Aerosol meteorology and Philippine receptor observations of Maritime Continent aerosol emissions for the 2012 7SEAS southwest monsoon intensive study

The largest 7 Southeast Asian Studies (7-SEAS) operations period within the Maritime Continent occurred in the 2012 August–September biomass burning season. Included where an enhanced deployment of Aerosol Robotic Network (AERONET) sun photometers, of multiple lidars, and of a Singapore supersite. Simultaneously, a ship was dispatched to the Palawan Archipelago and Sulu Sea of the Philippines for September 2012 to observe transported smoke and pollution as it entered the southwest monsoon monsoonal trough. Here we describe the nature of the overall 2012 southwest monsoon biomass burning season, but focus on the findings of the research cruise and the aerosol meteorology of this convectively active region. This 2012 cruise followed a 2 week cruise in 2011, and was in part consistent with the findings of that cruise for how smoke emission and transport relate to monsoonal flows, the propagation of the Madden Julian Oscillation (MJO), tropical cyclones, and covariance between smoke transport events and the atmosphere’s thermodynamic structure. Aerosol observations in the 2011 cruise also highlighted the importance of squall lines and cold pools as they propagate across the South China Sea, scavenging aerosol particles in their path. For 2012, the cruise experienced differing environments. The monsoonal flow direction was perturbed by easterly waves, leading at times to total flow reversal in the South China Sea. Two category 5 typhoons just east of the Philippines also modulated flow patterns and convection. Whereas in 2011 large synoptic scale aerosol events transported high concentrations of smoke into the Philippines over days, in 2012, measured aerosol events exhibited a much more short term variation, sometimes only over 3–12 hours. Navy Aerosol Analysis and Prediction System (NAAPS) simulations captured longer wavelength aerosol events quite well, but largely failed to capture the timing in high frequency phenomena. Also observed were nucleation events in cleaner and polluted conditions, as well as in urban plumes. Combined, observations indicate pockets of high particle counts are not uncommon in the region. Perhaps most interestingly, several cases of squall lines heralding major aerosol events were observed, as opposed to the previous observations in 2011 of these lines largely scavenging aerosol particles from the marine boundary layer. We hypothesize that these phenomena may originate from weakly forced convection ahead of polluted land breeze fronts caught in strong monsoonal flows. Ultimately, the research findings of the 2012 cruise nicely complement the narrative started by the 2011 research cruise, and point to the importance of small scale phenomena such as sea breezes and squall lines embedded in the large scale monsoonal flow patterns in dominating aerosol lifecycle and potentially effects. “Pure” biomass burning plumes are relatively rare and are usually mixed with significant amounts of anthropogenic pollution.