Characterizing lightning NOx production over South Africa

Abstract

Nitrogen oxides (NOx= nitric oxide (NO) + Nitrogen dioxide (NO2)) are toxic air pollutants and play a significant role in tropospheric chemistry. Global NOx hotspots are the industrialized regions of the United States, Europe, the Middle East, East Asia and the eastern parts of South Africa. Lightning is one of the tropospheric NOx sources and the only natural source far from the Earth’s surface. NOx poses a threat to air quality and the health of humans; as a result, NO2 is regulated by the National Ambient Air Quality Standard (NAAQIS). It plays a role in the formation of particulate matter (PM) and tropospheric ozone (O3), which are both linked to adverse health and climate effects. The NOx budget mainly comprises of anthropogenic rather-than natural sources. However, lightning is known to be the main source of tropospheric NOx globally, it is, therefore important to understand its contribution to the national and global NOx budget. This study characterizes the spatiotemporal variability of lightning and the associated NOx production over South Africa. The South African Weather Service operates a network of lightning detectors (LDN) over South Africa, which monitors cloud-to-ground lightning strikes. The system theoretically has a detection efficiency of 90% and a location accuracy of 0.5 km. The lightning events recorded by the LDN are used to approximate the influence of lightning on the NOx load over South Africa, and to develop a gridded dataset of lightning-produced NOx (LNOx) emissions over the country for the period 2008-2015. An emission factor of 11.5 kg NO2/flash was employed to calculate the LNOx budget of 270 kt NO2/year. The calculated LNOx was 14% of the total NOx emission estimates published in the EGDARv4.2 dataset for the year 2008. The results indicate that the NOx production is not distributed uniformly, with elevated areas having high LNOx production and the western parts of the country depicting lower emissions. LNOx production peaks in the late afternoon when thunderstorm activity starts to occur. Seasonally, the LNOx production was higher during summer, due to meteorological conditions being favourable for thunderstorm occurrence. In winter, the production was low due to most rainfall occurring because of frontal systems passing along the coastline of the country. Overall, the results indicate that both lightning and industrial NOx sources are essential in evaluating NOx and tropospheric O3 chemistry over South Africa. LNOx emissions are projected to increase with climate change, which can lead to a rise in tropospheric O3. Having LNOx emission inventory as input into air quality modelling will improve model performance and forecasting, and the understanding of the sensitivity of ambient pollution to changes in lightning emission. This will improve the regional emission inventories to inform chemical transport modelling so that the contribution of natural and anthropogenic sources can be better understood.