# Monte Carlo Simulations in Aviation Contrail Study: A Review
## 2 Contrails in Aviation: Generalities
### Experimental Campaigns and Microscopically Characterization of Contrials  
Microphysical parameters identified in experimental campaigns typically include the ice particles' effective radii, size distribution (PSD), shape, and number concentration (Nt) [21]. As natural clouds, a contrail cirrus is often characterized in terms of ice water content (IWC), that is, the cloud's total mass of ice particles, usually expressed as the percentage of total moist air mass, which is computed by adding dry air mass and ice, liquid water, and steam total mass. The total ice content of a cloud is usually employed in models for parameterizing clouds' optical properties and is then an interesting quantity for estimating the radiative forcing induced by cloudiness. The first studies on the microphysical characteristics of contrail cirrus parameters date back to the 1970s [22; 23; 24]. Early campaigns were limited to measuring ice crystals with linear dimensions above 20 \(\upmu\)m [25; 26; 27], even if the existence of a large number of ice particles with a mean diameter below 30 \(\upmu\)m [28; 29; 30; 31] had been demonstrated. The impact of small particles on the radiative transfer through the cirrus is not negligible because particle size is related to the wavelength of the radiation they scatter, and, below 20 \(\upmu\)m, one finds the thermal infrared region (8 - 15 \(\upmu\)m), which includes the black-body radiation of an object at room temperature. An initial breakthrough in terms of small-particles' observation capabilities was achieved in the second half of the 1990s with the Sulfur 4, Sulfur 5, Contrail, and Aerocontrail airborne experiments [5; 32], which were performed with the DLR Dassault Falcon 20-E research aircraft and the NASA SUCCESS (Subsonic Aircraft: Contrail and Cloud Effects Special Study) missions [33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54], employing the DC-8 aircraft of the Armstrong (Dryden) Flight Research Center (AFRC). Both of these airplanes (which are still in operation) are multipurpose flying research laboratories equipped with several different instruments. In particular, at the time of the experiments mentioned above, both mounted a number of systems for airborne particles' size and shape measurements, mainly 1D (the Forward Scattering Spectrometer Probe, FSSP [55; 56], for size determination) and 2D optical devices (2D optical arrays, used for particles' size measurements and shape recording, which has evolved considerably over time [57; 58; 59; 60; 61]). Optical probes are still the gold standard for instruments for cloud microphysical parameter measurements and have been exploited in other experimental campaigns after those mentioned above, notably the CONCERT (CONtrail and Cirrus ExperRimenT) [62] and ML-CIRRUS [63] experiments carried out by DLR. In particular, CONCERT allowed the collection of new data on contrail formation beyond commercial flights with the innovative measurements of optical properties' distribution within the mixing fluids [62] and of the vertical particle concentration [64], whereas ML-CIRRUS provided one of the first integrated measures on natural and contrail-induced cirrus employing a HALE (high-altitude long dndurance) airplane. An analysis of the entire set of experimental data measured with airborne probes, complemented with remote sensing [65] and (remote) lidar acquisition [66; 67], is out of the scope of this review and has been the subject of some dedicated works [68; 69]. What can be observed here is that strong evidence exists in contrails confirming the presence of a large initial number (\(N_{l}\sim 10^{4}\) - \(10^{5}\) cm\({}^{-3}\)) of very small (mean of PSD \(<\) 1 \(\upmu\)m) ice particleswith a non-linear evolution in time of particle size and number, respectively decreasing by about four orders of magnitude (\(N_{t}\sim 10\) cm\({}^{-3}\)) and increasing up to a mean size of 10 \(\upmu\)m during a one-hour time evolution.