10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Composition and Characteristics of NR-PM1 Using HR-TOF-AMS over a Big Urban City of Western India
ATINDERPAL SINGH, Rangu Venkata Satish, Neeraj Rastogi, Physical Research Laboratory, Ahmedabad, India
Abstract Number: 461 Working Group: Aerosol Chemistry
Abstract Poor understanding on the characteristics and composition of non-refractory PM1 (mainly secondary aerosol) leads to large uncertainty in the assessment of their effects on air quality and climate. Towards this, it is important to study and understand their composition, characteristics and temporal evolution in different regions. Present study reports real-time characteristics of non-refractory submicron aerosol (NR-PM1) using high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) during post-monsoon season over a big urban city (Ahmedabad, 23.0 oN, 72.6 oE, 49m amsl) situated in a semi-arid region of western India.
Study region receives most of annual rainfall during monsoon season (June-September), which cleans up the atmosphere. Subsequently, primary and secondary aerosol start accumulating in the atmosphere. During a month-long study (September 19th to October 19th, 2017) started right after monsoon season, NR-PM1 varied from 1.9 to 102 µg m-3 (14±9.8 µg m-3; Avg±1σ), where NR-PM1 is composed of organic aerosol (OA), SO42-, NO3-, NH4+ and Cl-. All the species of NR-PM1 showed increasing trends in their mass concentrations with higher slope for OA. These changes in concentrations of species reflect the role of local/regional sources and atmospheric processes in charging up the atmosphere. OA was observed to be dominant contributor to NR-PM1 with the contribution of 55%, followed by sulfate (31%), and contribution of other ionic species were ≤ 11%. Aerosol neutralization ratio (ANR, defined as the molar ratio of NH4+ to sum of SO42-, NO3- and Cl-), an indicative of aerosol acidity, has a period average value of 0.85±0.07 with the highest value during afternoon hours (~0.91).
Diurnal trends exhibited lower O/C and higher H/C ratios during rush hours (peak at 08:00 and 20:00 hrs IST), which indicates the dominance of primary organics from vehicular emissions at these hours. Strong photochemical activity during afternoon hours was evident from high O/C ratios that leads to enhancement in organic mass due to secondary formation during these hours. Moderate peak in O/C at 04:00 hrs could be due to oxidation of precursors by nocturnal oxidants. Slope of Van Krevelen (O/C vs H/C) diagram is often used to understand the bulk composition of OA. In this study, it is found to be -0.77±0.35, which suggests that ambient OA is aging towards widely observed slope (-1) of OA i.e., carboxylation. It also infers that the observed OA was a variable mixture of fresh and aged OA during the study period. This study provides deeper insights of secondary aerosol abundances in ambient air, hither to lacking from this region.