Photochemical transformation of residential wood combustion emissions: dependence of organic aerosol composition on OH exposure
Authors: Hartikainen Anni , Tiitta Petri , Ihalainen Mika , Yli-Pirilä Pasi , Orasche Jürgen , Czech Hendryk , Kortelainen Miika , Lamberg Heikki , Suhonen Heikki , Koponen Hanna , Hao Liqing , Zimmermann Ralf , Jokiniemi Jorma , Tissari Jarkko , Sippula Olli 
Published: June 2020

Abstract. Residential wood combustion (RWC) emits large amounts of
gaseous and particulate organic aerosol (OA). In the atmosphere, the
emission is transformed via oxidative reactions, which are under daylight
conditions driven mainly by hydroxyl radicals (OH). This continuing
oxidative ageing produces secondary OA and may change the health- and
climate-related properties of the emission. However, it is not well known
how the composition of RWC-originated OA changes as the function of OH
exposure. In this work, emissions from two modern residential logwood
combustion appliances were photochemically aged in an oxidation flow reactor
(OFR) with various OH exposure levels, reaching up to 6×1011 s cm−3 (equivalent to 1 week in the atmosphere). Gaseous
organic compounds were analysed by proton transfer reaction time-of-flight
mass spectrometry (PTR-ToF-MS), while particulate OA was analysed online by
a high-resolution soot particle aerosol mass spectrometer (SP-HR-ToF-AMS) and offline by in situ derivatization thermal desorption–gas chromatography–time-of-flight mass spectrometry (IDTD-GC-ToF-MS). Photochemical reactions increased the mass of particulate
organic carbon by a factor of 1.3–3.9. The increase in mass took place
during the first atmospheric equivalent day of ageing, after which the
enhancement was independent of the extent of photochemical exposure.
However, ageing increased the oxidation state of the particulate OA linearly
throughout the assessed range, with ΔH:C/ΔO:C slopes between
−0.17 and −0.49 in van Krevelen space. Ageing led to an increase in acidic fragmentation products in both phases, as measured by the IDTD-GC-ToF-MS for
the particulate and PTR-ToF-MS for the gaseous phase. For the gaseous
organic compounds, the formation of small carbonylic compounds combined with
the rapid degradation of primary volatile organic compounds such as aromatic
compounds led to a continuous increase in both the O : C and H : C ratios.
Overall, the share of polycyclic aromatic compounds (PACs) in particles
degraded rapidly during ageing, although some oxygen-substituted PACs, most
notably naphthaldehydic acid, increased, in particular during relatively
short exposures. Similarly, the concentrations of particulate nitrophenols
rose extensively during the first atmospheric equivalent day. During
continuous photochemical ageing, the dominant transformation mechanisms
shifted from the initial gas-phase functionalization/condensation to the
transformation of the particulate OA by further oxidation reactions and
fragmentation. The observed continuous transformation of OA composition
throughout a broad range of OH exposures indicates that the entire
atmospheric lifetime of the emission needs to be explored to fully assess
the potential climate and health effects of RWC emissions.