Characteristics of secondary aerosol formation during shortened multiday reaction experiments in a smog chamber: Effects of relative humidity and ammonia

Shortened multiday reaction experiments were conducted using the KIST chamber for atmospheric processes simulation (K-CAPS) to characterize the effects of ammonia (NH₃) and relative humidity (RH) on the formation of secondary organic aerosols (SOA) due to photooxidation of a mixture of toluene and inorganic gases such as NO_x, SO₂, and NH₃. UV lamps were repeatedly turned on for 3 h (daytime) and off for 6 h (nighttime), and precursors were injected to a reaction bag once (Multiday Initial injection, MI) or repeatedly (Multiday Cyclic injection, MC) to simulate high particulate matter episode due to foreign inflow episode and domestic stagnation episodes, respectively. As a result, the amount of SOA formed in the humid (RH 80 %) MI experiments with ammonia was approximately 1.1 times more than in the traditional single day experiment and approximately 1.6 times more than in the MC experiment, implying that aging processes including nighttime effects without additional emission of precursors during transport can produce more SOA as reactions progressed further under the experimental conditions of this study. The higher the initial RH, the more SOA was formed, with a slope increasing approximately 1.2 μg/m³ per unit RH, and the shorter run time required for SOA to increase to 30 μg/m³ (twice the WHO PM₁₀ standard), with a slope decreasing approximately 0.3 h per unit RH, implying that more humid condition caused during long-range transport across the oceans is one of the possible reasons of high secondary aerosol formation. The SOA formation was reduced by approximately 60 % in the absence of ammonia, suggesting that ammonia reduction is needed to decrease not only secondary inorganic aerosols but also SOA. These results are useful to understand the major reason of high pollution of particulate matters by episode cases in urban areas.