Linear and Non-Linear Trends for Seasonal NO2 and SO2 Concentrations in the Southern Hemisphere (2004−2016)

Adrián Yuchechen,Pablo Canziani,Susan Lakkis

doi:10.3390/rs9090891

Abstract

In order to address the behaviour of nitrogen dioxide (NO2) and sulphur dioxide (SO2) in the context of a changing climate, linear and non-linear trends for the concentrations of these two trace gases were estimated over their seasonal standardised variables in the Southern Hemisphere—between the Equator and 60° S—using data retrieved by the Ozone Monitoring Instrument, for the period 2004–2016. A rescaling was applied to the calculated linear trends so that they are expressed in Dobson units (DU) per decade. Separately, the existence of monotonic—not necessarily linear—trends was addressed by means of the Mann-Kendall test. Results indicate that the SO2 exhibits significant linear trends in the planetary boundary layer only; they are present in all the analysed seasons but just in a small number of grid cells that are generally located over the landmasses or close to them. The SO2 concentrations in the quarterly time series exhibit, on average, a linear trend that is just below 0.08 DU decade−1 when significant and not significant values are considered altogether, but this figure increases to 0.80 DU decade−1 when only the significant trends are included. On the other hand, an important number of pixels in the lower troposphere, the middle troposphere, and the lower stratosphere have significant monotonic upward or downward trends. As for the NO2, no significant linear trends were found either in the troposphere or in the stratosphere, yet monotonic upward and downward trends were observed in the former and latter layers, respectively. Unlike the linear trends, semi-linear and non-linear trends were seen over the continents and in remote regions over the oceans. This suggests that pollutants are transported away from their sources by large-scale circulation and redistributed hemispherically. The combination of regional meteorological phenomena with atmospheric chemistry was raised as a possible explanation for the observed trends. If extrapolated, these trends are in an overall contradiction with the projected emissions of both gases for the current century.

Highlights

There is widespread consensus that changes in the composition of the Earth’s atmosphere caused by human activities play a relevant role in the Earth’s climate system
The number of research efforts devoted to identifying different linkages between the concentration of pollutants and the meteorological conditions is on the rise in both hemispheres, but they are more numerous in the Northern Hemisphere due to a number of factors that include this hemisphere having a large number of megacities, and the highest concentration of regions with strong industrial activity
A study carried out for China found that the discrepancy in the relative values of the emissions and the SO2 concentrations in the lower troposphere owing to a change in local meteorological conditions can represent up to 20–30%

Summary

Introduction

There is widespread consensus that changes in the composition of the Earth’s atmosphere caused by human activities play a relevant role in the Earth’s climate system. Unlike the greenhouse gases that induce a positive radiative forcing, aerosol particles influence the global radiation budget causing a net negative radiative forcing associated with a cooling effect on the atmosphere The radiative contribution of the aerosols can be centred in three main categories depending on what they interact with: aerosols-surface, aerosols-radiation and aerosols-clouds, or considering the aerosols’ influence on the atmosphere as a direct, indirect, or semi-direct effect [2,3,4,5]. In addition to the radiative influence, aerosols are significant contributors to air pollution and they have a direct linkage with the biogeochemical cycles of the atmosphere, the oceans and the surfaces, acting as micronutrients for the marine and terrestrial biosphere. Aerosol deposition can have detrimental environmental effects (e.g., the acidification of precipitation by sulphurs [6]) with impacts on the aquatic and terrestrial ecosystems [7,8,9], yet the benefits or the detrimental effects on ecological processes depend upon both the amount and composition of deposition and the underlying ecosystem conditions.

Methods

Results

Discussion

Conclusion