Source and variability of formaldehyde (HCHO) at northern high latitude: an integrated satellite, ground/aircraft, and model study

Here we use satellite observations of HCHO vertical column densities (VCD) from the TROPOspheric Monitoring Instrument (TROPOMI), ground-based and aircraft measurements, combined with a nested regional chemical transport model (GEOS-Chem at 0.5° × 0.625° resolution), to understand the variability and sources of summertime HCHO better in Alaska. We first evaluate GEOS-Chem with in-situ airborne measurements during Atmospheric Tomography Mission 1 (ATom-1) aircraft campaign and ground-based measurements from Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS). We show reasonable agreement between observed and modeled HCHO, isoprene and monoterpenes. In particular, HCHO profiles show spatial homogeneity in Alaska, suggesting a minor contribution of biogenic emissions to HCHO VCD. We further examine the TROPOMI HCHO product in Alaska during boreal summer, which is in good agreement with GEOS-Chem model results. We find that HCHO VCDs are dominated by free-tropospheric background in wildfire-free regions. During the summer of 2018, the model suggests that the background HCHO column, resulting from methane oxidation, contributes to 66–80 % of the HCHO VCD, while wildfires contribute to 14 % and biogenic VOC contributes to 5–9 % respectively. For the summer of 2019, which had intense wildfires, the model suggests that wildfires contribute to 40 to 65 %, and the background column accounts for 30 to 50 % of HCHO VCD in June and July. In particular, the model indicates a major contribution of wildfires from direct emissions of HCHO, instead of secondary production of HCHO from oxidation of larger VOCs. We find that the column contributed by biogenic VOC is often small and below the TROPOMI detection limit. The source and variability of HCHO VCD above Alaska during summer is mainly driven by background methane oxidation and wildfires. This work discusses challenges for quantifying HCHO and its precursors in remote pristine regions.

Measurement report: Biogenic VOC emissions profiles of Rapeseed leaf litter and their SOA formation potential

We analysed the biogenic volatile organic compounds (BVOC) emissions from rapeseed leaves litter and their potential to create secondary organic aerosols (SOA) under three different conditions i.e., (i) in presence of UV light irradiation; (ii) in presence of ozone, and (iii) with both ozone and UV light. These experiments have been performed in a controlled atmospheric simulation chamber containing leaves litter samples, where BVOC and aerosol number concentrations have been measured for 6 days. Our results show that BVOC emission profiles were affected by UV light irradiation, which increased the summed BVOC emissions compared to the experiment with solely O3. Furthermore, the diversity of emitted VOCs from the rapeseed litter increased also in presence of UV light irradiation. SOA formation was observed when leaves litter were exposed to both UV light and O3, indicating a potentially large contribution to particle formation or growth at local scales. To our knowledge, this study investigates for the first time the effect of UV irradiation and O3 exposure on both VOC emissions and SOA formation for leaves litter samples. A detailed discussion about the processes behind the biological production of the most important VOC is proposed.

Herbivore Gender Effects on Volatile Induction in Aspen and on Olfactory Responses in Leaf Beetles

Hybrid aspen (Populus tremula × tremuloides Michx.) is a fast-growing tree species used for short-rotation forestry in northern latitudes. Aspen species have a rich herbivore fauna, including defoliating leaf beetles that induce emissions of volatile organic compounds (VOCs) when feeding on aspen leaves. We investigated the differential induction of VOCs by male and female Phratora laticollis leaf beetles feeding on hybrid aspen and the differences in the orientation of beetles in response to gender-specific induced VOCs. The hypotheses for the study were (1) the VOCs in the headspace of plants infested with beetles of the two genders individually and in mixed aggregates would vary subtly, and (2) foraging adult beetles would be able to detect differences in VOC blends and use them to fine-tune their orientation choices. In Y-tube bioassays, both females and males preferred VOCs from leaves damaged by one gender (females or males) over undamaged leaves. However, if leaves were damaged by a two-gender population, neither females nor males indicated a preference over volatiles of undamaged leaves. Leaves damaged by both beetle genders simultaneously had significantly increased green leaf volatile (GLV), benzenoid and homoterpene emissions compared to undamaged leaves. Emissions of these compounds possibly indicate higher herbivore pressure and a higher risk of attack by parasitoids and predators and could thus be the cause of the lack of beetle preference. Our findings provide new basic information on gender-based host plant selection by herbivores and may be helpful in the development of sustainable biogenic VOC-based herbivore-control methods for intensive short-rotation hybrid aspen production.

14 years of OMI HCHO observations reveal VOC emission trends over large cities worldwide

<p>Formaldehyde (HCHO) observations from satellites have been widely used to constrain volatile organic compound (VOC) emission estimates. The oxidation of anthropogenic organic compounds accounts for only a small fraction(~7%) of the total HCHO column on global average (Stavrakou et al., 2009). Therefore, the use of satellite observations to infer information about anthropogenic VOC emissions is generally very challenging . However, the relative contribution of anthropogenic VOCs in and around metropolitan centers is expected to be significant. In this study, we use HCHO column data retrieved from the OMI sensor between 2005 and 2018, and calculate monthly averages for every city of more than 500,000 inhabitants based on data within 20 km of the city centers. Because of the dependence of the background and especially of the biogenic VOC source on temperature and solar radiation, and because these contributions might be significant even around large cities, it is not possible to directly infer the anthropogenic contribution to the long-term observed HCHO trends based on HCHO data. To remove these non-anthropogenic contributions, we first regress the monthly averaged columns either onto the monthly maximum surface temperature, obtained by ECMWF reanalysis data, or onto the monthly isoprene flux, calculated with the MEGAN-MOHYCAN model (Guenther et al., 2012, Stavrakou et al. 2018). Only cities for which anthropogenic emissions are estimated to exceed biogenic emission by more than a factor of 3 are considered. In this way, positive trends of up to 3% yr<sup>-1</sup> are found over many Asian cities, especially in China and in the Indo-gangetic Plain, whereas over European cities, South Africa and South America negative trends up to -2% yr<sup>-1</sup> are derived. The deduced trends are compared to the corresponding trends of global bottom-up anthropogenic VOC emission inventories and are found to be in good overall agreement. Model simulations are further needed to quantify the relationship between anthropogenic emission trends and HCHO columns, accounting for the effect of non-anthropogenic emissions and potential changes in the oxidizing capacity.</p>

Climate-driven chemistry and aerosol feedbacks in CMIP6 Earth system models

Feedbacks play a fundamental role in determining the magnitude of the response of the climate system to external forcing, such as from anthropogenic emissions. The latest generation of Earth system models include aerosol and chemistry components that interact with each other and with the biosphere. These interactions introduce a complex web of feedbacks which it is important to understand and quantify. This paper addresses the multiple pathways for aerosol and chemical feedbacks in Earth system models. This is achieved by extending previous formalisms which include CO2 concentrations as a state variable to a formalism which in principle includes the concentrations of all climate-active atmospheric constituents. This framework is demonstrated by applying it to the Earth system models participating in CMIP6 with a focus on the non-CO2 reactive gases and aerosols (methane, ozone, sulphate aerosol, organic aerosol and dust). We find that the overall climate feedback through chemistry and aerosols is negative in the CMIP6 Earth system models due to increased negative forcing from aerosols with warmer temperatures. Through diagnosing changes in methane emissions and lifetime we find that if Earth system models were to allow methane to vary interactively, methane positive feedbacks (principally wetland methane emissions and biogenic VOC emissions) would offset much of the aerosol feedbacks.