Impact of Biomass Burning Plumes on Photolysis Rates and Ozone Formation at the Mount Bachelor Observatory

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Abstract

In this paper, we examine biomass burning (BB) events at the Mt. Bachelor Observatory (MBO) during the summer of 2015. We explored the photochemical environment in these BB plumes, which remains poorly understood. Because we are interested in understanding the effect of aerosols only (as opposed to the combined effect of aerosols and clouds), we carefully selected three cloud‐free days in August and investigate the photochemistry in these plumes. At local midday (solar zenith angle (SZA) = 35°), j(NO ₂) values were slightly higher (0.2–1.8%) in the smoky days compared to the smoke‐free day, presumably due to enhanced scattering by the smoke aerosols. At higher SZA (70°), BB aerosols decrease j(NO ₂) by 14–21%. We also observe a greater decrease in the actinic flux at 310–350 nm, compared to 360–420 nm, presumably due to absorption in the UV by brown carbon. We compare our measurements with results from the Tropospheric Ultraviolet‐Visible v.5.2 model. As expected, we find a good agreement (to within 6%) during cloud‐free conditions. Finally, we use the extended Leighton relationship and a photochemical model (Aerosol Simulation Program v.2.1) to estimate midday HO ₂ and RO ₂ concentrations and ozone production rates ( P(O ₃)) in the fire plumes. We observe that Leighton‐derived HO ₂ and RO ₂ values (49–185 pptv) and instantaneous P(O ₃) (2.0–3.6 ppbv/h) are higher than the results from the photochemical model.

Plain Language Summary

Biomass burning can emit huge amounts of aerosol particles and trace gases to the atmosphere. These plumes are rich in nitrogen oxides and volatile organic compounds that can react with sunlight to produce ozone, a greenhouse gas and a health hazard to sensitive individuals. However, photochemistry in biomass burning plumes is poorly understood. While most studies rely on model simulations, there are very few in situ measurements aimed at investigating how these aerosols affect photolysis rates. Our study aims to fill this knowledge gap. We use measurements from a mountaintop station in central Oregon (Mount Bachelor Observatory, 2.8 km above sea level) and run a box model to verify our estimates of radicals and ozone production rates. Our results show that biomass burning aerosols increase photolysis rates during midday and decrease the rates during early morning/late afternoon. The ozone production rates derived from the photochemical model are in the same order of magnitude but lower than our calculations. In our paper, we present explanations for the discrepancies between measured and modeled values.

Key Points

Biomass burning (BB) aerosols increase local noontime j(NO ₂)
At high solar zenith angle, BB plumes decrease j(NO ₂) by 14–21%
We calculate 49–185 pptv of HO ₂ and RO ₂ in BB plumes and an instantaneous O ₃ production rate of 2.0–3.6 ppbv/h

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Most cited references 64

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Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles.

P. CRUTZEN, M. Andreae (1990)

Biomass burning is widespread, especially in the tropics. It serves to clear land for shifting cultivation, to convert forests to agricultural and pastoral lands, and to remove dry vegetation in order to promote agricultural productivity and the growth of higher yield grasses. Furthermore, much agricultural waste and fuel wood is being combusted, particularly in developing countries. Biomass containing 2 to 5 petagrams of carbon is burned annually (1 petagram = 10(15) grams), producing large amounts of trace gases and aerosol particles that play important roles in atmospheric chemistry and climate. Emissions of carbon monoxide and methane by biomass burning affect the oxidation efficiency of the atmosphere by reacting with hydroxyl radicals, and emissions of nitric oxide and hydrocarbons lead to high ozone concentrations in the tropics during the dry season. Large quantities of smoke particles are produced as well, and these can serve as cloud condensation nuclei. These particles may thus substantially influence cloud microphysical and optical properties, an effect that could have repercussions for the radiation budget and the hydrological cycle in the tropics. Widespread burning may also disturb biogeochemical cycles, especially that of nitrogen. About 50 percent of the nitrogen in the biomass fuel can be released as molecular nitrogen. This pyrdenitrification process causes a sizable loss of fixed nitrogen in tropical ecosystems, in the range of 10 to 20 teragrams per year (1 teragram = 10(12) grams).