Colette Heald


Last Name

Heald

First Name

Colette

ORCID

0000-0003-2894-5738

Organisational unit

09810 - Heald, Colette L. / Heald, Colette L.

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20253
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Publications 1 - 3 of 3
  • Re-Examining Chemical Conditions of Past Chamber Studies of Secondary Organic Aerosol Formation
    Item type: Journal Article
    Goss, Matthew B.; Kenagy, Hannah S.; Heald, Colette; et al. (2025)
    ACS ES&T Air
    The simulation of secondary organic aerosol (SOA) in 3D models generally relies on measurements made in laboratory chamber studies. However, many of the laboratory studies that underpin the aerosol parametrizations used in these models were carried out more than 15 years ago, before recent developments in our understanding of peroxy radical (RO2) chemistry and its role in aerosol formation. As a result, limitations of past chamber experiments and the incomplete understanding of their chemical conditions (e.g., the initiating oxidants, RO2 fate) may skew SOA representation in models. In this work, literature SOA chamber studies, specifically those referenced by the SOA scheme in the GEOS-Chem global model, are simulated using a modified version of the Master Chemical Mechanism. This enables explicit estimation of typically unconstrained parameters that affect experimental outcomes, including the relative importance of different oxidants, and the relative loss of RO2 to different unimolecular and bimolecular processes. This work demonstrates that reaction conditions are dynamic, changing within and between experiments, and that many experimental conditions involve more than one oxidant or RO2 fate, complicating model parametrizations. However, we also find that RO2 isomerization is important under many of the experimental conditions used, meaning that some RO2 isomerization processes are to some extent embedded into 3D model SOA estimates, despite no explicit representation of this chemistry.
  • Exploring the processes controlling secondary inorganic aerosol: evaluating the global GEOS-Chem simulation using a suite of aircraft campaigns
    Item type: Journal Article
    Norman, Olivia G.; Heald, Colette; Bililign, Solomon; et al. (2025)
    Atmospheric Chemistry and Physics
    Secondary inorganic aerosols (sulfate, nitrate, and ammonium, SNA) are major contributors to fine particulate matter. Predicting concentrations of these species is complicated by the cascade of processes that control their abundance, including emissions, chemistry, thermodynamic partitioning, and removal. In this study, we use 11 flight campaigns to evaluate the GEOS-Chem model performance for SNA. Across all the campaigns, the model performance is best for sulfate (R2 = 0.51; normalized mean bias (NMB) = 0.11) and worst for nitrate (R2=0.22; NMB = 1.76), indicating substantive model deficiencies in the nitrate simulation. Thermodynamic partitioning reproduces the total particulate nitrate well (R2=0.79; NMB = 0.09), but actual partitioning (i.e., ε(NO3-)= NO3- / TNO3) is challenging to assess given the limited sets of full gas- and particle-phase observations needed for ISORROPIA II. In particular, ammonia observations are not often included in aircraft campaigns, and more routine measurements would help constrain sources of SNA model bias. Model performance is sensitive to changes in emissions and dry and wet deposition, with modest improvements associated with the inclusion of different chemical loss and production pathways (i.e., acid uptake on dust, N2O5 uptake, and NO3- photolysis). However, these sensitivity tests show only modest reduction in the nitrate bias, with no improvement to the model skill (i.e., R2), implying that more work is needed to improve the description of loss and production of nitrate and SNA as a whole.
  • Investigating fire-induced ozone production from local to global scales
    Item type: Journal Article
    Palmo, Joseph O.; Heald, Colette; Blake, Donald R.; et al. (2025)
    Atmospheric Chemistry and Physics
    Tropospheric ozone (O3) production from wildfires is highly uncertain; previous studies have identified both production and loss of O3 in fire-influenced air masses. To capture the total ozone production attributable to a smoke plume, we bridge the gap between near-field fire plume chemistry and aged smoke in the remote troposphere. Using airborne measurements from several major campaigns, we find that fire-ozone production increases with age, with a regime transition from NOx-saturated to NOx-limited conditions, showing that O3 production in well-aged plumes is largely controlled by nitrogen oxides (NOx). Observations in fresh smoke demonstrate that suppressed photochemistry reduces O3 production by ∼ 70 % in units of ppb Ox (O3 + NO2) per ppm CO in the near-field (age < 20 h). We demonstrate that anthropogenic NOx injection into VOC-rich fire plumes drives additional O3 production, sometimes exceeding 50 ppb above background. Using a box model, we explore the evolving sensitivity of O3 production to fire emissions and chemical parameters. We demonstrate the importance of aerosol-induced photochemical suppression over heterogeneous HO2 uptake, validate HONO's importance as an oxidant precursor, and confirm evolving NOx sensitivity. We evaluate GEOS-Chem's performance against these observations, finding the model captures fire-induced O3 enhancements at older ages but overestimates near-field enhancements, fails to capture the magnitude and variability of fire emissions, and does not capture the chemical regime transition. These discrepancies drive biases in normalized ozone production (ΔO3/ΔCO) across plume lifetime, though the model generally captures observed absolute O3 enhancements in fire plumes. GEOS-Chem attributes 2.4 % of the global tropospheric ozone burden and 3.1 % of surface ozone concentrations to fire emissions in 2020, with stronger impacts in regions of frequent burning.
Publications 1 - 3 of 3