See Brief request syntax to understand each keyword.

CAMS real-time surface forecast

Total aerosol optical depth at 550 nm, 120 steps 11 first days of June 2016, see also ECMWF Public Dataset web interface

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
   
server = ECMWFDataServer()
   
server.retrieve({
    'stream'    : "oper",
    'levtype'   : "sfc",
    'param'     : "aod550",
    'dataset'   : "cams_nrealtime",
    'step'      : "0/3/6/9/12/15/18/21/24/27/30/33/36/39/42/45/48/51/54/57/60/63/66/69/72/75/78/81/84/87/90/93/96/99/102/105/108/111/114/117/120",
    'expver'    : "0001",
    'time'      : "00",
    'date'      : "2016-06-01/to/2016-06-11",
    'type'      : "fc",
    'class'     : "mc",
    'target'    : "cams_nrealtime.grib"
})