Evidence Of Organonitrate Formation At A High Altitude Site, Mahabaleshwar, During The Pre-Monsoon Season

The presence of organonitrate and organosulfate was found at Mahabaleshwar, a high altitude site, during the pre-monsoon season of 2016. A Time of Flight Aerosol Chemical Speciation Monitor (ToF-ACSM) was used to measure the organic and inorganic components of non-refractory particulate matter (NR-PM1) aerosol. Positive Matrix Factorization (PMF) was performed on the (i) organics mass spectra of the aerosol (PMFOA), (ii) organics mass spectra merged with inorganics (PMFOA+IOA) and (iii) integrated mass spectra of organics with NO+ and NO2+ ions (PMFOA+NOx) to derive the chemical information on organonitrate and organosulfate. The results of PMFOA were used as a reference for validating the factors obtained through the PMFOA+IOA and PMFOA+NOx results. The analysis of PMFOA resolved four PMF factors: hydrocarbon-like OA (HOA); biomass burning OA (BBOA); oxygenated OA-1 (OOA-1) and OOA-2. The analysis of PMFOA+IOA identified two additional inorganic factors: sulfate organic aerosol (OA) and nitrate OA. Sulfate OA and nitrate OA contributed 36% and 6%, respectively, to the total aerosol mass. Although both originated as secondary organic aerosol, they displayed different diurnal profiles. The results of PMFOA+NOx were used for the quantification and apportionment of nitrate aerosol in two forms, organic nitrate and inorganic nitrate, which contributed 38% and 62%, respectively, to the total nitrate aerosol mass. The diurnal variation in organic nitrate highlights photochemical oxidation and nocturnal oxidation by the nitrate radical as the two major sources. This source apportionment study using a combined (organic and inorganic) dataset provides new source factors and improves our understanding about the sources and chemical nature of submicron aerosols in the atmosphere. However, uncertainties in the quantification of organosulfate remain a limitation.