Characteristics of size-resolved organic aerosols using offline aerosol mass spectrometry: NYC-METS

To study the size-resolved organic aerosol composition in New York City, daily aerosol samples were collected using a Micro-Orifice Uniform Deposition Impactor (MOUDI) during the Summer 2023 New York City Metropolitan Measurements of Emissions and TransformationS (NYC-METS) campaign. Organic aerosol mass loading, size distribution, composition, and elemental ratios were measured using offline analysis with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-TOF-AMS). Most organic aerosol mass was found in the size range of 0.32–0.56 µm and a unimodal size distribution was observed (0.056–5.6 µm). Higher aerosol mass loadings were observed under fire and hazy conditions compared to cleaner conditions. Across all conditions, more highly oxidized species (CHO and CHOx) were found in the 0.18–1 µm size range. A novel method for functional group quantification from AMS data was applied. This showed a higher percentage of carboxylic acid groups on stages 6–8 (0.18–1 µm) which suggests more extensive secondary organic aerosol formation or aging processes influencing that size range. The offline-AMS results were compared to measurements from an in situ online-AMS and both datasets exhibited similar trends in the mass concentrations with time and composition. However, for aerosol samples collected during fire and hazy conditions, some volatile components were likely lost before offline sample analysis, leading to higher oxygen-to-carbon (O/C) ratios in the offline measurements. The combination of MOUDI with the offline-AMS provides unique insights into the size-resolved composition of organic aerosol particles in New York City.