The California Air Resources Board (CARB) has released the Discussion Draft 2025 Mobile Source Strategy, outlining a comprehensive plan to address emissions from vehicles and equipment that remain the largest contributors to the state’s air pollution and greenhouse gas emissions. Building on the approach of its integrated 2016 and 2020 strategies, CARB’s latest framework charts a course for the transition to cleaner, zero-emission technologies to meet California’s ambitious air quality and climate goals.

The Scope of the Problem

Mobile sources — including cars, trucks, ships, trains, aircraft, and off-road machinery — continue to generate a majority of California’s diesel particulate matter, smog-forming oxides of nitrogen (NOx), and particulate matter (PM) emissions. These sources also account for the largest portion of the state’s greenhouse gas (GHG) emissions. According to CARB, tackling these emissions is critical to meeting California’s goals for improving air quality, mitigating climate change, and reducing health risks in vulnerable communities.

“The State of California has many air quality, climate, and community risk reduction goals to achieve over the next thirty years,” CARB notes on its website. The 2025 Mobile Source Strategy is designed to identify the scale of the transition required to achieve these targets.

Key Focus Areas

The Discussion Draft provides an in-depth look at emissions across various mobile source categories, summarizing current emissions, control programs, and the potential for reductions through advanced technologies. Among the priority areas:

• Light- and Heavy-Duty Vehicles: The strategy emphasizes accelerating the adoption of zero-emission vehicles (ZEVs) for personal, commercial, and freight use.
• Off-Road Equipment: Transitioning construction, agricultural, and industrial machinery to cleaner alternatives remains a key challenge.
• Marine, Rail, and Aviation: Efforts include electrification, cleaner fuels, and emissions reduction strategies tailored to these sectors.

CARB also highlights the importance of strengthening regulations, expanding infrastructure for ZEVs, and supporting the development of innovative technologies.

Legislative and Historical Context

The 2025 Mobile Source Strategy builds on the progress made under the 2016 and 2020 strategies. Both efforts introduced key regulatory measures, including the Advanced Clean Fleets initiative and updates to the In-Use Off-Road Diesel-Fueled Fleets Regulation (Off-Road Regulation).

The current effort stems from Senate Bill (SB) 44, enacted in 2019, which requires CARB to update its Mobile Source Strategy every five years to address the evolving vehicle market and the state’s air quality challenges. “SB 44 acknowledges the ongoing need to evaluate opportunities for mobile source emissions reductions,” CARB explains, underscoring the importance of regular assessments to adapt to changing conditions.

Engaging the Public

CARB’s development of the 2025 Mobile Source Strategy incorporates robust public engagement. A second public webinar, held in October 2024, provided stakeholders an opportunity to learn about the strategy and offer feedback. This input will inform the final strategy, which CARB aims to complete by January 2026.

Challenges and the Road Ahead

While the strategy sets ambitious goals, CARB acknowledges challenges such as technological feasibility, cost, and the need for infrastructure development. The Discussion Draft emphasizes the urgency of transitioning to zero-emission technologies to address climate change and improve public health via environmental protection, particularly in disadvantaged communities disproportionately affected by pollution.

With its 2025 Mobile Source Strategy, CARB reaffirms its commitment to leading the state’s efforts to reduce emissions, enhance air quality, and support a sustainable transportation future.