Abstract
Abstract. We analyzed observations from the Multi-angle Imaging SpectroRadiometer (MISR) to determine the injection heights of biomass-burning smoke plumes over peninsular Southeast Asia (PSEA, here defined as Vietnam, Cambodia, Thailand, Laos, and Myanmar) in the spring, with the goal of evaluating the impacts on long-range pollutant transport. We retrieved the heights of 22 000 MISR smoke pixels from 607 smoke plumes over PSEA during February to April of the years 2001–2010. Forty-five percent of the analyzed smoke pixels were above the local mean boundary layer (1 km) at MISR overpass time (10:30 a.m. local time). We used the GEOS–Chem model to simulate the transport of PSEA biomass-burning pollutants in March 2001. On a monthly mean basis, we found that the direct injection of 40% of the PSEA biomass-burning emissions had little impact on the long-range transport of CO to downwind regions, compared to a control simulation where all biomass-burning emissions were released in the boundary layer. This was because CO at the surface over PSEA was efficiently lifted into the free troposphere by deep convection associated with synoptic-scale weather systems. For pollutants with lifetimes shorter than the synoptic timescale, such as black carbon aerosol (BC), their long-range transport was much more sensitive to the initial plume injection height. The direct injection of NOx from PSEA biomass burning into the free troposphere drove increased formation and transport of peroxyacetyl nitrate (PAN), which in turn led to a small increase in ozone over downwind southern China and the northwestern Pacific. The Pacific subtropical high transported BC emitted from PSEA biomass burning to the marine boundary layer over the tropical northwestern Pacific. We compared our model results to aircraft measurements over the northwestern Pacific during the TRACE-P campaign (March 2001). The direct injection of 40% of the PSEA biomass-burning pollutants into the free troposphere in the model led to a more pronounced BC peak at 3 km over the northwestern Pacific. Our analysis highlights the point that the injection heights of smoke plumes presents great uncertainty over the interpretation of BC measurements downwind of biomass-burning regions.
Highlights
Open burning of biomass emits large amounts of trace gases and aerosols into the atmosphere, affecting atmospheric chemistry and climate
Our criteria for identifying a biomass-burning smoke plume were: (1) a plume-shaped area of high reflectivity was visible from the MISR INteractive eXplorer software (MINX) animation; (2) that high-reflectivity area showed a shift in position relative to the surface between cameras, indicating that it was above the surface; and (3) the vertex of that high-reflectivity area was marked by one or more Moderate Resolution Imaging Spectroradiometer (MODIS) thermal anomalies
Why was the long-range transport of biomass-burning CO from PSEA insensitive to the smoke plume injection height? We found that the surface circulation over PSEA in March was dominated by a persisting low-pressure system, which converged surface air and induced deep convection over the region (Fig. 4b)
Summary
Open burning of biomass emits large amounts of trace gases and aerosols into the atmosphere, affecting atmospheric chemistry and climate. Several studies embedded high resolution models or parameterization schemes to resolve the plume-rising process in largescale chemical transport models (Freitas et al, 2006, 2007; Rio et al, 2010) These studies found that the injections of biomass-burning emissions into the free troposphere have a large impact on CO concentrations downwind (Freitas et al, 2006, 2007). Chen et al (2009) found that the injection heights of North American boreal fire emissions had limited impacts on CO concentrations over the downwind areas in North America They attributed this lack of sensitivity to frequent strong convective activities occurring along the transport pathway, which would lift CO into the free troposphere regardless of the initial injection height. We compared model results against aircraft measurements during the TRACE-P campaign in spring 2001 to further examine the impacts on long-range transport and the interpretation of measurements
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