Poor air quality is a persistent challenge in Mexico
City, and
addressing this issue requires an understanding of the chemical composition
of PM2.5 (particulate matter less than 2.5 μm in
diameter). Sulfate and secondary organic aerosol (SOA) are two of
the largest contributors to PM2.5 in Mexico City, but uncertainties
exist regarding their sources, distribution across individual particles,
and ability to form organosulfates. Herein, we show using electron
dispersive x-ray spectroscopy that only 41 ± 1% and 25 ±
1% of particles (aerodynamic diameter, 0.32–0.56 μm)
by number at two sites in Mexico City, respectively, contain sulfur.
Vibrational spectroscopy (Optical-Photothermal Infrared + Raman Microspectroscopy)
shows that these sulfur-containing particles consist of inorganic
sulfate (SO4
2–) and organosulfates (ROSO3
–). In addition, we unexpectedly measured
abundant isoprene-derived SOA from low nitric oxide reaction pathways,
specifically organosulfates (methyltetrol sulfates = avg. 50 ng/m3, max. 150 ng/m3) and polyols (methyltetrols =
avg. 70 ng/m3, max. 190 ng/m3) using liquid
chromatography with high-resolution mass spectrometry. Differences
in SO2 and NO
x
concentrations
between sites likely contribute to these spatial differences in sulfate,
organosulfate, and SOA formation. These findings improve understanding
of sulfur distribution and sources of SOA in Mexico City, which can
inform efforts to improve air quality.