Abstract
Abstract. Ship-based measurements of aerosol and cloud condensation nuclei (CCN) properties are presented for 2 weeks of observations in remote marine regions of the South China Sea/East Sea during the southwestern monsoon (SWM) season. Smoke from extensive biomass burning throughout the Maritime Continent advected into this region during the SWM, where it was mixed with anthropogenic continental pollution and emissions from heavy shipping activities. Eight aerosol types were identified using a k-means cluster analysis with data from a size-resolved CCN characterization system. Interpretation of the clusters was supplemented by additional onboard aerosol and meteorological measurements, satellite, and model products for the region. A typical bimodal marine boundary layer background aerosol population was identified and observed mixing with accumulation mode aerosol from other sources, primarily smoke from fires in Borneo and Sumatra. Hygroscopicity was assessed using the κ parameter and was found to average 0.40 for samples dominated by aged accumulation mode smoke; 0.65 for accumulation mode marine aerosol; 0.60 in an anthropogenic aerosol plume; and 0.22 during a short period that was characterized by elevated levels of volatile organic compounds not associated with biomass burning impacts. As a special subset of the background marine aerosol, clean air masses substantially scrubbed of particles were observed following heavy precipitation or the passage of squall lines, with changes in observed aerosol properties occurring on the order of minutes. Average CN number concentrations, size distributions, and κ values are reported for each population type, along with CCN number concentrations for particles that activated at supersaturations between 0.14 and 0.85 %.
Highlights
In the Southeast Asian Maritime Continent (MC) and South China Sea/East Sea (SCS) aerosol particles are expected to play an important role modulating cloud development, precipitation, and radiative properties that affect heat transfer through the atmosphere (Reid et al, 2013)
Accumulation mode aerosol mass concentration estimates (Fig. 1b) were initially generated from the passive cavity aerosol spectrometer probe (PCASP) measurements using a density of 1.4 μg m−3 (Levin et al, 2010), assumed to be representative of a combination of smoldering peat and agricultural fire emissions typical in the MC (Reid et al, 2012) that constituted the largest plumes observed during the study (Reid et al, 2016)
The time line of dN/dlogDp size distributions, as measured by the cloud condensation nuclei (CCN) system and shown in Fig. 2a, indicates that most of the particle number concentration fell within the 17–500 nm measurement range, except possibly during the highest-concentration periods
Summary
In the Southeast Asian Maritime Continent (MC) and South China Sea/East Sea (SCS) aerosol particles are expected to play an important role modulating cloud development, precipitation, and radiative properties that affect heat transfer through the atmosphere (Reid et al, 2013). The impact of various aerosol sources on surface properties and concentrations in remote marine regions, and their relationship to expected transport pathways and the few remotely sensed column measurements that exist, is not well understood. Over these remote ocean regions the aerosol optical and physical properties, their variability in time and space, and the processes controlling aerosol life cycle have not been well constrained. This uncertainty in the aerosol environment itself comes in addition to uncertainty about its impacts on meteorological processes. Improved knowledge of the aerosol environment and aerosol–cloud–climate relationships in the Southeast Asian region has been identified as important regionally, and in regards to links with global climate and large-scale aerosol budgets (Reid et al, 2013)
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