of organic aerosols (Murphy et al, 2006) and ammonia species (Höpfner et al, 2019) that were frequently found in aerosol particles at upper troposphere (UT)–tropical transition layer (TTL) heights in the Asian monsoon anticyclone (AMA) during StratoClim 2017, NPF is likely promoted by such species in the UT and TTL region
Was analysed as it was encountered in the upper troposphere–lowermost stratosphere (UTLS) region of the Asian monsoon anticyclone (AMA) over northern India, Nepal, and Bangladesh
Summary
The process of gas-to-particle conversion, denoted as homogeneous aerosol nucleation and most commonly known as new particle formation (NPF), is a major source of atmospheric aerosols and cloud condensation nuclei, which could promote the cloud formation at intermediate and upper tropospheric altitudes (e.g. Spracklen et al, 2006; Merikanto et al, 2009; Dunne et al, 2016; Gordon et al, 2017). The region above tropospheric clouds seems favourable for NPF to occur, and possible reasons for this are discussed by Wehner et al (2015) They found that the majority of their near-cloud NPF observations correlated with increased ultraviolet irradiance, so they concluded cloud edges to be a favourable environment for the production of precursor gases for the formation of new particles (Wehner et al, 2015). In the region of the tropical transition layer (TTL) over South America, Australia, and West Africa, the in situ measurements by Weigel et al (2011) revealed nucleation-mode particles in elevated number concentrations from recent NPF. Regarding the occurrence of NPF in conjunction with the presence of upper tropospheric ice clouds, several unspecified details remain
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