diff --git a/doc/project/ssp.rst b/doc/project/ssp.rst
index 7946d20370..9b4a09151e 100644
--- a/doc/project/ssp.rst
+++ b/doc/project/ssp.rst
@@ -41,3 +41,16 @@ Data
    .. autosummary::
       SSPOriginal
       SSPUpdate
+
+2024 update
+===========
+
+Transport
+---------
+
+.. currentmodule:: message_ix_models.project.ssp.transport
+
+.. automodule:: message_ix_models.project.ssp.transport
+   :members:
+
+   Use :program:`mix-models ssp transport --help in.xlsx out.xlsx` to invoke :func:`.main`.
diff --git a/message_ix_models/project/ssp/transport.py b/message_ix_models/project/ssp/transport.py
index 474a405d5a..148e7efbae 100644
--- a/message_ix_models/project/ssp/transport.py
+++ b/message_ix_models/project/ssp/transport.py
@@ -106,7 +106,8 @@ def extract_dims(
 def extract_dims1(qty: "AnyQuantity", dim: dict) -> "AnyQuantity":  # pragma: no cover
     """Extract dimensions from IAMC-like ‘variable’ names expressions.
 
-    .. note:: This incomplete, non-working version uses :mod:`xarray` semantics.
+    .. note:: This incomplete, non-working version of :func:`extract_dims` uses
+       :mod:`xarray` semantics.
     """
     from collections import defaultdict
 
@@ -148,12 +149,27 @@ def select_re(qty: "AnyQuantity", indexers: dict) -> "AnyQuantity":
     return qty.sel(new_indexers)
 
 
+#: Expression for IAMC ‘variable’ names used in :func:`main`.
 EXPR = r"^Emissions\|(?P<e>[^\|]+)\|Energy\|Demand\|Transportation(?:\|(?P<t>.*))?$"
 
 
 def main(path_in: "pathlib.Path", path_out: "pathlib.Path"):
     """Postprocess aviation emissions for SSP 2024.
 
+    1. Read input data from `path_in`.
+    2. Select data with variable names matching :data:`EXPR`.
+    3. Calculate (identical) values for:
+
+       - ``Emissions|*|Energy|Demand|Transportation|Aviation``
+       - ``Emissions|*|Energy|Demand|Transportation|Aviation|International``
+
+       These are currently calculated as the product of :func:`aviation_share` and
+       ``Emissions|*|Energy|Demand|Transportation``.
+    4. Subtract (3) from:
+       ``Emissions|*|Energy|Demand|Transportation|Road Rail and Domestic Shipping``
+    5. Recombine with all other, unmodified data.
+    6. Write to `path_out`.
+
     Parameters
     ----------
     path_in :
@@ -208,13 +224,12 @@ def main(path_in: "pathlib.Path", path_out: "pathlib.Path"):
     c.add(
         "broadcast:t:AIR emissions",
         genno.Quantity(
-            [1, 1, -1, -1],
+            [1, 1, -1],
             coords={
                 "t": [
                     "Aviation",
                     "Aviation|International",
                     "Road Rail and Domestic Shipping",
-                    "_T",
                 ]
             },
         ),