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Atmospheric Environment 38 (2004) 12311232

New Directions: The role of bioaerosols in atmospheric

chemistry and physics

Bioaerosols, a group of organic aerosols ranging from

B10 nm to 100 mm, are airborne particles or large

molecules that are either alive, carry living organisms

or are released from living organisms (e.g., bacteria,

fungi, virus, pollen, cell debris, and biofilms;Fig. 1). The

presence of various types of bioaerosols in indoor air, in

the troposphere and even in the stratosphere has long

been established (e.g., Gidlen, 1948, Biological Reviews23, 109126). Although most research on bioaerosols

has focused on issues related to health hazards, there is a

substantial body of work in progress on the importance

of bioaerosols as (a) ice nuclei (IN) and (b) cloud

condensation nuclei (CCN), and implicating them in the

alteration of cloud coverage and hence the global

climate. In this communication, in addition to their IN

and CCN capability, we would like to briefly discuss the

potential role of bioaerosols (c) in altering the chemistry

of the atmosphere via microbiological degradation, (d)

in modifying the chemical composition of other organic

compounds upon collision or contact, and hence

inducing changes in the IN or CCN ability of organics

in atmosphere, and (e) in driving the chemistry (includ-

ing photochemistry) at environmental interfaces such as

air/snow interface.

It was during the late 1950s (Soulage, 1957, Ann.

Geophys. 13, 103134) that bioaerosols were initially

identified within ice nuclei. Soon after, they were

recognized as active IN at near 0C (e.g., TIN(Pseudomonas syringae bacteria) =2C). Within the

atmosphere bioaerosols can be dead, dormant or

actively reproducing (Fuzzi et al., 1997, Atmospheric

Environment 31(2), 287290; Sattler et al., 2001,

Geophysical Research Letters 28(2), 239242). See-mingly, some airborne taxa are quite resilient against

harsh atmospheric conditions such as UV radiation and

low H2O concentration. Clouds and fogs can attenuate

some of these environmental stressors and provide a

medium for growth. Although the observed mass of

bioaerosols is minute compared to other aerosols in

atmosphere, their density (e.g.,B103104 bacteria/m3) is

approximately the same order of magnitude as that of

ice nuclei, hinting to the potential significance of

bioaerosols as effective IN. Several types of biological

organisms (e.g., fungi, bacteria, and algae) and their

debris have been identified as effective CCN. Note that

the measured number density of microorganism in cloud

water appear to be a few orders of magnitude lower than

the average number of cloud droplet (e.g.;B1500 bacteria ml1 in contrast to typical cloud droplet

density ofB2 108 ml1). This has led some researchers

to conclude that bacteria are not an important source

for cloud nuclei. A caveat should be noted that since

physical processes rendering bacteria (or other types of

bioaerosols) effective CCN have yet to be understood,

the role of bioareosols in general, and bacteria in

particular, in CCN formation, deserves further studies.

Recently, we observed evidence for chemical reactions

induced by bioaerosols. We discovered that several

atmospherically measured dicarboxylic acids (DCA),

one of the predominant groups of organic aerosols, can

efficiently be transformed by airborne bacteria and fungi

(Ariya et al., 2002, Geophysical Research Letter 29(22),

20772081). Isotopic studies indicated that microbiolo-

gical entities can transform and use DCA as nutrients.

Several observed products were relatively benign (e.g.,

butanoic acid), and several were toxicants or pathogens

(e.g., kojic acid). Identified volatile products indicate

that newly deposited DCA can be recycled back to the

atmosphere via microbiological processes. Our addi-

tional unpublished data indicate that DCA are not the

only chemicals that can undergo microbiological degra-

dation. Several other dominant atmospherically mea-

sured organic functional groups can be transformed viabioaerosols. We also measured, isolated, and identified

over 38 airborne taxa. We carried out kinetic experi-

ments to determine the degradation rates of model

organics in presence of each type of bioaerosol, along

with products. We observed a wide range of variation in

degradation lifetimes depending on the given taxum, as

well as factors like pH and temperature. Besides the

microbiological degradation, bioaerosols can alter the

chemical composition of atmospheric organic com-

pounds while they are in contact with them, through

processes such as cell lysis, and desorption. Once the

chemical composition of organics along with particle

ARTICLE IN PRESS

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uea.ac.uk, or go to www.uea.ac.uk/Be044/apex/newdir2.html

for further details.

