Abstract

ABSTRACT 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.

Highlights

  • Organic aerosol (OA) makes up a substantial fraction of tropospheric fine particulate matter, accounting for 20–90% of the total aerosol mass

  • This study mainly focuses on the identification of organic sulfate and organic nitrate in the ambient air at High Altitude Cloud Physics Laboratory (HACPL) during the pre-monsoon season

  • To obtain intrinsic information on the interactions between organic and inorganic components of the aerosol, Positive Matrix Factorization (PMF) analysis was applied to both the organic mass spectra merged with inorganics (PMFOA+inorganic aerosol (IOA)) and the organic mass spectra integrated with NO+ and NO2+ ions (PMFOA+NOx)

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Summary

Introduction

Organic aerosol (OA) makes up a substantial fraction of tropospheric fine (submicron) particulate matter, accounting for 20–90% of the total aerosol mass This important component of submicron aerosol is originated from a wide variety of natural (wildfires) and anthropogenic sources (fossil fuel combustion and biomass burning). Organonitrates have been identified as significant component of NOy in the gas phase (Nielsen et al, 1998; Day et al, 2003; Perring et al, 2009) and is likely to affect the nitrogen cycle and ozone production (Rosen et al, 2004; Mao et al, 2012; Perring et al, 2013). Despite lower ON yields during the daytime, model studies including ON production have reported that photochemically produced ON accounts for the significant fraction of SOA functionalization (Camredon et al, 2007)

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