Spatial-Temporal Variations and Influencing Factors of Carbon Monoxide Exposure In Underground Parking Facility: A Case Study of Meskel Square, Addis Ababa, Ethiopia.

Urban underground parking facilities, while managing surface congestion, pose significant indoor air quality risks from vehicular Carbon Monoxide (CO). This study comprehensively assessed CO spatial-temporal variations, identified influencing factors, and evaluated existing control mechanisms' effectiveness and compliance in the Meskel Square Underground Parking Facility, Addis Ababa. Employing a mixed-methods approach, 432 hourly CO readings from 18 fixed sensors across six zones were integrated with traffic/ventilation logs and qualitative observations from five key informant interviews. Quantitative data were analyzed through descriptive statistics and a Linear Mixed-Effects Model (LMM). The study revealed significant spatial heterogeneity, with Central Parking Zone 1 exhibiting the highest mean CO at 11.92 ppm and entrance/exit zones the lowest (e.g., 5.29 ppm). Temporally, CO progressively increased from morning to evening, peaking at 13.05 ppm during evening hours, with weekend bazaars consistently showing the highest pollution burden (mean 13.07 ppm). CO concentrations frequently exceeded World Health Organization (WHO) and Federal Democratic Republic of Ethiopia Environmental Protection Authority (EPA) permissible limits; Central Parking Zone 1, for instance, reached 31.5 ppm mean CO level or 40 ppm highest individual measurement (1-hour) and 18.3 ppm (8-hour, 103% exceedance of 9 ppm limit) during weekend bazaars. The LMM confirmed that zone, time period, day type, traffic count (β = 0.109 ppm/vehicle), and ventilation status all significantly influenced CO levels. Qualitative insights revealed critical ventilation management challenges, including manual control, low-speed operation, lack of automation, and maintenance gaps, resulting in poor Air Changes per Hour (ACH) performance below American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 62.1 standards in multiple zones. The study concludes that inconsistent ventilation, institutional inaction, and inadequate protective measures contribute significantly to acute and chronic CO exposure risks for workers and visitors. It recommends automated sensor-based ventilation systems, fan redesign, stricter traffic/idling controls, enhanced occupational safety protocols, and alignment with international health and ventilation standards.