Driving parameters of biogenic volatile organic compounds and consequences on new particle formation observed at an eastern Mediterranean background site
Authors: Debevec Cécile , Sauvage Stéphane , Gros Valérie , Sellegri Karine , Sciare Jean , Pikridas Michael , Stavroulas Iasonas , Leonardis Thierry , Gaudion Vincent , Depelchin Laurence , Fronval Isabelle , Sarda-Esteve Roland , Baisnée Dominique , Bonsang Bernard , Savvides Chrysanthos , Vrekoussis Mihalis , Locoge Nadine 
Published: October 2018

Abstract. As a part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx) and
Cyprus Aerosols and Gas Precursors (ENVI-Med CyAr) programs, this study aims
primarily to provide an improved understanding of the sources and the fate of
volatile organic compounds (VOCs) in the eastern Mediterranean. More than 60
VOCs, including biogenic species (isoprene and eight monoterpenes) and
oxygenated VOCs, were measured during a 1-month intensive field campaign
performed in March 2015 at the Cyprus Atmospheric Observatory (CAO), a
regional background site in Cyprus. VOC measurements were conducted using
complementary online and offline techniques. Biogenic VOCs (BVOCs) were
principally imputed to local sources and characterized by compound-specific
daily cycles such as diurnal maximum for isoprene and nocturnal maximum for
α- and β-pinenes, in connection with the variability of emission
sources. The simultaneous study of pinene and isoprene temporal evolution and
meteorological parameters has shown that BVOC emissions were mainly
controlled by ambient temperature, precipitation and relative humidity. It
was found that isoprene daytime emissions at CAO depended on temperature and
solar radiation changes, whereas nocturnal BVOC concentrations (e.g., from
oak and pine forests) were more prone to the relative humidity and
temperature changes. Significant changes in monoterpene mixing ratios
occurred during and after rainfall. The second part of the study focused on
new particle formation (NPF) events at CAO. BVOCs are known to potentially
play a role in the growth as well as in the early stages of formation of new
atmospheric particles. Based on observations of the particle size
distribution performed with a differential mobility particle sizer (DMPS) and
the total number concentrations of particles larger than 1 nm diameter
measured by particle size magnifier (PSM), NPF events were found on 14 out of
20 days of the field campaign. For all possible proxy parameters
(meteorological parameters, calculated H2SO4 and measured gaseous
compounds) having a role in NPF, we present daily variations of different
classes during nucleation event and non-event days. NPF can occur at various
condensational sink (CS) values and both under polluted and clean atmospheric
conditions. High H2SO4 concentrations coupled with high BVOC
concentrations seemed to be one of the most favorable conditions to observe
NPF at CAO in March 2015. NPF event days were characterized by either
(1) a predominant anthropogenic influence (high concentrations of
anthropogenic source tracers observed), (2) a predominant biogenic influence
(high BVOC concentrations coupled with low anthropogenic tracer
concentrations), (3) a mixed influence (high BVOC concentrations coupled with
high anthropogenic tracer concentrations) and (4) a marine influence (both
low BVOC and anthropogenic tracer
concentrations). More pronounced NPF events were identified during mixed
anthropogenic–biogenic conditions compared to the pure anthropogenic or
biogenic ones, for the same levels of precursors. Analysis of a specific NPF
period of the mixed influence type highlighted that BVOC interactions with
anthropogenic compounds enhanced nucleation formation and growth of newly
formed particles. During this period, the nucleation-mode particles may be
formed by the combination of high H2SO4 and isoprene amounts, under
favorable meteorological conditions (high temperature and solar radiation and
low relative humidity) along with low CS. During the daytime, growth of the
newly formed particles, not only sulfate but also oxygen-like organic aerosol
(OOA) mass contributions, increased in the particle phase. High BVOC
concentrations were observed during the night following NPF events,
accompanied by an increase in CS and in semi-volatile OOA contributions,
suggesting further BVOC contribution to aerosol nighttime growth by
condensing onto pre-existing aerosols.