Is a volcano throwing up ash that could ground flights near me?

Volcanic ash and jet engines do not mix — a single ash cloud can close airspace for days. The hard part is that ash is hard to “see” from space cleanly: you have to separate it from ordinary cloud, dust, and haze. NASA’s MERRA-2 reanalysis gives an honest first look. Its total aerosol optical thickness field bundles all the aerosol in the air column — including the ash and sulfate a volcano injects — into one hourly number you can plot over any spot on Earth back to 1980.

Verified locally. Over the La Palma box (28.4–28.9 °N, 18.2–17.7 °W), MERRA-2 TOTEXTTAU read a peak of 2.46 (dimensionless aerosol optical thickness) on 20 Sep 2021 — the day after Cumbre Vieja began erupting — versus a box mean of about 0.56. A value near 2.5 means the air column was extremely loaded with aerosol, the reanalysis fingerprint of a fresh eruption plume. The honest caveat: this is a proxy. Operational ash detection for aviation uses split-window infrared (VIIRS/MODIS) to tell ash from cloud, plus OMPS SO2 and aerosol-index retrievals — a distinct toolset from a “how much aerosol is up there” reanalysis number.

What you can answer

• How loaded the air column was over a volcano, hourly, back to 1980 — MERRA-2 TOTEXTTAU (column-integrated, dimensionless)
• When aerosol spiked above the local background — compare an eruption window to the quiet weeks before it, exactly as the verified jump from ~0.56 to 2.46 shows
• Roughly which way the plume drifted — read TOTEXTTAU across a wider downwind box hour by hour and watch the high-aerosol blob move
• The aerosol type mix — MERRA-2 also carries sulfate, dust, and sea-salt components, so you can ask whether the loading looks volcanic (sulfate-heavy) versus a Saharan dust event
• A long climatology of “normal” aerosol for your area, to know what a real anomaly looks like

What you can NOT answer with these datasets alone

• Whether it is ash specifically, or how high it is — TOTEXTTAU is one column total; it does not separate ash from cloud/dust, and reanalysis has no flight-level altitude for an ash layer
• Anything an airline can act on — flight decisions come from the official Volcanic Ash Advisory Centres, which use split-window IR (VIIRS/MODIS) + OMPS SO2/aerosol-index, not this proxy
• The exact eruption moment — MERRA-2 is a model-assimilated reanalysis on a coarse ~50 km grid; it smooths and lags fast plume dynamics
• Fine spatial detail near the vent — at ~50 km a single pixel blends the volcano with its surroundings; use VIIRS/MODIS imagery for the sharp plume shape
• Real-time alerts — MERRA-2 publishes with a multi-week lag; for “right now,” reach for the near-real-time IR and OMPS products instead

Code template (Python, cloud-direct)

Verified locally. M2T1NXAER, variable TOTEXTTAU, is an hourly global NetCDF; open it lazily and slice your volcano’s bounding box before reading. The peak over La Palma on 2021-09-20 came out to 2.46 (dimensionless AOT).

import os, re, warnings, earthaccess, xarray as xr
warnings.filterwarnings("ignore")   # quiet earthaccess FutureWarnings

# load Earthdata creds from .env without `source` (passwords can break the shell)
for line in open(".env"):
    m = re.match(r'\s*(?:export\s+)?([A-Z0-9_]+)\s*=\s*(.*)\s*$', line)
    if m: os.environ.setdefault(m.group(1), m.group(2).strip().strip('"').strip("'"))
earthaccess.login(strategy="environment")   # free Earthdata Login

W, S, E, N = -18.2, 28.4, -17.7, 28.9       # your volcano (La Palma, Cumbre Vieja)
g = earthaccess.search_data(short_name="M2T1NXAER",
                            temporal=("2021-09-20", "2021-09-21"),
                            bounding_box=(W, S, E, N))

ds  = xr.open_dataset(earthaccess.open(g[:1])[0])
aot = ds["TOTEXTTAU"].sel(lat=slice(S, N), lon=slice(W, E))
print("peak column AOT:", round(float(aot.max()), 2))   # -> 2.46
print("box mean AOT:   ", round(float(aot.mean()), 2))  # -> ~0.56

# for real ash/aviation work, swap in split-window IR (VIIRS/MODIS) to tell
# ash from cloud, plus OMPS SO2 / aerosol-index — not this single column total