Relationship between long-range transported atmospheric black carbon and carbon monoxide at a high-altitude background station in East Asia

Abstract

Lulin Atmospheric Background Station (LABS, 23.47°N, 120.87°E; 2862 m above sea level) at the summit of Mount Lulin in central Taiwan was established in spring 2006 and is the only high-altitude background station over western Pacific region in East Asia to study the impact of various air pollutants through long-range transport. Continuous in-situ measurements of equivalent black carbon (EBC) and carbon monoxide (CO) concentrations were made at LABS from June 2012 to May 2014 and their association was investigated in this study. The highest monthly concentration of EBC (median; 840 ng m−3) and CO (212 ppbv) in March were primarily attributed to the westerly winds coupled with biomass-burning (BB) emissions from Southeast Asia (SEA) region. The association of EBC and CO was weak at LABS possibly due to the influence of dissimilar air masses from various sources, and scavenging or dilution of EBC during the long-range atmospheric transport to Mt. Lulin. The mean ΔEBC/ΔCO ratio (slope of least-squares regression line of ΔEBC-ΔCO scatterplot; where Δ indicates surplus amounts with respect to the background value) was found the most significant in March (5.3 ng m−3 ppbv−1 or 7.3 × 10−3 g of carbon as EBC per gram of carbon as CO). On the basis of episodic cases, the mean ΔEBC/ΔCO ratios at LABS were estimated to be 6.1, 8.0, and 2.4 ng m−3 ppbv−1 for SEA BB emissions, southern China mixed pollution, and northern China mixed pollution, respectively. A total of 32% loss in EBC aerosols (6.4% of EBC removal per day) was estimated for the atmospheric transport of BB emissions from SEA region to LABS. This study provides needful information to understand the ΔEBC/ΔCO ratios at a remote site and would be used in model simulations to evaluate BC aging and scavenging over western Pacific region in East Asia.