Transformation of logwood combustion emissions in a smog chamber: formationof secondary organic aerosol and changes in the primary organic aerosol upondaytime and nighttime aging

摘要

Organic aerosols (OA) derived from small-scale woodcombustion emissions are not well represented by current emissionsinventories and models, although they contribute substantially to theatmospheric particulate matter (PM) levels. In this work, a 29 m smogchamber in the ILMARI facility of the University of Eastern Finland wasutilized to investigate the formation of secondary organic aerosol (SOA)from a small-scale modern masonry heater commonly used in northern Europe.Emissions were oxidatively aged in the smog chamber for a variety of dark(i.e., O and NO) and UV (i.e., OH) conditions, with OHconcentration levels of (0.5–5)  ×  10 molecules cm,achieving equivalent atmospheric aging of up to 18 h. An aerosol massspectrometer characterized the direct OA emissions and the SOA formed fromthe combustion of three wood species (birch, beech and spruce) using twoignition processes (fast ignition with a VOC-to-NO ratio of 3 and slowignition with a ratio of 5).Dark and UV aging increased the SOA mass fraction with average SOAproductions 2.0 times the initial OA mass loadings. SOA enhancement wasfound to be higher for the slow ignition compared with fast ignitionconditions. Positive matrix factorization (PMF) was used to separate SOA,primary organic aerosol (POA) and their subgroups from the total OA mass spectra. PMF analysisidentified two POA and three SOA factors that correlated with the threemajor oxidizers: ozone, the nitrate radical and the OH radical.Organonitrates (ONs) were observed to be emitted directly from the woodcombustion and additionally formed during oxidation via NO radicals(dark aging), suggesting small-scale wood combustion may be a significant ONsource. POA was oxidized after the ozone addition, forming aged POA, andafter 7 h of aging more than 75 % of the original POA was transformed.This process may involve evaporation and homogeneous gas-phase oxidation aswell as heterogeneous oxidation of particulate organic matter. The resultsgenerally prove that logwood burning emissions are the subject of intensivechemical processing in the atmosphere, and the timescale for thesetransformations is relatively short, i.e., hours.