# Hygroscopicity and Cloud Condensation Nuclei Activity of Fresh and Aged Biomass Burning Particles

## TL;DR

This study examines how particles from biomass burning change in their ability to form clouds when aged, showing that oxidation increases their hygroscopicity.

## Contribution

The study introduces new insights into the hygroscopicity and CCN activity of aged biomass burning particles through controlled experiments and oxidation.

## Key findings

- Oxidized primary organic aerosol (OPOA) has higher hygroscopicity than primary organic aerosol (POA).
- Secondary organic aerosol (SOA) has intermediate hygroscopicity between POA and OPOA.
- A strong correlation exists between hygroscopicity and the oxygen-to-carbon (O/C) ratio in aged particles.

## Abstract

Biomass burning (BB)
is a major source of atmospheric particles
and trace gases, influencing climate change, air quality, and human
health. During the Georgia Wildland-Fire Simulation Experiment, we
measured the hygroscopicity (κ) and size-resolved cloud condensation
nuclei (CCN) activity of BB particles from controlled burns of fuel
beds representative of three ecoregions in Georgia, United States.
Primary BB particles were predominantly organic, and photooxidation
in an oxidation flow reactor produced secondary organic aerosol (SOA)
in a new nucleation mode while transforming primary organic aerosol
(POA) into oxidized POA (OPOA) in the pre-existing accumulation mode.
We measured hygroscopic growth from 20% to 90% relative humidity using
a quartz crystal microbalance and assessed size-resolved CCN activity
for particles from 30 to 350 nm at supersaturation between 0.13% and
0.99%. We found that the hygroscopicity parameter of OPOA (κOPOA = 0.10–0.19) was higher than that of POA (0.04–0.10),
reflecting the influence of heterogeneous oxidation, whereas the hygroscopicity
parameter of SOA (κSOA = 0.07–0.14) fell between
the two. Both fresh and aged BB particles displayed size-dependent
κ values and evidence of external mixing, likely because of
complex emission characteristics of fuel beds and size-dependent deposition
processes. Growth factor-derived and CCN-derived κ values were
consistent when accounting for such heterogeneity. A strong positive
correlation was found between the mass-averaged κ and O/C ratio,
described by the regression κ = 0.31 ± 0.02­(O/C) –
0.05 ± 0.02, which broadly agrees with previous findings for
a wide range of laboratory SOA and ambient oxidized organic aerosols.
This suggests the potential applicability of a generalized hygroscopicity
parameterization across organic aerosols within acceptable uncertainty.
Our results highlight the role of BB particles as significant CCN
sources during atmospheric aging and emphasize the importance of heterogeneous
oxidation in physicochemical evolution of BB particles.