Assessing the sensitivity of the hydroxyl radical to model biases in composition and temperature using a single-column photochemical model for Lauder, New Zealand

Abstract

Abstract. We assess the major factors contributing to local biases in the hydroxyl radical (OH) as simulated by a global chemistry-climate model, using a single-column photochemical model (SCM) analysis. The SCM has been constructed to represent atmospheric chemistry at Lauder, New Zealand, which is representative of the background atmosphere of the Southern Hemisphere (SH) mid-latitudes. We use long-term observations of variables essential to tropospheric OH chemistry, i.e. ozone (O 3 ), water vapour (H 2 O), methane (CH 4 ), carbon monoxide (CO), and temperature, and assess how using these measurements affect OH calculated in the SCM, relative to a reference simulation only using modelled fields. The analysis spans 1994 to 2010. Results show that OH responds approximately linearly to correcting biases in O 3 , H 2 O, CO, CH 4 , and temperature. The biggest impact on OH is due to correcting an overestimation by approximately 20 to 60 % of H 2 O, using radiosonde observations. Correcting this moist bias leads to a reduction of OH by around 5 to 35 %. This is followed by correcting predominantly overestimated O 3 . In the troposphere, the model biases are mostly in the range of −10 to 30 %. The impact of changing O 3 on OH is due to two pathways; the OH responses to both are of similar magnitude but different seasonality: correcting in situ tropospheric ozone leads to changes in OH in the range −14 to 4 %, whereas correcting the photolysis rate of O 3 in accordance with overhead column ozone changes leads to increases of OH of 8 to 16 %. The OH sensitivities to correcting CH 4 , CO, and temperature biases are all minor effects. The work demonstrates the feasibility of quantitatively assessing OH sensitivity to biases in longerlived species, which can help explain differences in simulated OH between global chemistry models and relative to observations. In addition to clear-sky simulations, we have performed idealized sensitivity simulations to assess the impact of clouds (ice and liquid) on OH. The results indicate that the impacts on the ozone photolysis rate and OH are substantial, with a general decrease of OH below the clouds of up to 30 % relative to the clear-skies situation, and an increase of up to 15 % above. Using the SCM simulation we calculate recent OH trends at Lauder. For the period of 1994 to 2010, all trends are insignificant, in agreement with previous studies. For example, the trend in total-column OH is 0.5 ± 1.3 % over this period.

Changes in Global Tropospheric OH Expected as a Result of Climate Change Over the Last Several Decades

“…Montzka et al (2011) attribute the high interannual variability from previous studies to large uncertainties in the emissions of MCF and methane. The previously mentioned work of López-Comí et al (2016) found no significant trend in OH between 1994 and 2010, based on their single column analysis in the remote southern Pacific Ocean. Two recent chemical inversion studies indicate increases in [OH] TROP by~7%-10% between the 1990s and early 2000s, leading to the plateau in methane growth rate during this time, but differ in their attribution of the rise in methane burden since 2007 (Rigby et al, 2017;Turner et al, 2017).…”

“…Strode et al (2015) found that substituting observed values of ozone and reanalysis values of water vapor for the simulated ones improved their estimate of the methane lifetime using a chemistry-climate model. López-Comí et al (2016) used a single-column photochemical model to determine that water vapor and ozone biases within a global chemistry-climate model have the largest impact on OH over New Zealand. Finally, Nicely et al (2017) showed that the primary causes of variations in [OH] TROP simulated by global models were, in order of importance, different chemical mechanisms, various treatments of the photolysis frequency of O 3 that leads to production of O( 1 D), and modeled O 3 and CO.…”

Advancing Scientific Understanding of the Global Methane Budget in Support of the Paris Agreement

“…Global Biogeochemical Cycles estimates, and emissions inventories (i.e., for reactive gases which participate in the formation of OH). Recent studies have quantified sensitivities of global model [OH] to various inputs and parameterizations, which may help modelers better understand how global [OH] simulations could be improved in future (López-Comí et al, 2016;Nicely et al, 2017;Ryan et al, 2018). Alternatively, more comprehensive observations of key tropospheric gases (e.g., O 3 and H 2 O) may allow estimation of [OH] variability using relatively simplified chemical schemes (e.g., Nicely et al, 2018).…”