Missing SO<sub>2</sub> oxidant in the coastal atmosphere? – observations from high-resolution measurements of OH and atmospheric sulfur compounds

Abstract

Abstract. Diurnal and seasonal variations of gaseous sulfuric acid (H 2 SO 4 ) and methane sulfonic acid (MSA) were measured in NE Atlantic air at the Mace Head atmospheric research station during the years 2010 and 2011. The measurements utilized selected-ion chemical ionization mass spectrometry (SI/CIMS) with a detection limit for both compounds of 4.3 × 10 4 cm −3 at 5 min signal integration. The H 2 SO 4 and MSA gas-phase concentrations were analyzed in conjunction with the condensational sink for both compounds derived from 3 nm to 10 µm (aerodynamic diameter) aerosol size distributions. Accommodation coefficients of 1.0 for H 2 SO 4 and 0.12 for MSA were assumed, leading to estimated atmospheric lifetimes on the order of 7 and 25 min, respectively. With the SI/CIMS instrument in OH measurement mode alternating between OH signal and background (non-OH) signal, evidence was obtained for the presence of one or more unknown oxidants of SO 2 in addition to OH. Depending on the nature of the oxidant(s), its ambient concentration may be enhanced in the CIMS inlet system by additional production. The apparent unknown SO 2 oxidant was additionally confirmed by direct measurements of SO 2 in conjunction with calculated H 2 SO 4 concentrations. The calculated H 2 SO 4 concentrations were consistently lower than the measured concentrations by a factor of 4.7 ± 2.4 when considering the oxidation of SO 2 by OH as the only source of H 2 SO 4 . Both the OH and the background signal were also observed to increase significantly during daytime aerosol nucleation events, independent of the ozone photolysis frequency, J (O 1 D), and were followed by peaks in both H 2 SO 4 and MSA concentrations. This suggests a strong relation between the unknown oxidant(s), OH chemistry, and the atmospheric photolysis and photooxidation of biogenic iodine compounds. As to the identity of the atmospheric SO 2 oxidant(s), we have been able to exclude ClO, BrO, IO, and OIO as possible candidates based on ab initio calculations. Nevertheless, IO could contribute significantly to the observed CIMS background signal. A detailed analysis of this CIMS background signal in context with recently published kinetic data currently suggests that Criegee intermediates (CIs) produced from ozonolysis of alkenes play no significant role for SO 2 oxidation in the marine atmosphere at Mace Head. On the other hand, SO 2 oxidation by small CIs such as CH 2 OO produced photolytically or possibly in the photochemical degradation of methane is consistent with our observations. In addition, H 2 SO 4 formation from dimethyl sulfide oxidation via SO 3 as an intermediate instead of SO 2 also appears to be a viable explanation. Both pathways need to be further explored.

Contribution from biogenic organic compounds to particle growth during the 2010 BEACHON-ROCS campaign in a Colorado temperate needleleaf forest

“…It includes all basic aerosol processes, including nucleation, condensation, coagulation and dry deposition. The model performance has been validated against field measurements from Hyytiälä, Finland, in various studies (Boy et al, 2013;Mogensen et al, 2011Mogensen et al, , 2015Smolander et al, 2014).…”

“…In the case of linalool we have included its reaction with OH and NO 3 to form acetone and "condensable material" and its reaction with O 3 to additionally produce formaldehyde. For the reactions of the stabilized Criegee intermediates (sCI) from alpha-and beta-pinene and limonene, we used the rates from Mauldin III et al (2012), similar to "Scenario C" in Boy et al (2013). For the sCI from isoprene, we used the rates from Welz et al (2012) as done in "Scenario D" in Boy et al (2013).…”

“…For the reactions of the stabilized Criegee intermediates (sCI) from alpha-and beta-pinene and limonene, we used the rates from Mauldin III et al (2012), similar to "Scenario C" in Boy et al (2013). For the sCI from isoprene, we used the rates from Welz et al (2012) as done in "Scenario D" in Boy et al (2013). Sulfuric acid and nitric acid are removed from the gas phase based on the condensation sinks calculated from background aerosol loading.…”

“…This growth step determines the formation rate of detectable particles (usually > 3 nm) as well as the impact of NPF on cloud condensation nuclei populations . Organic compounds are the main drivers of the growth step and are thus critical for aerosol formation (Kerminen et al, 2000;Sellegri et al, 2005;Boy et al, 2005;Allan et al, 2006;Laaksonen et al, 2008;Ehn et al, 2014).…”

“…It provides an opportunity to study biogenic SOA formation at a rural-urban interface . Various studies have indicated that biogenic SOA formation in forest environments can be enhanced by the inflow of anthropogenic pollutants (Boy et al, 2008;Hoyle et al, 2011;Jung et al, 2013). The modeling tool used in this study is the chemical-transport column model SOSAA (model to Simulate Organic vapors, Sulfuric Acid and Aerosols; Zhou et al, 2014).…”