There
is a need to understand the impacts of ambient aerosol acidity
on the photosynthetic process in terrestrial plants and its connection
to carbon storage. To help address this need, we conducted a study
to investigate the acidity of ambient aerosol on the stability of
chlorophyll, the main indicator of photosynthetic processes in leaves.
Ambient fine particulate matter (<2.5 μm in aerodynamic diameter
or PM2.5) sampled at a suburban site of Nanjing during
cold seasons of 2019 and 2020 was collected. Ambient levels of gaseous
pollutants including NO2, SO2, and CO; daily
maximum 8 h average O3 concentration (O3-8h);
and daily meteorological parameters were also recorded. Water-soluble
ions of F–, Cl–, NO3
–, SO4
2–, NH4
+, Na+, K+, Mg2+, and
Ca2+ in collected PM2.5 samples were determined
using an ion chromatograph. The Extended Aerosol Inorganics Model
version IV (E-AIM IV) was used to estimate the total acidity and in
situ pH of collected PM2.5 samples, while Positive Matrix
Factorization 5.0 (PMF) was used to apportion emission sources. Depletion
of chlorophyll a and b was determined
using water extracts of collected PM2.5 samples. Estimated
in situ pH levels of collected PM2.5 samples were in the
range of pH 2–4. Associations were found between the concentration
of SO4
2– and the total acidity of ambient
PM2.5 particles. Source apportionment results showed that
five PM sources including dust source (DS), secondary source (SS),
industrial source (IS), mobile source (MS), and biomass burning (BB)
contributed to aerosol acidity of ambient PM2.5 particles.
Aerosol acidity, rather than PM2.5 mass, resulting from
water-soluble ions in gaseous pollutants was the dominant factor influencing
the stability of chlorophyll a. Alternatively, chlorophyll b was found to be stable with the exposure to water extracts
of ambient PM2.5 particles within 60 h. This study provides
the first field evidence that aerosol acidity could result in the
depletion of chlorophyll a.