The oxidative capacity of the atmosphere controls the residence time, and thus the spatial distribution of trace gases. Oxidation of volatile organic compounds (VOCs) primarily occurs via reactions involving the hydroxyl (OH) radical, nitrate (NO3) radical, and ozone (O3). The oxidized VOCs are generally less reactive, but more strongly greenhouse forcing. Additionally, they enable the rapid interconversion of nitric oxide and nitrogen dioxide (collectively known as NOx), resulting in production of tropospheric ozone. One of the main components of urban smog, tropospheric ozone is a lung irritant, causes damage to plants, and is a greenhouse gas.
Our research aims to understand the oxidation and tropospheric ozone production
processes on a regional scale. We investigate NOx lifetime and understanding
the physical and chemical processes that control nitrogen oxide abundances and
the partitioning of nitrogen oxides between more reactive (e.g. NO3, NO2, NO)
and less reactive (e.g. HNO3, alkylnitrates) species. We are also interested
in how reduction of higher nitrogen oxides, such as nitrous acid (HONO), affects
NOx concentrations. We accomplish this through in-situ measurements and satellite
and modeling studies. Our three main areas of focus are Biosphere-Atmosphere
Interactions, Regional
Transport, and Intercontinental
and Transcontinental Transport.
We use gradient and flux measurements of nitrogen oxides to understand the oxidation
processes taking place within and directly above a forest canopy. The in-canopy
oxidation of nitrogen produces less reactive compounds, more rapidly depositing
compounds such as nitric acid, peroxy nitrates, and alkyl nitrates. In the local
environment, these compounds prevent the NOx from participating in ozone production.
However, after time, these compounds may decompose, releasing NOx, and resulting
in ozone production downwind. In addition to affecting the chemistry downwind,
the oxidation capacity of the forest canopy may also affect secondary organic
aerosol formation, rate of trace gas removal, and nitrogen availability to the
forest.
Some Field Measurements: Big Hill,
Blodgett Forest, BEARPEX
field campaign
We use our own measurements as well as those made by the California Air Resources
Board to understand the evolution of the Sacramento urban plume as it travels
towards the Sierra Nevada Mountains. Of particular interest to us, is the weekday/weekend
difference in the emissions in the city center and how those differences affect
photochemical ozone production downwind. This analysis has shown that the differences
anthropogenic produced nitrogen oxides on the weekdays and weekends leads to
a weekday/weekend difference in the oxidative capacity of the atmosphere (specifically
the OH radical concentration) and thus to differences in ozone production downwind
of Sacramento.
In addition to the ground-based measurements, we also use models and satellite
measurements to understand regional processes. The satellite measurements have
the advantage of providing long term data sets of high spatial resolution and
coverage. With the satellite measurements, we can observe variations (such as
weekday/weekend) in NO2 concentrations. We couple these observations to models
to understand the resulting variations in emissions and lifetime of NO2 and
the effect on the oxidative capacity of the atmosphere.
Some Field Measurements: Granite
Bay, Blodgett Forest
Understanding how nitrogen oxides are partitioned and transported long distances
is key to understanding atmospheric chemical processes on a global scale. We
look at the vertical and spatial distribution of nitrogen oxides and use these
measurements to try and infer the processing age of air plumes as well as the
chemical mechanisms that have led to the nitrogen partitioning. We are also
interested in how convection and lightning redistribute NOx and other ozone
precursors.
Some Field Measurements: TOPSE,
PAVE,
Intex-NA,
Intex-B, TC4,
ARCTAS