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Séminaire de Luke Surl le 01/12/2017

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An investigation into the atmospheric chemistry of a volcanic plume using WRF Chem

Volcanoes are the principal way by which volatiles are transferred from the solid Earth to the atmosphere-hydrosphere system. Once released into the atmosphere, volcanic emissions rapidly undergo a complex series of chemical reactions, with particularly interesting halogen-chemistry phenomena. This talk discusses attempts to further our understanding of such processes by both observation and numerical modelling.

The core of this project is the adaptation of the chemistry and transport model WRF-Chem to model passive degassing from Mount Etna, the chemistry of its plume, and its influence on the wider atmosphere. This investigation considers model plumes from the point of emission up to a day’s travel from the vent and is able to reproduce observed phenomena of BrO formation and O3 depletion within volcanic plumes.

The model plume influences several atmospheric chemistry systems, including reactive nitrogen and organic chemistry. Plume chemistry is driven by sunlight, and the modelled phenomena identified in this investigation vary with the diurnal cycle.

In the modelled plume all of the bromine is involved in O3-destructive cycling. The cycling of bromine has the capacity to incorporate chlorine, furthering its O3-destrictive capabilities. The modelling investigates what factors are rate-limiting in these systems.

I find qualitative differences between the chemistry of low and high intensity plumes, with the bromine chemistry in the latter case being limited by O3 depletion.

This modelling investigation is complemented by an observational study of O3 in a young Etnean plume from which I estimate the rate of in-plume O3 destruction within seconds to minutes after emission.

Which processes drive observed variations of HCHO columns over India ?

We interpret HCHO column variations observed by the Ozone Monitoring Instrument (OMI), aboard the NASA Aura satellite, over India during 2014 using the GEOS-Chem atmospheric chemistry and transport model. We use a nested version of the model with a spatial resolution of approximately 25 km. HCHO columns are related to local emissions of volatile organic compounds (VOCs) - the oxidation of almost all VOCs results in production of HCHO, though HCHO yields and VOC lifetimes varies significantly between species. We use a 0-D box model to investigate these variations.

Over India, HCHO has biogenic, pyrogenic, and anthropogenic VOC sources.
We find that forested regions that neighbours major urban conurbations are exposed to high levels of nitrogen oxides. This results in depleted hydroxyl radical concentrations and a delay in the production of HCHO from isoprene oxidation. We find that propene is the only anthropogenic VOC emitted in major Indian cities that produces HCHO at a comparable (slower) rate to isoprene.

The GEOS-Chem model reproduces the broadscale annual mean HCHO column distribution observed by OMI (r = 0.6), which is dominated by a distinctive meridional gradient in the northern half of the country, and by localized regions of high columns that coincide with forests. Major discrepancies are over the Indo-Gangetic Plain and Delhi. We find that the model has more skill at reproducing observations during winter (JF) and pre-monsoon (MAM) months with Pearson correlations r > 0.5 but with a positive model bias of 1 × 10^15 molec/cm^2. During the monsoon season (JJAS) we reproduce only a diffuse version of the observed meridional gradient (r = 0.4).

Generally, we find that on a continental scale most of the seasonal cycle is explained by monthly variations in surface temperature (r = 0.9), suggesting a strong role for biogenic VOCs, in agreement with the 0-D and GEOS-Chem model calculations. We also find that the seasonal cycle during 2014 is not significantly different from the 2008–2015 mean seasonal variation but there are large year to year variations. There are two main loci for biomass burning (states of Punjab and Haryana, and northeastern India), which we find only contributes a significant contribution (up to 1 × 10^15 molec/cm2) to observed HCHO columns during March to April over northeastern India. The slow production of HCHO from propene oxidation results in a smeared hotspot over Delhi that we resolve only on an annual mean timescale by using a temporal oversampling method.

Using a linear regression model to relate GEOS-Chem isoprene emissions to HCHO columns we infer seasonal isoprene emissions over two key forest regions from the OMI HCHO column data. We find that the a posteriori emissions are typically lower than the a priori emissions, with a much stronger reduction of emissions during the monsoon season. We find that this reduction in emissions during monsoon months coincides with a large drop in satellite observations of leaf phenology that recovers in post monsoon months. This may signal a forest-scale response to monsoon conditions.

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