Command line harpmerge | sequence of filters

Good evening Sander,

I wanted to ask about the sequence of operations/options in harpmerge, command line edition.

I am using

harpmerge -a ‘SO2_column_number_density>0.8; SO2_column_number_density_validity!& 255;scan_direction_type==0’ -ap ‘bin_spatial(721,-90.0,0.25,1441,-180.,0.25);derive(latitude {latitude}); derive(longitude {longitude});derive(SO2_column_number_density [DU]);keep(latitude, longitude, latitude_bounds,longitude_bounds,SO2_column_number_density,solar_zenith_angle, cloud_fraction,SO2_column_number_density)’ *.HDF5

On the Raikoke eruptive day of 23.06.2019, with GOME2C GDP4.9

I want to exclude all pixels with SO2 load of less than 0.8 D.U.

However, I seem to be left with a number of values lower that that. Same if I do not bin_spatial.

There are actual values between 0 and 0.8D.U. in the data, it is not a case of panoply not being able to deal with NaNs or whatever.

What do you think? maybe the sequence of operations has to change?

Furthermore, I just realise that the derive(SO2_column_number_density [DU]) is not needed since this is done automatically for GOME2.

Many thanks

The default unit for SO2 as ingested by HARP is [molec/cm2], so, since you are filtering before doing a unit conversion, you are filtering with the wrong unit. The best approach is to just provide an explicit unit with your filter: SO2_column_number_density > 0.8 [DU]

1 Like

You are absolutely right, as usual… I was reading the description “unit is converted from DU to molec/cm2” in its vice versa form. From molec to DU.

Let’s see if I get something sensible out this way.

Best wishes,