Characterization and dark oxidation of the emissions of a pellet stove

Pellet combustion in residential heating stoves has increased globally during the last decade. Despite their high combustion efficiency, the widespread use of pellet stoves is expected to adversely impact air quality. The atmospheric aging of pellet emissions has received even less attention, focusing mainly on daytime conditions, while the degree to which pellet emissions undergo night-time aging as well as the role of relative humidity remain poorly understood. In this study, environmental simulation chamber experiments were performed to characterize the fresh and aged organic aerosol (OA) emitted by a pellet stove. The fresh pellet stove PM1 (particulate matter with an aerodynamic diameter less than 1 μm) emissions consisted mainly of OA (93 ± 4%, mean ± standard deviation) and black carbon (5 ± 3%). The primary OA (POA) oxygen-to-carbon ratio (O : C) was 0.58 ± 0.04, higher than that of fresh logwood emissions. The fresh OA at a concentration of 70 μg m−3 (after dilution and equilibration in the chamber) consisted of semi-volatile (68%), low and extremely low volatility (16%) and intermediate-volatility (16%) compounds. The oxidation of pellet emissions under dark conditions was investigated by injecting nitrogen dioxide (NO2) and ozone (O3) into the chamber, at different (10–80%) relative humidity (RH) levels. In all dark aging experiments secondary organic aerosol (SOA) formation was observed, increasing the OA levels after a few hours of exposure to NO3 radicals. The change in the aerosol composition and the extent of oxidation depended on RH. For low RH, the SOA mass formed was up to 30% of the initial OA, accompanied by a moderate change in both O : C levels (7–8% increase) and the OA spectrum. Aging under higher RH conditions (60–80%) led to a more oxygenated aerosol (increase in O : C of 11–18%), but only a minor (1–10%) increase in OA mass. The increase in O : C at high RH indicates the importance of heterogeneous aqueous reactions in this system, that oxidize the original OA with a relatively small net change in the OA mass. These results show that the OA in pellet emissions can chemically evolve under low photochemical activity (e.g. the wintertime period) with important enhancement in SOA mass under certain conditions.


Aerosol acidity
Four different cases have been simulated and are shown here: a) Base case: measured OA and a potassium concentration estimated as 15% of the PM nitrate 2 ; b) both OA and potassium concentrations were assumed to be zero; c) measured OA and a zero assumed potassium concentration; d) a zero assumed OA and scaled potassium concentrations.
For the high RH experiments, the different sensitivity tests showed that, when including neither OA water uptake nor potassium in the model (case b), the pH of the pellet aerosol stayed constant throughout the experiment for all cases with an average value of 2.8 ± 0.3.This is consistent with the fact that the inorganic components of the BB aerosol (which control the acidity in this case) is not affected by the aging process.When OA water uptake alone is included (case c), the pH of the OA decreased after ozone injection, but only in experiment 4 (Figure S5a) from 3.8 to 3.1, while for experiments 5 and 6 pH was constant and equal to 3.1 for both.This indicates that the water uptake from organics in many of the experiments is not significant enough to have a large impact on nitrate/ammonium partitioning and ozone injection did not affect it.Similarly, when an estimate of potassium emissions is included (but without OA water uptake; case d), the pH decreases after t=0 h, from 3.6 to 3.2 only in the case of exp. 4, while for the other two high RH experiments, pH remains constant at 2.9 ± 0.1.
In the low RH cases, when including neither OA water uptake nor potassium in the model (case b), the pH of the pellet aerosol stays constant for the whole experiment, and the same applies for exp.7 (UV ref.), in which pH equals 1 even after the lights are turned on.When OA water uptake alone is included (case c), the pH of the OA increased after ozone injection for all three dark dry casesowing to the dilution effect that the organic water has on acidity.For experiments 2 and 3 (where ozone and NO2 injections were higher than 100 ppb) pH increased for both from 2.7 to 3.7, while in experiment 1 (under lower oxidants conditions) increased from 2.5 to 3.1.In the case where only scaled potassium is included (case d), pH remained constant for the whole experimentreflecting that the amount and partitioning of inorganics is largely unaffected by the aging process.
The simulated inorganic nitrate matches the observations for the whole experiment for the base case simulation (OA water uptake and potassium are included) (Exp.4; Figure 5b), which confirms that the pH estimates here are realistic.When the OA water uptake and potassium are neglected, nitrate is underestimated by a factor of 2, but follows the trend of observationswhich points to the need of including organic water in partitioning and pH calculations.Finally, when only potassium is included, inorganic nitrate is underestimated only for the period before the ozone injection and matches the measurements right after the start of dark oxidation.

Figure S1 .
Figure S1.Average OA mass spectra of fresh pellet emissions (red bars) and standard deviation22

Figure S3 .
Figure S3.Fractional enhancement of inorganic nitrate for the dark aging experiments under dry (brown line; experiments 1 to 3) and high RH (blue line; experiments 4 to 6).The shaded light blue (dark high RH experiments) and orange (dark dry experiments) regions correspond to the variability across all experiments due to differences in injected NO2 and O3 concentration, while the solid blue and brown lines are the mean across the RH and dry experiments, respectively.

Figure S4 .
Figure S4.Change in concentration (in %) of certain VOCs for UV, humid and low RH experiments after aging.

Figure S5 .
Figure S5.Average OA mass spectra of aged pellet emissions (red bars) and standard deviation (black error bars) for a) low and b) high RH experiments.

Figure S6 .
Figure S6.The Van Krevelen (VK) triangle diagram presents the relation of the H:C and O:C ratio for the pellet-burning experiments shown here.The OA components from this dataset mostly fall outside the VK-triangle region. 1

Figure S7 .
Figure S7.Measurements for the photooxidation (Experiment 7 in Table 1): a) wall-loss corrected OA, PM inorganic nitrate, and PM organic nitrate, b) gas-phase species NO2, and O3, c) the change in the O:C ratio and theta angle d) representative VOCs showing the largest decrease (MEK, m/z 73; furfural, 97 and creosol, m/z 139).

Figure S8 .
Figure S8.Estimated (a) aerosol pH for experiment 4 including sensitivity simulations for base case (red), neither OA water uptake nor K + (light blue), only OA water uptake (orange), and only potassium (black), and (c) PM inorganic nitrate for experiment 4 for the different simulations, with measurements from the HR-ToF-AMS presented as blue dots.

Figure S10 .
Figure S10.Theta angles between the fresh pellet mass spectrum and BBOA/ wood spectra from the literature.

Figure S11 .
Figure S11.Comparison of aged pellets (red bars; this work) and olive wood (blue squares; Kodros et al. 3 ) spectra for a) low RH and b) high RH conditions.

Table S1 .
Theta angle of HR spectra during the emissions period.

Table S2 .
Assumed reaction rate constants used to calculate typical lifetime of the VOCs with the largest observed decrease.Reaction rate constants are taken from the listed publications and the Master Chemical Mechanism, MCM v3.3.117.

Table S3 .
Average concentrations of oxidants and estimated lifetimes for the VOCs in Experiment

Table S4 .
Theta angle of HR spectra for the 4 h period (aged).

Table S5 .
Calculated pH for emissions and after 4 h of time zero.

Table S6 .
Comparisons (quantified by the theta angle in degrees) of dark-dry and humid pellet spectra 117 from this study to AMS OA factors from literature.The same spectra are used for Fig.9in the main text.