Characteristics and influence of biosmoke on the fine‐particle ionic composition measured in Asian outflow during the Transport and Chemical Evolution Over the Pacific (TRACE‐P) experiment

Abstract

We investigate the sources, prevalence, and fine‐particle inorganic composition of biosmoke over the western Pacific Ocean between 24 February and 10 April 2001. The analysis is based on highly time‐resolved airborne measurements of gaseous and fine‐particle inorganic chemical composition made during the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment. At latitudes below approximately 25°N, relatively pure biomass burning plumes of enhanced fine‐particle potassium, nitrate, ammonium, light‐absorbing aerosols, and CO concentrations were observed in plumes that back trajectories and satellite fire map data suggest originated from biomass burning in southeast Asia. Fine‐particle water‐soluble potassium (K+) is confirmed to be a unique biosmoke tracer, and its prevalence throughout the experiment indicates that approximately 20% of the TRACE‐P Asian outflow plumes were influenced, to some extent, by biomass or biofuel burning emissions. At latitudes above 25°N, highly mixed urban/industrial and biosmoke plumes, indicated by SO42−and K+, were observed in 5 out of 53 plumes. Most plumes were found in the Yellow Sea and generally were associated with much higher fine‐particle loadings than plumes lacking a biosmoke influence. The air mass back trajectories of these mixed plumes generally pass through the latitude range of between 34° and 40°N on the eastern China coast, a region that includes the large urban centers of Beijing and Tianjin. A lack of biomass burning emissions based on fire maps and high correlations between K+and pollution tracers (e.g., SO42−) suggest biofuel sources. Ratios of fine‐particle potassium to sulfate are used to provide an estimate of relative contributions of biosmoke emissions to the mixed Asian plumes. The ratio is highly correlated with fine‐particle volume (r2= 0.85) and predicts that for the most polluted plume encounter in TRACE‐P, approximately 60% of the plume is associated with biosmoke emissions. On average, biosmoke contributes approximately 35–40% to the measured fine inorganic aerosol mass in the mixed TRACE‐P plumes intercepted north of 25°N latitude.