An improved understanding of the optical properties of secondary organic aerosol (SOA) particles is needed to better predict their climate impacts. Here, SOA was produced by reacting 1-methylnaphthalene or longifolene with hydroxyl radicals (OH) under variable ammonia (NH₃), nitrogen oxide (NO_x), and relative humidity (RH) conditions. In the presence of NH₃ and NO_x, longifolene-derived aerosols had relatively high single scattering albedo (SSA) values and low absorption coefficients at 375 nm independent of RH, suggesting that the longifolene SOA is mostly scattering. In 1-methylnaphthalene experiments, the resulting SSA and SOA mass absorption coefficient (MAC_org) values suggest the formation of light-absorbing SOA, and the addition of high NO_x and high NH₃ enhanced the SOA absorption. Under intermediate-NO_x dry conditions, the MAC_org values increased from 0.13 m² g⁻¹ in NH₃-free conditions to 0.28 m² g⁻¹ in high-NH₃ conditions. Under high-NH₃ conditions, the MAC^org value further increased to 0.36 m² g⁻¹ with an increase in RH. Under dry high-NO_x conditions, the MAC^org value increased from 0.42 to 0.67 m² g⁻¹ with the addition of NH³, while with elevated RH, the MAC^org value reached 0.70 m² g⁻¹. The time series of MAC_org showed increasing trends only in the presence of NH₃. Composition analysis of SOA suggests that organonitrates, nitroorganics, and other nitrogen-containing organic compounds (NOCs) are potential chromophores in the 1-methylnaphthalene SOA. Significant formation of NOCs was observed in the presence of high-NO_x and NH₃ and was enhanced under elevated RH.

The data have been collected in an environmental chamber, oxidizing 30-100 ppbv of 1-methylnaphthalene and 70-100 ppbv of longifolene under different levels of nitrogen oxides, ammonia, and relative humidity. Microphysical properties (size distribution and absorption and scattering coefficients) of SOA along with its composition were monitored throughout the experiment. Submitted data include the time series of the calculated mass absorption coefficient and single scattering albedo at 375 nm, derived real and imaginary components of the refractive index, organic nitrate to organic ratio, and organic and nitrate mass concentrations.

The data have been analyzed using Wavemetrics Igor Pro, specifically the software written specifically to analyze the data obtained from Aerodyne's aerosol mass spectrometers (i.e., Squirrel and Pika). Other calculations were also carried out in Igor Pro. More details are provided in the related manuscripts.

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