## 1 Introduction  
Lead halide perovskites are receiving much attention as potentially interesting novel active materials for electronic, optoelectronic, and photonic devices, due to their unique combination of the active properties typical of semiconductors, wide degrees of freedom for their engineering, and possible realization and deposition with easy techniques. In particular, perovskite nanocrystals (NCs) are very interesting due to the possibility to engineer the optical properties by acting on the chemical composition [1], the size, the shape [2, 3], and even the crystalline packing dimensionality within each NC [4]. Moreover, efficient NCs electroluminescence [5] and low threshold amplified spontaneous emission [6, 7, 8, 9] have been demonstrated, thus proposing them as possible active materials in LEDs and lasers. Despite the very rapid performance increase of perovskite-based solar cells [10], LEDs [11], and lasers [12], the applicative perspectives of these materials are currently limited by the lack of long term stability, thus strongly stimulating the research on the understanding of the processes leading to their degradation. To date, it has been demonstrated that lead halide perovskites can be degraded by many factors such as temperature [13], light exposure [13; 14; 15; 16; 17], and interaction with oxygen and moisture [18; 19; 20; 21]. Interestingly, despite the reports of material degradation due to its interaction with air, several experiments demonstrated that the interaction with oxygen and wet air can also improve the light emission and the device performances of bulk polycrystalline thin films [22; 23; 24; 25] and single crystals [26; 27]. This effect, typically reversible, is generally ascribed to the oxygen passivation of halide vacancies taking place at the air-perovskite interface, which decreases the excitation non-radiative decay rate, thus increasing the photoluminescence (PL) intensity and decreasing the PL decay rate [22; 23; 24; 25; 26; 27]. The evidence that the optical properties variation is mainly related to the interaction between the perovskite surface and the atmosphere allows expecting a strong environment dependence of the emission properties of NCs, characterized by a much higher surface/volume ratio. Despite this, the effects of the environment on the optical properties of lead halide perovskites NCs have been surprisingly poorly investigated to date, with only few experiments on CsPbBr\({}_{3}\) nanocubes grown by hot-injection method, curiously finding contradictory results. In particular, the PL intensity in air has been reported to be lower than in vacuum [3; 28], or not affected by the environment [27]. These results are different from the ones on single crystals, bulk polycrystalline thin films, and even nanosheets and nanowires [3], all showing a PL intensity enhancement in air. In this study, we investigated the role of the synthesis method and of the sample aging on the PL spectra and on the exciton relaxation dependence on the environment in thin films of high quality CsPbBr\({}_{3}\) NCs grown by a co-precipitation method. We demonstrate that, differently from what is observed in NCs grown by hot-injection, the PL intensity is higher in wet air (WA, relative humidity \(\approx\)50%) than in vacuum (V) both in fresh (#F) and in aged (#A) films, ascribed to reversible defects passivation. We also show that the interaction with WA results both in static and in dynamic effects, leading to a variation of the number of emitting NCs and of the exciton relaxation pathways. In Sample #F, the main effects are a decrease of free exciton (FE) trapping, increasing the relative weight of FE radiative recombination, and the suppression of a non-radiative decay process for bound excitons (BE). The increase of surface defects in Sample #A [29] results in a PL intensity increase in WA about nine times stronger than that of Sample #F, due the combination of a strong static increase of the number of emitting NCs and of a reduction of the non-radiative decay rate of FE.