Analysis of black carbon and carbon monoxide observed over the Indian Ocean: Implications for emissions and photochemistry

Abstract

Air from South Asia carries heavy loadings of organic and light‐absorbing aerosol but low concentrations of ozone. We investigate ambient pollutant concentrations measured during the Indian Ocean Experiment (INDOEX), and we estimate emissions to determine the origin, magnitude, and impacts of air pollution from South Asia and to understand better the uncertainty in emission inventories. In India, the preponderance of motorcycles with small, two‐stroke engines and the practice of adulterating gasoline with kerosene lead to high CO emission factors; for 1999, we estimate release of 15 Tg yr−1 from fossil fuel use and 40 Tg yr−1 from biomass burning. With the addition of isoprene oxidation, the total CO emissions were 67 Tg yr−1 from India and 87 Tg(CO) yr−1 from all of South Asia. These values indicate a somewhat larger contribution from fossil fuels but slightly lower overall emissions when compared to prior emission inventories. Two‐stroke engines also exhibit high emission factors for volatile organic compounds (VOC) and particulate organic matter but produce only modest amounts of NOx. Near sources in India, VOC to NOx ratios appear too high for efficient O3 formation, although other factors probably contribute to observed low O3 mixing ratios. An inventory based on source characteristics and known emission factors for black carbon (BC) from South Asia yields 0.7 Tg yr−1 (upper limit of about 1.0 Tg yr−1) with biomass burning as the dominant source. We can test this inventory with measurements of ambient CO and BC—ship, island, and aircraft observations of air from South Asia all show a positive correlation between CO and BC (r2 = 0.71–0.81). Such strong correlations have also been observed over North America and Europe, but with a lower BC/CO slope. Ambient concentrations indicate high BC emission from South Asia: 2–3 Tg(BC) yr−1. This disagreement with emission inventories demonstrates the need for direct measurements from sources in India.