3. Sampler preparation NO2 was collected using a silica/C-18 cartridge (300 mg, Waters, USA) impregnated by passing around 10 mL of triethanolamine solution through it using a syringe. Excess TEA solution was then removed by passing pure NO2-free air through the cartridge for around 10 min. The prepared C-18 cartridges were then stored in sealed plastic bags in the refrigerator, until used.