Abstract
During September 1990 we sampled coarse (>18‐μm diameter) and fine (18‐ to 5.5‐μm diameter) droplets and liquid‐water content (LWC) in cloud from a tower on a forested ridge top in Shenandoah National Park, Virginia. Cloud‐water pH and aqueous‐ and vapor‐phase concentrations of carboxylic acids (HCOOH and CH3COOH) and formaldehyde (HCHO) were measured in parallel over 1‐ to 1.5‐hour intervals. Both size fractions of cloud droplets contained similar concentrations of carboxylic species and H+ during most sampling; most cloud water was in coarse droplets. The pH of coarse (3.27–4.76) and fine (3.22–4.70) droplets coupled with total LWC of 0.04–0.56 g m−3 STP (standard m3 at 0°C and 1 atm) resulted in the partitioning of carboxylic acids primarily in the vapor phase. The observed phase partitioning for CH3COOH was within the uncertainty range of thermodynamic data. However, HCOOH exhibited significant phase disequilibria, which could not be explained by artifacts from variable LWC or from mixing droplets of different acidities. We hypothesize that the large volume of liquid water deposited on the forest canopy interacted with the near‐surface cloud leading to apparent disequilibria based on time‐integrated samples. HCOOH was selectively depleted relative to CH3COOH in cloud, particularly at higher pH, suggesting rapid removal of HCOOH by cloud‐water deposition. We saw no evidence for significant production of HCOOH from the aqueous‐phase oxidation of HCHO.
Highlights
Carboxylicacidsareubiquitouschemicacl onstituentosf Wetanddrydepositioanretheprimaryatmosphersicinksfor the globaltroposphertehatcontributesignificantlyto the HCOOHandCH3COOH[Sanhuezeatal., 1992];gas-phase acidityof precipitatioanndcloudwater,especiallyinregions oxidationby OH is slow[Atkinson1, 990].farfromanthropogenincfluences[e.g.,KeeneandGalloway, Several field studies have investigatedmultiphase1988]
Aqueous-phacsheemistry aciditiesmix [Pandisand Seinfeld,1991]or LWC changes in cloudsprovideseithera sourceor a sink for HCOOH duringsampling[Winiwarteret al., 1992; Pandisand
We sampledcloudwaterandtracegasesfroma 15-m reverse-phaseh,igh-performancliequid chromatography toweron a mountainridgeat Pinnacles(1037-melevation) (I-tPLLD;overall measuremenut ncertaintiesfor HCHO were in the north central section of ShenandoahNational Park abou2t 0%[Hungeretal., thisissue].AnySOV)adductisn (SNP),Virginia(38ø37'N, 78ø2I'W)
Summary
Carboxylicacidsareubiquitouschemicacl onstituentosf Wetanddrydepositioanretheprimaryatmosphersicinksfor the globaltroposphertehatcontributesignificantlyto the HCOOHandCH3COOH[Sanhuezeatal., 1992];gas-phase acidityof precipitatioanndcloudwater,especiallyinregions oxidationby OH is slow[Atkinson1, 990]. Therearemanycarboxylicacidsin thetroposphere, chemicapl roc involvingHCOOHandCH3COOHin butHCOOHandCH3COOHaretypicallyin thehighest cloudsand fogs [e.g., Winiwarteret al., 1988, 1992; concentration[es.g.,Keeneetal., 1983]. Concentrations atmospherHicCOOHandCH3COOHhavebeenhypothe- ofHCOOHandCH3COOHmeasuresdimultaneouisnltyhe sized,includinggas-phasoexidationof precursoor rganic vaporphaseand in bulk cloudwateror fog wateroften compound[JsacobandWojy,1988;Gallowayetal., 1989; divergesignificantlfyromthosebasedon thermodynamic. Madronichand Calvert, 1990;Moodyet al., 1991];direct propertiese,speciallyat pH greaterthan 5. Observed emissionfromplants[Talbotet al., 1990],soils[Sanhueza disequilibribaetweenphaseshavebeenattributetdo organic andAndreae,1991]andformicheants[GraedelandEisner, films on dropletsthatlimit masstransfer[Facchiniet al., 1988];andfossil-fuelandbiomasscombustion[Kawamura 1992]andto artifactsresultingwhendropletswithdifferent etal., 1985;Talbotet al., 1988]. Aqueous-phacsheemistry aciditiesmix [Pandisand Seinfeld,1991]or LWC changes in cloudsprovideseithera sourceor a sink for HCOOH duringsampling[Winiwarteret al., 1992; Pandisand [ChameideasndDavis,1983;Jacob,1986]butis thoughto Seinfeld1, 992]. Severasl tudiesuggestthatcloudsmaybea of of VN2iIernwgsinHtiiaatmu,CftophesrathrhlioreSetDt,teuusrvdhioyallmfeE..arthO, ceanasn, dSpacUe,niversitynSeantshiunekzfaoerttarlo.,p1o9s9p1h].erHicCOOH [Jacoband. Wofsy1, 988; 3Divisioonf AppliedScieaceasndDepartmeoaftEarthand The ShenandoahCloud and PhotochemistrEyxperiment.
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