Elevated airborne submicron particle counts linked to direct evaporative cooling in greenhouses

• Direct evaporative cooling elevated airborne nanoparticle counts in greenhouses • The counts increased markedly from 0.15 m to 2.0 m downstream of cooling pads • Evaporation of saline droplets in greenhouses contributed to nanoparticle formation • Gas-particle conversion due to lower temperature and higher humidity is also likely Direct evaporative cooling (DEC) systems are widely used for climate control in agricultural, industrial, and residential environments. However, prolonged operation can lead to mineral scaling and biofilm fouling on cooling pads. To mitigate these issues, chemical additives are commonly introduced into the recirculating water. During system operation, fine saline droplets, similar to those produced by atomization, may contribute to airborne submicron particle (SP) formation, a phenomenon not yet investigated. This study compared airborne SP counts and size distributions in a greenhouse during active versus idle periods and accessed spatial distributions using three outdoor and nine indoor monitoring locations. Results revealed that indoor SP counts averaged 3,117±223 particles/cm 3 with the DEC system off and increased to 3,437±720 particles/cm 3 when on, as measured using a TSI NanoScan SMPS. Similar trends were observed using a CPC 3007 (off: 5,369±271 particles/cm 3 ; on: 6,416±1,128 particles/cm 3 ). SP counts increased significantly from 0.15 m downstream of the cooling pads to 2.1 m downstream, followed by a slight decrease at 4.0 m. This pattern contrasts with the gradual decline observed at these locations when the system was off. Particle size analysis revealed increased SP counts at 2.0 m downstream primarily occurred in the 11.5-27.5 nm, consistent with nanoparticles generated via atomization of diluted salt solutions. Overall, findings indicate that DEC operation can elevate airborne SP concentrations in built environments.