In
a region heavily influenced by anthropogenic and biogenic atmospheric
emissions, recent field measurements have attributed one-third of
urban organic aerosol by mass to isoprene epoxydiols (IEPOX). These
aerosols arise from the gas-phase oxidation of isoprene, the formation
of IEPOX, the reactive uptake of IEPOX by particles, and finally the
formation of new compounds in the aerosol phase. Using a continental-scale
chemical transport model, we find a strong temporal correspondence
between the simulated formation of IEPOX-derived organic aerosol and
these measurements. However, because only a subset of isoprene-derived
aerosol compounds have been specifically identified in laboratory
studies, our simulation of known IEPOX-derived organic aerosol compounds
predicts a mass 10-fold lower than the field measurements, despite
abundant gas-phase IEPOX. Sensitivity studies suggest that increasing
the effective IEPOX uptake coefficient and including aerosol-phase
reactions that lead to the addition of functional groups could increase
the simulated IEPOX-derived aerosol mass and account for the difference
between the field measurements and modeling results.
A rising source of outdoor emissions in northwestern India is crop residue burning, occurring after the monsoon (kharif ) and winter (rabi) crop harvests. In particular, post-monsoon rice residue burning, which occurs annually from October to November and is linked to increasing mechanization, coincides with meteorological conditions that enhance short-term air quality degradation. Here we examine the Global Fire Emissions Database (GFED), whose bottom-up emissions are based on the 500-m burned area product, MCD64A1, derived from Moderate Resolution Imaging Spectroradiometer (MODIS) observations. Using a household survey from 2016, we find that MCD64A1 tends to underestimate burned area in many surveyed villages, leading to poor representation of small, scattered fires and consequent spatial biases in model results. To more accurately allocate such small fires and resolve sub-village heterogeneity, we use an experimental hybrid MODIS-Landsat method (ModL2T) to map burned area at 30-m spatial resolution, which results in 44±21% higher burned area than MCD64A1 and up to 105±52% increase in dry matter emissions over GFEDv4s. In our validation and assessments, we find that ModL2T performs better relative to MCD64A1 in terms of bias and omission error, but may introduce commission error due to conflation of burning with harvest and still underestimate burned area due to Landsat's coarse temporal resolution (every 16 days). We conclude that while MODIS and Landsat provide more than two decades worth of observations, their spatio-temporal resolution is too coarse to overcome several region-specific challenges: small median landholding size (1-3 ha), quick harvest-to-sowing turnover period, prevalence of partial burning, and increasing haziness. To further constrain agricultural fire emissions in northwestern India and improve model estimates of associated public health impacts, integration of finer resolution imagery, as well as better understanding of the spatial patterns in burn rates, burn practices, and fuel loading, is requisite.
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