M. Le Breton, M. R. McGillen, J. B. A. Muller, †D. E. Shallcross, †P. Xiao, ‡L. G. Huey, ‡D. Tanner and C. J. Percival

Centre for Atmospheric Science, School of Earth, Atmospheric and Environmental Science, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. † Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK. ‡School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA

Organic acids are ubiquitous in the gas and aerosol phase and they are common constituents of global precipitation. Formic acid (HCOOH) can dominate the free acidity of precipitation and it therefore influences pH-dependent chemical reactions and OH cloud chemistry. The presence of organic acids, such as formic acid, in cloud condensation nuclei (CCN) can also act to decrease the critical supersaturations at which CCN activate to cloud droplets, therefore affecting the radiative balance of clouds. Sources of atmospheric formic acid include biogenic and anthropogenic primary emissions, e.g. direct emissions by plants, biomass burning and vehicle exhausts; as well as secondary emissions, i.e. in-situ production of formic acid from hydrocarbon oxidation, which is estimated to be an important source term. Major sinks for formic acid are wet and dry deposition, as well as the chemical sink with OH. Overall, the atmospheric formic acid budget is not well quantified and e.g. global models underpredict formic acid by up to a factor of 50 in marine locations.

Here, the first detailed formic acid measurements performed by a chemical ionisation mass spectrometer (CIMS) onboard the FAAM BAe-146 research aircraft are presented. The detection of formic acid was enabled using I- ionisation chemistry, which produces adducts of iodine with a range of carboxylic acids, including formic acid. The first case study shows daytime data of formic acid in the UK boundary layer on 16 March 2010. For this flight, the sensitivity of the CIMS to formic acid was 35 ± 6 ion counts s-1 pptv-1 and the limit of detection was 25 pptv. Mean concentrations were 142 ±72 pptv. Higher levels of formic acid were observed in plumes, which could be linked to main urban and industrial centres (London, Humberside and Tyneside regions) using trajectory analysis. However, formic acid showed no clear correlation with anthropogenic pollution markers such as NOx, CO and O3. The highest formic acid concentrations were measured at lower altitudes which points to the importance of surface emission sources. The lack of clear correlation with pollutants such as NOx indicates that a range of formic acid sources and sinks exist that must have different time-constants compared to e.g. NOx in the atmosphere.

To further investigate sources and sinks, formic acid was also estimated by a trajectory model that showed an underprediction of concentrations by up to a factor of 2. This underestimation can be explained by missing emission sources in the model that are considered to be the primary emissions of formic acid (of both biogenic and mainly anthropogenic origin) as well as insufficient or lack of formic acid precursor emissions, such as isoprene from biogenic sources.

The second case study focuses on formic acid measurements obtained from night time flying as part of the NERC funded RONOCO campaign (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the AtmOsphere) in summer 2010 and winter 2011. Formic acid night-time concentration during January 2011 stayed mostly below 100 pptv, whereas during the summer, night time concentrations of above 1 ppbv could be observed. The drivers for the observed difference in formic acid concentrations will be discussed. wever, formic acid showed no clear correlation with anthropogenic pollution markers such as NOx, CO and O₃. The highest formic acid concentrations were measured at lower altitudes which points to the importance of surface emission sources. The lack of clear correlation with pollutants such as NOx indicates that a range of formic acid sources and sinks exist that must have different time-constants compared to e.g. NOx in the atmosphere.

To further investigate sources and sinks, formic acid was also estimated by a trajectory model that showed an underprediction of concentrations by up to a factor of 2. This underestimation can be explained by missing emission sources in the model that are considered to be the primary emissions of formic acid (of both biogenic and mainly anthropogenic origin) as well as insufficient or lack of formic acid precursor emissions, such as isoprene from biogenic sources.

The second case study focuses on formic acid measurements obtained from night time flying as part of the NERC funded RONOCO campaign (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the AtmOsphere) in summer 2010 and winter 2011. Formic acid night-time concentration during January 2011 stayed mostly below 100 pptv, whereas during the summer, night time concentrations of above 1 ppbv could be observed (Figure 2). The drivers for the observed difference in formic acid concentrations will be discussed.