Abstract
Observations of new particle formation events in free troposphere are rather seldom and limited in time and space, mainly due to the complexity and the cost of the required on-board instrumentation for airplane field campaigns. In this paper, a calibrated (UV, VIS) polarization elastic lidar (2β + 2δ) is used to remotely sense new particle formation events in the free troposphere in the presence of mineral dust particles. Using very efficient (UV, VIS) light polarization discriminators (1:107) and after robust calibration, the contribution of mineral dust particles to the co-polarized (UV, VIS) lidar channels could be removed, to reveal the backscattering coefficient of the newly nucleated particles after these numerous particles have grown to a size detectable with our lidar. Since our polarization and wavelength cross-talks are fully negligible, the observed variation in the (UV, VIS) particle backscattering time–altitude maps could be related to variations in the particle microphysics. Hence, day and nighttime differences, at low and high dust loadings, were observed in agreement with the observed nucleation process promoted by mineral dust. While light backscattering is more sensitive to small-sized particles at the UV lidar wavelength of 355 nm, such new particle formation events are here for the first time also remotely sensed at the VIS lidar wavelength of 532 nm at which most polarization lidars operate. Moreover, by addressing the (UV, VIS) backscattering Angstrom exponent, we could discuss the particles’ sizes addressed with our (UV, VIS) polarization lidar. As nucleation concerns the lowest modes of the particles’ size distribution, such a methodology may then be applied to reveal the lowest particle sizes that a (UV, VIS) polarization lidar can address, thus improving our understanding of the vertical and temporal extent of nucleation in free troposphere, where measurements are rather seldom.
Highlights
As underscored by the latest IPCC report [1], atmospheric aerosols contribute to the Earth’s radiative budget by interacting with solar radiation and participating in the formation of clouds [2,3,4]
In [32], we identified the required sensitivity and accuracy, ensuring a lidar backscattering detector was sensitive to the subsequent particles’ growth following nucleation, in the case of new particle formation (NPF) events promoted by mineral dust [7]
We here propose (UV, VIS) lidar remote sensing observations of NPF events promoted by mineral dust in the free troposphere in the form of time–altitudes maps of particle backscattering, providing the vertical and temporal extent of NPF
Summary
As underscored by the latest IPCC report [1], atmospheric aerosols contribute to the Earth’s radiative budget by interacting with solar radiation and participating in the formation of clouds [2,3,4]. Field observations of NPF in free troposphere are rather scarce and restricted to only a few mountain locations [16] or to very interesting but sporadic airplane field campaigns [23], which can hardly provide the spatial and temporal extent of nucleation [24], due to their intrinsics limit duration In this context, lidar remote sensing, based on time-resolved detection of laser light elastic backscattering, is really interesting as it is the only technique providing continuous vertical profiles of particles’ backscattering as a function of altitude, under in-situ conditions of temperature and humidity [25], which govern the spatial and temporal extension of NPF events.
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