The higher value range for the same satellite product (NO2_OMI_TEMIS)

Question

I'm dealing with NO2 column density of troposphere, and my data source is TEMIS.

The NO2 level3 data can be derived from the original information of OMI instrument.

I have downloaded two types of data, .grd and .kml for the same month.

The data I have downloaded are uploaded here1, 2

KML data

I open it in Google Earth, the figure shows like this:

We can see the data range of NO2 trop. column is 0~20 which are idential to the templete figure on the website.

GRD data

I have not found any detailed information about this data. In the contents of it, -999 is deemed as no_data place.

I use python to read and plot the spatial distribution.

filename  = './CH2O-NO2/no2_201306.grd'
def read_grd(filename):
    ncols     = np.array(linecache.getline(filename, 1)[6:10]).astype(float)
    nrows     = np.array(linecache.getline(filename, 2)[6:10]).astype(float)
    xllcorner = np.array(linecache.getline(filename, 3)[10:14]).astype(float)
    yllcorner = np.array(linecache.getline(filename, 4)[10:14]).astype(float)
    cellsize  = np.array(linecache.getline(filename, 5)[9:14]).astype(float)
    nan_value = np.array(linecache.getline(filename, 6)[13:17]).astype(float)
    longitude = xllcorner + cellsize * np.arange(ncols)
    latitude = yllcorner + cellsize * np.arange(nrows)
    value = np.loadtxt(filename, skiprows=7)
    value = value[::-1]
    return value, longitude, latitude, nan_value

no2,lon_no2, lat_no2, nan_value = read_grd(filename)
no2[no2 == nan_value] = np.nan

def isnt_NaN(num):
    return num == num

no2[isnt_NaN(no2)].max()
> 9999.0

It seems that the high value in grd format data are out of the regular condition which can be learned from previous resarch (Hot spots of NO2 column are about 15~20 10^15 molec/cm2).

Does anyone familiar with the OMI-NO2 data? I don't know how to deal with the irregular value which are way too higher than realistic.

Answer 1

It seems that you are concerned that your values in the data go above the max value on the color bar shown in the kml file. I wouldn't worry about that. You are using the "level 3" monthly average product, which by definition is highly quality controlled. For most of the globe, typical values will vary between 1 x 10^14 and 5 x 10^15 molec/cm2. Values over 20 x 10^15 molec/cm2 only occur in highly polluted areas. Very large values in the OMI NO2 product are realistic, especially in SE Asia, where the largest NO2 columns are obtained. I have even seen values over 1 x 10^17 on occasion. As the signal gets larger, there is actually more confidence in the retrieval. Really it's the low values (e.g. less than 8 x 10^14 molec/cm2) that you should treat cautiously, as anything lower than that could be "noise".

In order to show the variance of values (which can span 4 orders of magnitude) across the whole globe, the color scale chosen works well. Note that the color bar is a logarithmic scale, which helps give definition to the plots in the regions with typical values. You could rewrite the kml file though, and pick a different max value, if you are interested in better plots of southeast Asia.

If you are interested in learning more about the product, I highly suggest you see Boersma (2011) and also refer to the user manual.