Hi everyone:
I am using SMOKE 4.9 with BEIS v3.70 to calculate BVOCs emissions, and I find that the emission factors of MODIS_13 (urban) are not zero?
Any recommendations for how to solve this or investigate further?
Thanks!
beisfacs_beld5_BEISv37.21Apr2020.csv (414.7 KB)
Others like @jbash, @jvukovich, and @pouliot.george can give a more detailed answer on this specific dataset, but on a fundamental level urban areas aren’t 100% built-up with concrete but also have trees and other vegetation which do emit BVOCs. Since the MODIS “urban” category used in the BELD/BEIS system doesn’t distinguish different types of urban areas (e.g. low density, medium density, and high density like the NLCD LULC classification scheme available as an option in WRF and CMAQ), I would expect a non-trivial amount of BVOCs in grid cells classified as “urban” by the MODIS scheme, and this seems consistent with the contents of the file you posted.
In the screenshot below, I just selected the ISOP emissions from the file you posted and added the plain text definitions of the 20 MODIS categories (as they are defined in CMAQ, I would expect them to be the same in BELD/BEIS but I don’t have first hand experience with that). To me, everything seems conceptually consistent with expectations (i.e. the emission factor for urban areas is lower than for forest types but higher than for shrublands, tundra, etc.), but as mentioned above, I am not familiar with the details of this.
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Thanks. In line 723 or 728 of the code, emissions of each biotype in each grid are the sum of emission factor of each emitted biotype () * each vegetation area (e.g., MODIS_1, MODIS_2., etc). If emission factor in the urban area is not zero (if its fraction is 90%), and BVOCs emissions in urban can be overestimated.
In my understanding, the MODIS land cover dataset classifies a pixel as “urban and built up” if it has at least 30% impervious surface area including building materials, asphalt, and vehicles (e.g. in this documentation, though this might not be the exact same version that went into developing BELD). Crucially, “urban and built up” does not mean 100% impervious surface area, only > 30% impervious surface area.
Put differently, the “urban and built up” MODIS_13 category should be thought of as a mix of impervious (> 30%) and pervious (< 70%) surfaces, with the latter including vegetation like trees, shrubs, grass, etc. which will produce non-zero BVOC emissions.
In your example, if a BELD grid cell has a MODIS_13 coverage of 90%, it means that >30% of this 90% is impervious with zero BVOC emissions while <70% is pervious and therefore likely at least partially vegetated and expected to have non-zero BVOC emissions.
Thank you for your detailed reply.