Abstract. Nitrous acid (HONO) can strongly affect atmospheric photochemistry in polluted regions through the production of hydroxyl radicals (OHs). In January 2017, a severe pollution episode occurred in the Pearl River Delta (PRD) of China, with maximum hourly PM2.5, ozone, and HONO levels reaching 400 µg m−3, 150 ppb, and 8 ppb, respectively, at a suburban site. The present study investigated the sources and processes generating such high HONO concentrations and the role of HONO chemistry in this severe winter episode. Four recently reported HONO sources were added to the Community Multiscale Air Quality (CMAQ) model, including RH-dependent (relative humidity) and light-enhancing effects on heterogeneous reactions, photolysis of particulate nitrate in the atmosphere, and photolysis of HNO3 and nitrate on surfaces. The revised model reproduced the observed HONO and significantly improved its performance for O3 and PM2.5. The model simulations showed that the heterogeneous generation on surfaces (with RH and light effects) was the largest contributor (72 %) to the predicted HONO concentrations, with the RH-enhancing effects more significant at nighttime and the light-enhancing effects more important in the daytime. The photolysis of total nitrate in the atmosphere and deposited on surfaces was the dominant HONO source during noon and afternoon, contributing above 50 % of the simulated HONO. The HONO photolysis was the dominant contributor to HOx production in this episode. With all HONO sources, the daytime average O3 at the Heshan site was increased by 24 ppb (or 70 %), compared to the simulation results without any HONO sources. Moreover, the simulated mean concentrations of TNO3 (HNO3+ fine particle NO3-) at the Heshan site, which was the key species for this haze formation, increased by about 17 µg m−3 (67 %) due to the HONO chemistry, and the peak enhancement reached 55 µg m−3. This study highlights the key role of HONO chemistry in the formation of winter haze in a subtropical environment.

and mass-sensitive sensors, [9] but few of these have been demonstrated to be cost, power, and size effective. For instance, the widely commercialized resistance-based metal oxide sensors must typically be operated at high temperatures to enable the adsorption interactions required for transduction. [8,[10][11][12] This results in higher power consumption as operation temperatures must be adjusted by a built-in heater. While other efforts have focused on the optimization of sensing materials [4,13,14] and sensor structure, [15] the resulting devices remain far from being practically applicable due to their limited detection sensitivity and poor reproducibility under mass fabrication. [16] Therefore, effective gas sensing systems with minimal baseline drift, good selectivity, low hysteresis, and the ability to simultaneously measure multiple gases still need to be developed. In this context, silicon transistor-based sensors have shown significant promise, with key advantages in overcoming size limitations, low power sensing, and high sensitivity, [17][18][19][20][21] making them useful for trace-level gas sensing applications required in food freshness monitoring.Ammonia (NH 3 ) and hydrogen sulfide (H 2 S) are two types of marker gases for spoiling food. For high-protein foods such as eggs, dairy, and meat, off-gassed NH 3 and H 2 S serve as quality indicators of freshness. [22][23][24][25] These gases can also be emitted from rotting vegetables such as corn and spinach. [26] For simplicity, eggs and pork samples are selected for monitoring food spoilage in this work. Based on reported data, 10 mL of egg whites produces ≈100 µg of H 2 S over multiple hours. [23] After accounting for food storage volume and temperature, this means that the sensor system must be able to identify H 2 S and NH 3 gases with lower than 100 ppb detection limits and negligible cross-sensitivity. While electrochemical, colorimetric, and other sensing schemes in previous work have shown promise for gas and adulteration detection, it still remains to detect gas signatures continually and at low concentration levels for monitoring spoiling food. [24][25][26][27][28][29] Multiplexed sensing is also important-for example, humidity is another important parameter that affects food storage and spoilage, [22,[30][31][32][33][34] and thus should be simultaneously monitored with H 2 S and NH 3 . All of these requirements necessitate the deployment of sensors with high selectivity and low detection limits.Multiplexed gas detection at room temperature is critical for practical applications, such as for tracking the complex chemical environments associated with food decomposition and spoilage. An integrated array of multiple silicon-based, chemical-sensitive field effect transistors (CSFETs) is presented to realize selective, sensitive, and simultaneous measurement of gases typically associated with food spoilage. CSFETs decorated with sensing materials based on ruthenium, silver, and silicon oxide are used to obtain stable room-temperature responses t...

This paper presents an investigation on the humidity sensitivity of deposited multi-walled carbon nanotube (MWCNT) networks using ac dielectrophoresis (DEP) between interdigitated electrodes (IDEs). MWCNTs dispersed in ethanol were trapped and enriched between IDEs on a Si/SiO2 substrate under a positive DEP force. After the DEP process, the ethanol was evaporated and the MWCNT network on a substrate with IDEs was put into a furnace for repeated thermal annealing. It was found that the resistance stability of the network was effectively improved through thermal annealing. The humidity sensitivity was obtained by measuring the resistance of the MWCNT network with different relative humidity at room temperature. The experimental results show the resistance increases linearly with increasing the relative humidity from 25% to 95% RH with a sensitivity of 0.5%/%RH. The MWCNT networks have a reversible humidity sensing capacity with response time and recovery time of about 3 s and 25 s, respectively. The resistance is dependent on temperature with a negative coefficient of about −0.33%/K in a temperature range from 293 K to 393 K.