Simulation of Photochemical Effects on Air Quality

Nitrogen dioxide is an atmospheric pollutant with largely anthropogenic sources; it is a combustion side product often produced by automobiles and industrial activity. It has negative effects on the respiratory system. In addition to being a pollutant itself, nitrogen dioxide is also a contributor to tropospheric ozone through photochemical reactions, both of which are EPA criteria air pollutants. Nitrogen dioxide and ozone participate in a net-zero reaction cycle in which the formation of ozone is rate-limited by the photolysis rate of nitrogen dioxide. The destruction of ozone and reformation of nitrogen dioxide is a fast reaction unaffected by photochemistry, therefore it occurs at relatively stable rates, regardless of available radiation. It is necessary to quantify the rate of nitrogen dioxide photolysis, the first half of this cycle, under varying atmospheric conditions, due to its dependence on radiation.

FastJX is a software package which simulates the effects of atmospheric aerosols on radiation, which then affects photolysis rates. FastJX uses optical properties at a range of wavelengths to simulate the extent of radiation absorption or scattering for an aerosol layer at a specified elevation. This project specifically uses aerosol optical properties measured during the NASA Langley ACTIVATE campaign. The optical properties were most closely examined in the UV-A and UV-B wavelengths, as shorter wavelengths penetrate minimally to the troposphere and longer wavelengths do not carry enough energy to drive nitrogen dioxide photolysis.

This project aims to correlate photolysis rates simulated by FastJX, measured aerosol optical depths, and measured concentrations of nitrogen dioxide. The measured aerosol optical depths and nitrogen dioxide concentrations come from co-located NASA AERONET and EPA monitoring sites, respectively, which allows for direct comparison. The sites, in Pasadena, California, were chosen due to their location in an urban center with consistently high levels of sunlight.

It was found that FastJX simulation results are most closely correlated with measured changes in nitrogen dioxide levels during hours of peak sunlight, which is when photolysis is most active. This is consistent with the expectation that time of day plays a significant role in nitrogen dioxide levels, due to the combined factors of nitrogen dioxide production during rush hour traffic and nitrogen dioxide destruction via photolysis when UV radiation is present.

Utilizing simulation allows for knowledge of photolysis rates under a greater range of atmospheric conditions without the resource-intensive steps of measuring actinic flux in a field environment. Instead, aerosol optical depth monitors, which require less man hours, can be combined with existing optical property sets. FastJX is additionally equipped to simulate aerosol effects on other photolysis reactions, which means that the sensitivity analysis conducted here may be applied to other chemical species.

The next steps in this work on FastJX software in simulating the effects photochemical reaction rates of nitrogen dioxide on urban air quality include accounting for the effects of geographical location and different aerosol types, such as brown carbon.