Sentinel 5P Aerosol Index outside range

Hi, I am trying to download Sentinel5P data but I noticed the values for layers AER_AI_354_388 and AER_AI_340_380 are outside the normal range specified here. It is reported that the minimum value for these layers is -1 but I often encounter values lower than this threshold. I am downloading the data using Python SDK in this way

request = SentinelHubRequest(

                evalscript=evalscript,

                input_data=[

                    SentinelHubRequest.input_data(

                        data_collection=DataCollection.SENTINEL5P,

                        time_interval=(ts, ts),

                        mosaicking_order=MosaickingOrder.MOST_RECENT,

                        upsampling="BICUBIC"

                    )

                ],

                responses=[SentinelHubRequest.output_response("default", MimeType.TIFF)],

                bbox=city_bbox,

                size=city_size,

                config=config,

            )

            img = request.get_data()[0]

where evalscript is the following

function setup() {

    return {

        input: [{

            bands: ["AER_AI_354_388", "dataMask"],

        }],

        output: {

            bands: 2,

            sampleType: "FLOAT32"

        }

    };

}



function evaluatePixel(sample) {

    return [

        sample.AER_AI_354_388,

        sample.dataMask

    ]

}

the same for AER_AI_340_380 . I very rarely noticed some negative values also for HCHO layer which uses the exact same script.

Am I doing something wrong ?

Thanks in advance for the help

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Hi,

Your script looks fine to me. As mentioned in the documentation, the Typical Range indicates what values are common for a given band and unit, however outliers can be expected.

I randomly downloaded an original file from Copernicus and the histogram did indicate that there’re values lower than -1 (Fig 1).

Fig 1

Hi, thanks for your reply.
I understand that Typical Range indicates common values but negative values do not make so much sense to me since they should be concentration values in mol/m^2. I would then consider those values as wrong acquisition and discard them, what do you think?

In addition, I noticed that the tile downloaded with my script and the values reported in EO Browser are different. More in detail, considering the metropolitan area of Milan on 2018-04-30 for some regions my downloaded tile has values lower than -1 while EO Browser shows values around 0.5 in the same area.
For this reason, I am still forced to think there is something wrong with downloaded data, or the alternative could be that the EO browser does some post-processing before visualisation. What do you think about this?

Thanks again for your support

Hi,

Could you provide your aoi and the timestamp which shows the discrepancy?

Hi,
my AOI is ( 9.0408867, 45.3867381, 9.2781103, 45.5358482 ).

Regarding AER_AI_340_380 and AER_AI_354_388 layers the timestamp in which I observed the discrepancy between my tile and EO Browser is 2018-04-30T10-27-52.

For what concerns HCHO layers I have to check the timestamp in which I obtained negative values but I can easily discard them as negative concentrations are for sure due to a noisy acquisition.

Thanks for your support