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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-1017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
22 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Which processes drive observed variations of HCHO columns over India?
Luke Surl1, Paul I. Palmer1,2, and Gonzalo González Abad3 1National Centre for Earth Observation, University of Edinburgh, Edinburgh, UK
2School of GeoSciences, University of Edinburgh, Edinburgh, UK
3Atomic and Molecular Physics Division, Harvard–Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA
Abstract. 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) with a spatial smearing that increases with the VOC lifetime. Over India, HCHO has biogenic, pyrogenic, and anthropogenic VOC sources. Using a 0-D photochemistry model, we find that isoprene has the largest molar yield of HCHO that is typically realized within a few hours. 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 × 1015 molec/cm2. 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 × 1015 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.

Citation: Surl, L., Palmer, P. I., and González Abad, G.: Which processes drive observed variations of HCHO columns over India?, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1017, in review, 2017.
Luke Surl et al.
Luke Surl et al.
Luke Surl et al.

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Short summary
We used observations of HCHO (formaldehyde columns from the OMI satellite instrument and the GEOS-Chem atmospheric chemistry model to investigate how and why HCHO vary over India. We find that emissions of isoprene from forests is the most powerful driver, with forests' response to seasonal temperature variations causing variation over time. Human-driven emissions of VOC and burning of vegetation have detectable, but more limited, impacts.
We used observations of HCHO (formaldehyde columns from the OMI satellite instrument and the...
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