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The first 5 hours of aging in biomass burning plumes can strongly affect ozone photochemistry. We examine how volatile organic compounds (VOCs), nitrogen oxides, and nitrous acid influence hydroxyl radical, ozone, and peroxyacetyl nitrate (PAN) based on three aircraft campaigns over the United States. Our analyses reveal variable, highly elevated hydroxyl radical concentrations in the first 2 hours, resulting in evident fire-to-fire variability in VOCs oxidation and in ozone and PAN production. About 40 to 70% of the variability is explained by chemical aging. Ozone production in the plumes is usually VOC-limited for the first 2 hours and then nitrogen oxide limited downwind. Box model results for hydroxyl radical, ozone, and most VOCs, using the full, explicit Master Chemical Mechanism (MCM) mechanism, suggest no major gaps in the current best knowledge of gas-phase chemistry. However, the MCM sometimes overestimates PAN due to underestimated nitrogen oxide sinks. GEOS-Chem, a widely used chemical transport model with a reduced mechanism, generally underperforms because of incomplete VOC representation. We identify these critical pathways to guide future model development. 

Rapid production of formic acid in biomass burning smoke is not captured by the Master Chemical Mechanism (MCM) nor simplified GEOS-Chem chemistry, likely due to missing secondary chemical production. 

Abstract. The impact of biomass burning (BB) on the atmospheric burden of volatile organic compounds (VOCs) is highly uncertain. Here we apply the GEOS-Chemchemical transport model (CTM) to constrain BB emissions in the western USA at ∼ 25 km resolution. Across three BB emission inventorieswidely used in CTMs, the inventory–inventory comparison suggests that the totals of 14 modeled BB VOC emissions in the western USA agree with eachother within 30 %–40 %. However, emissions for individual VOCs can differ by a factor of 1–5, driven by the regionally averaged emissionratios (ERs, reflecting both assigned ERs for specific biome and vegetation classifications) across the three inventories. We further evaluate GEOS-Chemsimulations with aircraft observations made during WE-CAN (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen) andFIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) field campaigns. Despite being driven by different global BBinventories or applying various injection height assumptions, the model–observation comparison suggests that GEOS-Chem simulations underpredictobserved vertical profiles by a factor of 3–7. The model shows small to no bias for most species in low-/no-smoke conditions. We thus attribute thenegative model biases mostly to underestimated BB emissions in these inventories. Tripling BB emissions in the model reproduces observed verticalprofiles for primary compounds, i.e., CO, propane, benzene, and toluene. However, it shows no to less significant improvements for oxygenatedVOCs, particularly for formaldehyde, formic acid, acetic acid, and lumped ≥ C3 aldehydes, suggesting the model is missing secondarysources of these compounds in BB-impacted environments. The underestimation of primary BB emissions in inventories is likely attributable tounderpredicted amounts of effective dry matter burned, rather than errors in fire detection, injection height, or ERs, as constrained by aircraftand ground measurements. We cannot rule out potential sub-grid uncertainties (i.e., not being able to fully resolve fire plumes) in the nestedGEOS-Chem which could explain the negative model bias partially, though back-of-the-envelope calculation and evaluation using longer-term groundmeasurements help support the argument of the dry matter burned underestimation. The total ERs of the 14 BB VOCs implemented in GEOS-Chem onlyaccount for half of the total 161 measured VOCs (∼ 75 versus 150 ppb ppm−1). This reveals a significant amount of missing reactiveorganic carbon in widely used BB emission inventories. Considering both uncertainties in effective dry matter burned (× 3) and unmodeledVOCs (× 2), we infer that BB contributed to 10 % in 2019 and 45 % in 2018 (240 and 2040 Gg C) of the total VOC primaryemission flux in the western USA during these two fire seasons, compared to only 1 %–10 % in the standard GEOS-Chem. 

Abstract. Fires emit a substantial amount of non-methane organic gases (NMOGs), theatmospheric oxidation of which can contribute to ozone and secondaryparticulate matter formation. However, the abundance and reactivity of thesefire NMOGs are uncertain and historically not well constrained. In thiswork, we expand the representation of fire NMOGs in a global chemicaltransport model, GEOS-Chem. We update emission factors to Andreae (2019) andthe chemical mechanism to include recent aromatic and ethene and ethyne modelimprovements(Bateset al., 2021; Kwon et al., 2021). We expand the representation of NMOGs byadding lumped furans to the model (including their fire emission andoxidation chemistry) and by adding fire emissions of nine species alreadyincluded in the model, prioritized for their reactivity using data from the Fire Influence on Regional to Global Environments (FIREX) laboratory studies. Based on quantified emissions factors, we estimatethat our improved representation captures 72 % of emitted, identified NMOGcarbon mass and 49 % of OH reactivity from savanna and temperate forestfires, a substantial increase from the standard model (49 % of mass,28 % of OH reactivity). We evaluate fire NMOGs in our model withobservations from the Amazon Tall Tower Observatory (ATTO) in Brazil, Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) and DC3 in the US, and Arctic Research of the Composition of theTroposphere from Aircraft and Satellites (ARCTAS) in boreal Canada. We show that NMOGs,including furan, are well simulated in the eastern US with someunderestimates in the western US and that adding fire emissions improves ourability to simulate ethene in boreal Canada. We estimate that fires provide15 % of annual mean simulated surface OH reactivity globally, as well as morethan 75 % over fire source regions. Over continental regions about half ofthis simulated fire reactivity comes from NMOG species. We find that furansand ethene are important globally for reactivity, while phenol is moreimportant at a local level in the boreal regions. This is the first globalestimate of the impact of fire on atmospheric reactivity. 

Abstract Agricultural and prescribed burning activities emit large amounts of trace gases and aerosols on regional to global scales. We present a compilation of emission factors (EFs) and emission ratios from the eastern portion of the Fire Influence on Regional to Global Environments and Air Quality (FIREX‐AQ) campaign in 2019 in the United States, which sampled burning of crop residues and other prescribed fire fuels. FIREX‐AQ provided comprehensive chemical characterization of 53 crop residue and 22 prescribed fires. Crop residues burned at different modified combustion efficiencies (MCE), with corn residue burning at higher MCE than other fuel types. Prescribed fires burned at lower MCE (<0.90) which is typical, while grasslands burned at lower MCE (0.90) than normally observed due to moist, green, growing season fuels. Most non‐methane volatile organic compounds (NMVOCs) were significantly anticorrelated with MCE except for ethanol and NMVOCs that were measured with less certainty. We identified 23 species where crop residue fires differed by more than 50% from prescribed fires at the same MCE. Crop residue EFs were greater for species related to agricultural chemical use and fuel composition as well as oxygenated NMVOCs possibly due to the presence of metals such as potassium. Prescribed EFs were greater for monoterpenes (5×). FIREX‐AQ crop residue average EFs generally agreed with the previous agricultural fire study in the US but had large disagreements with global compilations. FIREX‐AQ observations show the importance of regionally‐specific and fuel‐specific EFs as first steps to reduce uncertainty in modeling the air quality impacts of fire emissions.