Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Emission controls have substantially brought down aerosol pollution in China, however, aerosol mass reductions have slowed down in recent years in the Pearl River Delta (PRD) region, where secondary organic aerosol (SOA) formation poses a major challenge for air quality improvement. In this study, we characterized the roles of SOA in haze formation in urban Guangzhou City of the PRD using year-long aerosol mass spectrometer measurements for the first time and discussed possible pathways of SOA formations. On average, organic aerosols (OA) contribute dominantly (50 %) to non-refractory submicron aerosol mass (NR-PM₁). The average mass concentration of SOA (including by less and more oxidized OA, LOOA and MOOA) contributed most to NR-PM₁, reaching about 1.7 times that of primary organic aerosols (POA, including hydrocarbon-like and cooking-related OA) and accounting for 32 % of NR-PM₁, even more than sulfate (22 %) and nitrate (16 %). Seasonal variations of NR-PM₁ revealed that haze formation mechanisms differed much among distinct seasons. Sulfate mattered more than nitrate in fall, while nitrate was more important than sulfate in spring and winter, with SOA contributing significantly to haze formations in all seasons. Daytime SOA formation was weak in winter under low oxidant level and air relative humidity, whereas prominent daytime SOA formation was observed in fall, spring and summer almost on daily basis, suggesting for important roles of photochemistry in SOA formations. Further analysis showed that the coordination of gas-phase photochemistry and subsequent aqueous-phase reactions likely played significant roles in quick daytime SOA formations. Obvious nighttime SOA formations were also frequently observed in spring, fall and winter, and it was found that daytime and nighttime SOA formations together had resulted in the highest SOA concentrations in these seasons and contributed substantially to severe haze formations. Simultaneous increases of nitrate with SOA after sunset suggested the important roles of NO₃ radical chemistry in nighttime SOA formations, and confirmed by continuous increase of NO⁺/NO₂⁺ fragment ratio after sunset. Findings of this study have promoted our understanding in haze pollution characteristics of the PRD and laid down future directions on investigations of SOA formation mechanisms in urban areas of southern China that share similar emission sources and meteorological conditions.