Nitrogen dioxide (NO2) is a significant pollutant [1]. It is predominantly produced through combustion processes [2], including emissions from vehicles, industrial activities, and other forms of burning [[3], [4], [5], [6]]. The concentration of NO2 is continuously rising worldwide, which is toxic and a trace gas [7]can lead to increased respiratory diseases and other health issues [8,9]. Furthermore, NO2 participates in photochemical reactions in the atmosphere, facilitating ozone and particulate matter formation [10,11], which exacerbates environmental pollution and threatens ecosystem stability. Consequently, continuous and reliable monitoring of nitrogen dioxide is essential [12].

With societal advancements, gas sensors are widely used to detect toxic and harmful gases [13], and have become increasingly important in various fields [14], including environmental protection and monitoring [15], medical diagnostics [16], and industrial process control [17]. Photoacoustic spectroscopy is a powerful analytical technique [18]. It is increasingly recognized as an important method for trace gas detection due to its unique principles and excellent detection capabilities. It offers advantages such as high selectivity and a broad detection range [19]. It is widely recognized for its excellent performance in trace gas detection ranging from ppb to ppt levels [20]. In recent years, considerable progress has been achieved in developing NO2 measurement systems based on photoacoustic spectroscopy:Bernhardt developed an LED-based photoacoustic system. The separate signals generated in the two resonators achieved greater signal and common-mode noise suppression, enabling detection of NO2 concentrations down to 60 ppb [21]. In 2015, Peltola et al. utilized a cantilever beam sensor and a high-power 532 nm laser, reaching a minimum detection limit in the range of 10−12 [22]. Zheng developed a strategy to mitigate background noise caused by stray light in QEPAS NO2 sensors, achieving ppb-level detection of NO2 [23]. In 2021, Li and colleagues enhanced the intensity of photoacoustic signals by continuously reflecting the laser in a diffusive sphere, constructing a detection system for NO2 [24]. In 2023, Yuan et al. designed a measurement system that achieved a detection limit of 4.85 ppb with a photoacoustic cell volume of only 18.85 mL [25]. Panzard et al. proposed a cost-effective and simplified system capable of simultaneously detecting CO2 and NO2 in exhaust emissions, demonstrating significant potential for continuous emission monitoring [26].

Rounded-corner photoacoustic cells exhibit enhanced noise reduction and detection capabilities compared to traditional cylindrical designs. However, research on these rounded-corner configurations is still underexplored. Although higher incident laser power enhances photoacoustic signals, the use of highpower lasers is often constrained by their cost and bulkiness [[27], [28], [29], [30]].

Based on the above analysis, this study proposes several key contributions. A new rounded photoacoustic cell with optimized dimensions was manufactured using COMSOL software for modeling and structural design. A high refractive index plane mirror was added to the system and iterative testing was conducted to develop a dual path configuration. In addition, the system incorporates suitable optical isolators to protect the system from damage caused by reflected laser light. At the same time, this study conducted laboratory tests and long-term atmospheric environmental tests. The experimental results show that the proposed photoacoustic spectroscopy system has superior performance, robustness, high sensitivity, and ease of operation.