1352-2310/$- see front matterr 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.atmosenv.2003.12.006

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size and density are changed, their CCN capability will

also be affected. Note that the lifetime of organic

aerosols in the atmosphere can vary from days to weeks,

and that several types of airborne taxa are attached to

other aerosols such as dust or sea salt. Hence, the

interactions between organics and bioaerosols (which are

also organic) are likely. Interestingly, we observed that

organic compounds can also be transformed in snow by

microbiological processes (at the snowair interface, taxa

were dominantly airborne), while producing several

condensed and volatile compounds. Snow can therefore

be a source for the atmosphere of biologically produced

volatile compounds. On speculative ground, the presence

of photosynthetic organisms may contribute to the

photochemical reactions in snowpack.

Current knowledge regarding the physics and chem-

istry of bioaerosols is not very advanced. Definitely,

bioaerosol chemistry is very complex and at its early

stages of evolution. There are more questions than

answers, namely (a) what is the contribution of airborne

taxa on transformation of inorganic compounds (or

pollutants such as heavy metals), in atmosphere or at

environmental interfaces? (b) How does the chemical

heterogeneity of bioaerosols influence their ability to actas CCN or IN? (c) Can microbiological degradation

occur as aerosols are suspended, or are surfaces always

required? (d) What factors dictate the relative impor-

tance of chemistry driven by contact vs. microbiological

degradation (such as temperature, pH, humidity, irra-

diation)? (e) What are the roles of biofilms, viruses and

other airborne taxa? (f) What types of chemical

feedbacks, microbiological activities at air/snow/water

interface, supply to the atmosphere? There are a few

major challenges ahead including accurate and selective

measurement of various forms of bioaerosols. Most

available techniques provide information on cultur-

able taxa, which include only a small fraction of the

total bioaerosols. There have been promising advance-

ments in techniques to measure bioaerosols in recent

years, such as femtosecond adaptive spectroscopy for

coherent antistoke Raman spectroscopy or time-of-flight

mass spectrometry. However, methods capable of

accurate measurements of a wide range of bioaerosols

are yet to be developed. Moreover, further fundamental

laboratory chemical-biological research is required to

provide an understanding of the kinetics and mechanism

for chemical transformation including nature of sur-

faces, environmental conditions, enzymatic and non-

enzymatic transformations. Clearly, we also foresee

some need for modeling studies that will ultimately

evaluate whether bioaerosols significantly contribute to

the chemistry and physics of atmosphere, or not!

Parisa A. Ariya1

Departments of Chemistry, and Atmospheric and Oceanic

Sciences, McGill University, 801 Sherbrooke Street West,

Montreal, PQ, Que., Canada

E-mail address: parisa.ariya@mcgill.ca

Marc Amyot2

Department of Biological Sciences, University of Montreal,

90 Vincent DIndy, D-223, Montreal, PQ, Que.,

Canada H3A 2K6 H2 V 2S9

ARTICLE IN PRESS

Fig. 1. Simplified schematic of bioaerosols cycling in the Earths ecosystem.

1Parisa Ariya is a professor of Chemistry and Atmospheric

and Oceanic Sciences at McGill University, Canada. Her

research entails understanding atmospheric transformation of

selected organic compounds and trace metals.2Marc Amyot is a professor of biological sciences at

Universit!e de Montreal, Canada. His research focuses on

biogeochemistry of heavy metals in aquatic environment.

New Directions / Atmospheric Environment 38 (2004) 123112321232