Modeling of ground - level ozone formation in urban air - sheds of Sri Lanka

Abstract

Physical phenomenon of the relation among ground-level ozone (O 3 ), oxides of nitrogen (NO X ) and volatile organic compounds (VOC) is governed by complex nonlinear photochemistry. To predict and control O 3 concentration, it is vital to know, how O 3

concentration changes in response to prescribed changes in source emissions of NO X

and VOCs. In this research, a theoretical model was developed and validated for ground-level O 3 formation in urban air-sheds of Sri Lanka. Hourly averaged weekly results of ambient pollutant concentration data of eleven cities in the base years 2013, 2014 and 2015 in Sri Lanka was assessed and an urban air shed model was developed. The model was calibrated using influential parameters measured. Then Colombo as the most complicated urban air-shed in Sri Lanka was analyzed in detail. Model was validated using measured 24-hour air quality monitoring data from the mobile air quality monitoring stations at major traffic locations in Colombo in the year 2018 and 2019. Operational schedules of emission sources including train scheduled data, working hours of the thermal power plants, and vehicle counts were conducted at identified critical locations in Colombo to investigate the responsible sources. Gases from the exhaust line of different types of vehicles was collected and analyzed using Gas Chromatography Mass Spectroscopy (GCMS).

Results confirm that there exist two regimes of NO X -VOC-O 3 sensitivity as NO x sensitive regime

and VOC-sensitive regime. The urban air-shed model is capable of estimating the ground-level steady state ozone concentration (O 3ss ) and contributions from each regime. The univariate linear regression model using predicted and observed O 3 values confirmed that O 3ss concentration was significantly correlated with the predicted O 3 concentration. Analysis of urban air shed in Colombo also confirms the predicted and observed O 3ss concentration were significantly correlated. This research provides a detailed understanding of photochemical degradation on formation of ground-level O 3 in urban air-sheds of Sri Lanka and provides critical information for the scientific community and decision-makers to formulate air pollution mitigation policies.