Effect of relative humidity, NOx, and ammonia on the physical properties of naphthalene secondary organic aerosols

Abstract

Light-absorbing organic aerosols, commonly referred to as “brown carbon” (BrC), have important climate effects. The primary source of BrC is biomass burning, but additional BrC can be formed by less constrained secondary atmospheric reactions, such as photooxidation of aromatic volatile organic compounds (VOCs) and reactions of carbonyl compounds in secondary organic aerosols (SOAs) with ammonia (NH₃). As NH₃ and VOC emissions increase globally, these insufficiently understood BrC production pathways may gain in importance relative to the primary BrC sources. This study examines the effects of NH₃, NO_x, and relative humidity (RH) on the optical and molecular properties of naphthalene (NAP) photooxidation SOAs. NAP SOAs were prepared by OH-initiated oxidation in a smog chamber at high and low levels of NO_x, RH, and NH₃. SOA hygroscopicity and optical properties were monitored online, and SOA samples were collected on filters for offline analysis. UV-visible spectroscopy and high-resolution mass spectrometry were used to determine mass absorption coefficients and molecular composition, respectively. Of the three parameters studied, RH exhibited the largest effects on both composition and optical properties – with an increase in the 〈O/C〉 ratio, relative dimer abundance, and absorption at wavelengths longer than 350 nm. Elevated NO_x and NH₃ concentrations increased the amount of nitrogen incorporated into the SOA, and NO_x increased the amount of absorption at wavelengths longer than 350 nm while NH₃ had little effect. The combination of elevated humidity and NO_x led to the formation of 6-nitro-2-naphthol, a strong chromophore with a distinct peak at 400 nm in the absorption data. We find that RH, NO_x, and NH₃ all affect the characteristics of NAP SOAs in different ways: NO_x and RH control both molecular composition and optical properties, while NH₃ has a stronger effect on composition and a minor effect on the absorption coefficient.