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Price Unit Interpretation

How to interpret GCAM price units, the difference between output-unit and price-unit, and sector-specific unit quirks in GCAM-China.

Two Independent Unit Systems

Every GCAM sector has two units defined independently:

• output-unit: The unit of physical-output. Examples: Mt, EJ, Mt NH3.
• price-unit: The unit of sector/cost. Examples: 1975$/kg, 1975$/GJ, 1975$/kgNH3.

The 1975$ prefix means the price is in constant 1975 US dollars, the standard GCAM currency baseline.

Compatibility for Revenue Calculation

Direct multiplication cost * output only produces a meaningful monetary proxy when units are dimensionally compatible:

Case	Output Unit	Price Unit	Compatible	Notes
Mass-mass	Mt	1975$/kg	Yes	Mt * $/kg = 10^9 * $ (Mt->kg factor)
Energy-energy	EJ	1975$/GJ	Yes	EJ * $/GJ = 10^9 * $ (service proxy)
Mixed	Mt	1975$/GJ	No	Physical dimensions mismatch

GCAM-China Sector Unit Reference

Confirmed from A323.sector_China.csv:

Sector	output-unit	price-unit (GCAM-China)	price-unit (GCAM-Core)	Revenue Proxy?
cement	Mt	1975$/kg	1975$/kg	Yes
iron and steel	Mt	1975$/GJ	1975$/kg	No (mixed in China)
coke	Mt	1975$/GJ	N/A (not a sector)	No (mixed)
chemical	EJ	1975$/GJ	1975$/GJ	Yes (service proxy)
other industry	EJ	1975$/GJ	1975$/GJ	Yes (service proxy)
ammonia	Mt NH3	1975$/kgNH3	N/A	Yes (Mt->kg conversion)
aluminum	Mt	1975$/kg	1975$/kg	Yes
paper	Mt	1975$/kg	1975$/kg	Yes
food processing	Mt	1975$/kg	1975$/kg	Yes

Why Iron and Steel Uses 1975$/GJ in GCAM-China

In official GCAM-Core, iron and steel uses price-unit=1975$/kg. GCAM-China changed this to 1975$/GJ, likely because the iron and steel sector represents an energy service rather than a pure mass commodity in the China model. Input coefficients are defined as GJ/ton, and the model resolution uses energy flows.

This means:

• sector/cost for iron and steel is in 1975$/GJ
• physical-output is in Mt
• Direct multiplication is not interpretable as revenue
• For revenue comparison, either use marketplace prices (if available) or apply external unit conversion

Coke: A GCAM-China Extension

Coke does not exist in official GCAM-Core as a standalone sector. In GCAM-China:

• It is an independent supplysector name="coke"
• output-unit=Mt (physical mass)
• price-unit=1975$/GJ (energy-based price)
• Similarly mixed units as iron and steel

Where Marketplace Prices Help

The GCAM Marketplace (Marketplace/market nodes) records transaction prices and quantities for goods traded between sectors. Marketplace prices may use different units than sector costs:

let $outUnit := string(($m/Info/Pair[Key = "output-unit"]/Value)[1])
let $priceUnit := string(($m/Info/Pair[Key = "price-unit"]/Value)[1])

Always check marketplace output-unit and price-unit before using marketplace prices for revenue calculation.

Unit Override Strategy

When a sector has incompatible units but revenue comparison is needed:

1. Check if the marketplace reports compatible units
2. Check if external data (e.g., statistical yearbook prices) is available
3. Apply explicit overrides only with documented justification:

UNIT_OVERRIDES = {
    "coke": {"price_unit": "1975$/kg", "output_unit": "Mt"},
    "iron and steel": {"price_unit": "1975$/kg", "output_unit": "Mt"},
}

1. Document which overrides were applied in the analysis

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