Global and regional impacts of land cover changes on isoprene emissions derived from spaceborne data and the MEGAN model

Abstract

Abstract. Among the biogenic volatile organic compounds (BVOCs) emitted by plant foliage, isoprene is by far the most important in terms of both global emission and atmospheric impact. It is highly reactive in the air, and its degradation favours the generation of ozone (in the presence of NOx) and secondary organic aerosols. A critical aspect of BVOC emission modelling is the representation of land use and land cover (LULC). The current emission inventories are usually based on land cover maps that are either modelled and dynamic or satellite-based and static. In this study, we use the state-of-the-art Model of Emissions of Gases and Aerosols from Nature (MEGAN) model coupled with the canopy model MOHYCAN (Model for Hydrocarbon emissions by the CANopy) to generate and evaluate emission inventories relying on satellite-based LULC maps at annual time steps. To this purpose, we first intercompare the distribution and evolution (2001–2016) of tree coverage from three global satellite-based datasets, MODerate resolution Imaging Spectroradiometer (MODIS), ESA Climate Change Initiative Land Cover (ESA CCI-LC), and the Global Forest Watch (GFW), and from national inventories. Substantial differences are found between the datasets; e.g. the global areal coverage of trees ranges from 30 to 50×106 km2, with trends spanning from −0.26 to +0.03 % yr−1 between 2001 and 2016. At the national level, the increasing trends in forest cover reported by some national inventories (in particular for the US) are contradicted by all remotely sensed datasets. To a great extent, these discrepancies stem from the plurality of definitions of forest used. According to some local censuses, clear cut areas and seedling or young trees are classified as forest, while satellite-based mappings of trees rely on a minimum height. Three inventories of isoprene emissions are generated, differing only in their LULC datasets used as input: (i) the static distribution of the stand-alone version of MEGAN, (ii) the time-dependent MODIS land cover dataset, and (iii) the MODIS dataset modified to match the tree cover distribution from the GFW database. The mean annual isoprene emissions (350–520 Tg yr−1) span a wide range due to differences in tree distributions, especially in isoprene-rich regions. The impact of LULC changes is a mitigating effect ranging from 0.04 to 0.33 % yr−1 on the positive trends (0.94 % yr−1) mainly driven by temperature and solar radiation. This study highlights the uncertainty in spatial distributions of and temporal variability in isoprene associated with remotely sensed LULC datasets. The interannual variability in the emissions is evaluated against spaceborne observations of formaldehyde (HCHO), a major isoprene oxidation product, through simulations using the global chemistry transport model (CTM) IMAGESv2. A high correlation (R > 0.8) is found between the observed and simulated interannual variability in HCHO columns in most forested regions. The implementation of LULC change has little impact on this correlation due to the dominance of meteorology as a driver of short-term interannual variability. Nevertheless, the simulation accounting for the large tree cover declines of the GFW database over several regions, notably Indonesia and Mato Grosso in Brazil, provides the best agreement with the HCHO column trends observed by the Ozone Monitoring Instrument (OMI). Overall, our study indicates that the continuous tree cover fields at fine resolution provided by the GFW database are our preferred choice for constraining LULC (in combination with discrete LULC maps such as those of MODIS) in biogenic isoprene emission models.

Reply on RC2

“…When annually changing ESA-CCI data are implemented, the trend decreases to 0.24 % yr -1 . Similar observation was made by Opacka et al (2021), who used a modified MODIS land cover data in the MEGAN-MOHYCAN emission model to study the impact of land cover change on isoprene emissions. They found a 0.04 to 0.33 % yr -1 mitigating effect of land cover change on general positive trends of isoprene induced mainly by temperature and solar radiation.…”

Spaceborne evidence for significant anthropogenic VOC trends in Asian cities over 2005–2019

“…The discrepancy could be related to the large deforestation rates observed in the humid primary forests located within a radius of 150 km around the city (Hansen et al 2013). When considering the effects of land use changes on isoprene fluxes (Opacka et al 2021), the isoprene emissions around Phnom Penh over 2005-2019 are found to decline by about 1% yr −1 , suggesting that the trend in HCHO columns might be partly due to forest loss in the regions surrounding Phnom Penh.…”

Comparative assessment of TROPOMI and OMI formaldehyde observations and validation against MAX-DOAS network column measurements

“…Satellite observations of formaldehyde columns in the troposphere have been extensively reported in the literature from a number of nadir UV sensors, e.g. the Global Ozone Monitoring Experiment (GOME; Chance et al, 2000;Palmer et al, 2001;De Smedt et al, 2008), SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY; Wittrock et al, 2006;De Smedt et al, 2008, Ozone Monitoring Instrument (OMI;De Smedt et al, 2015Kaiser et al, 2018;Levelt et al, 2018), Global Ozone Monitoring Experiment-2 (GOME-2; De Vrekoussis et al, 2010;Hewson et al, 2013;Hassinen et al, 2016) and Ozone Mapping and Profiler Suite (OMPS; Li et al, 2015;González Abad et al, 2016). They are used in many studies related to air quality and climate change (e.g.…”