AMTD

Atmospheric Measurement Techniques Discussions

AMTD

Atmos. Meas. Tech. Discuss.

1867-8610

Göttingen, Germany

10.5194/amt-2020-219

Use of filter radiometer measurements to derive local photolysis rates and for future monitoring network application

Walker

Hannah L.

https://orcid.org/0000-0002-1531-4147

¹ ² Heal

Mathew R.

https://orcid.org/0000-0001-5539-7293

¹ Braban

Christine F.

https://orcid.org/0000-0003-4275-0152

² Coyle

Mhairi

https://orcid.org/0000-0002-9440-6524

² ³ Leeson

Sarah R.

² Simmons

Ivan

² Jones

Matthew R.

² Kift

Richard

https://orcid.org/0000-0003-1826-3653

⁴ Twigg

Marsailidh M.

https://orcid.org/0000-0002-5462-3348

School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, EH9 3FJ, UK

UK Centre for Ecology & Hydrology, Bush Estate, Penicuik, Edinburgh, EH26 0QB, UK

The James Hutton Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK

Department of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, UK

18 06 2020

2020 1 32

2020

This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/

This article is available from https://amt.copernicus.org/preprints/amt-2020-219/

The full text article is available as a PDF file from https://amt.copernicus.org/preprints/amt-2020-219/amt-2020-219.pdf

Production of hydroxyl (OH) radicals is frequently dominated by the photolysis of tropospheric ozone (O3). However, photolysis of nocturnal radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO2), also produces radicals (OH and Cl atoms) that contribute to the oxidising capacity of the local atmosphere, and initiate many radical-chain reactions that lead to the formation of harmful secondary pollutants. Photolysis of nitric acid (HNO3) is also a minor radical production mechanism. In this paper, locally representative photolysis rate constants (j-values) for these molecules are shown to be critical for quantifying and understanding the rate of radical production in a local atmosphere. The first long-term 4-π filter radiometer dataset in the UK (21 November 2018–20 November 2019) available for direct atmospheric model validation is reported. Measurements were made at Auchencorth Moss, a Scottish rural background site, and j(NO2) is used to generate a measurement-driven adjustment factor (MDAF) for calculated j-values that accounts for local changes in meteorological variables without significantly increasing computational cost. Modelled clear-sky j-values and actinic flux for Auchencorth Moss were generated using the Tropospheric Ultraviolet and Visible radiation model (TUV; v.5.3.1). Applying the MDAF metric resulted in the calculated photolytic production rate of OH radicals, from all sources considered, being ~40 % lower over the year. Photolysis of HONO resulted in an increased rate of OH production compared to that from O3 in low-light conditions, such as sunrise and sunset (Solar Zenith Angle > 80°). Hydroxyl radical production from HONO photolysis exceeded that from O3 consistently throughout the day during the winter and autumn (by a factor of 5 and 2.1, respectively). Radical production rates from HONO and ClNO2 reached maximum values during the early morning hours of summer (06:00–09:00 UTC), with OH produced at a rate of 1.06 × 106 OH radicals cm−3 s−1, and Cl radicals at 3.20 × 104 Cl radicals cm−3 s−1, with the MDAF metric applied. This first application of the MDAF j-values demonstrates an efficient measurement and computational approach to improve modelling of the local atmospheric photochemistry that drives NO2, O3 and PM pollution levels. The incorporation of local radiation measurements in measurement networks, and the consequent greater spatial resolution of locally-relevant photolysis coefficients in model photolysis parameterisations, will improve the accuracy of assessment of air pollution and policy-intervention impacts.