Monitoring of Glyoxal Column densities in different forest type of Pakistan by using MAX-DOAS

Abstract

Forests act as a contributor to the global emissions of biogenic volatile organic compounds (BVOCs). Measurements of their oxidation products, such as glyoxal (CHOCHO), provide useful indicators of fast photochemistry occurring in the lower troposphere. However, measurements of these species in forests location are extremely limited. Air pollution has become a serious issue in Pakistan and Forest also contribute in degrading the ambient air quality, therefore, air quality in forest ecosystem must be monitored continuously to understand the likely impacts of air pollutants on the atmosphere, environment and particularly human health. To address this, CHOCHO concentrations was measured using the multi-axis (MAX) differential optical absorption spectroscopy (DOAS) techniques in different Forest types, and Capital city of Pakistan. Glyoxal being the tiniest α-dicarbonyl forms during VOC oxidation reactions. It has residence time of few hours determined by photolysis and hydroxyl radical reactions during day time. In the presence of NOx, glyoxal photolysis results in the formation of secondary organic pollutants along with the tropospheric ozone. This is a pioneer study for glyoxal retrieval in Pakistan’s Forest. It presents a comparison of glyoxal retrieval from mini MAX-DOAS measurements at different monitoring sites of Pakistan Forest which include, Sub-Tropical Chir-pine Forest, Himalayan Moist Temperate Forest, Himalayan Dry Temperate Forest, Sub-Alpine Pasture and Alpine Pasture. At first CHOCHO data was obtained during Field campaigns. Next data set was generated from ground-based MAX-DOAS observations at IESE-NUST (Institute of Environmental Sciences and Engineering - National University of Sciences and Technology), Islamabad, Pakistan. Glyoxal differential slant column densities (DSCDs) were retrieved using DOAS (differential optical absorption spectroscopy) technique. And by using geometric air mass factor approach, tropospheric vertical column densities (VCDs) were derived from measured DSCDs. The retrieved glyoxal was compared over the all Forest sites in respective years. Diurnal, Weekly and Annual Seasonal cycle of glyoxal was investigated for data set of IESE from September-2016 to August-2018. Results showed that CHOCHO concentrations was higher in morning and evening but lower in the noon due to photolysis. It was also observed that CHOCHO was lowest on Friday, while annual seasonal cycle of CHOCHO shows highest average concentrations in Summers and lowest in Winters over Islamabad. A considerably good correlation of 0.72 and 0.59 was found between MAX-DOAS observations of CHOCHO with temperature and GHI and a positive correlation of 0.44 was observed between MAX-DOAS measurements and NDVI. During field campaigns of different forest types, the CHOCHO concentrations were found to be maximum at Alpine Pasture and lowest at Chir-pine Forest. An anticorrelation of -0.11 was found between the CHOCHO concentration and average temperature and anticorrelation of 0.70 was found between relative humidity and CHOCHO concentrations.