Table 2.

Themes 1 to 3 and subthemes together with summary of core findings related to general views and expectations for CO Smartphone Systems (CSSs) and their use.

Theme 1	CO Testing—General Views and Motivation to Use
1.1.	General Views on CSSs
1.2.	Motivation to use—a novel cessation aid Potentially helpful at increasing motivation to quit and remain abstinent Monitor and inform about health damages from smoking A long-term companion through the smoking and quitting journeys
1.3.	Motivation to use—other reasons The ‘quantified self’ Opportunity to learn new things about oneself Willingness to contribute to science Tech gadget; something to show off with friends
1.3.	Concerns over CSS accuracy of CO testing and possibility to manipulate results anxiety and worry over high results annoyance and demotivation due to lack of sufficient progress ‘moderate’ CO levels reassuring and permitting of continued smoking
Theme 2	Practicalities of CSS Use
2.1.	Commercial use vs. use as part of study study: acceptance to record personal details, share CO results, use CSS according to schedule outside of the study: expectations to use ad libitum and anonymously
2.2.	Smoking status and CO testing preferences for testing: when the results is expected to be low vs. high interest to test and record CO levels across a range of situations and smoking levels
2.3.	Location of use different preferences to use at home, in private vs. in front of friends and family vs. in public
2.4.	Sharing the device device is private, not to be shared, vs. interested to share with family and friends
2.5.	Timing and duration of use morning and evening most likely times for testing, especially for home-only testing different preferences for duration of CSS use (only during a quit attempt vs. long term to document smoking and quitting journey)
2.6.	Barriers to CSS use annoyance or inconvenience of blowing into the device annoyance or inconvenience of needing to connect the device to a phone dislike for carrying around or displaying the cable anticipated embarrassment to test in public limited battery life low relevance for light smokers or abstainers
Theme 3	Factors Potentially Affecting Preferences, Views and Engagement with CSSs
3.1.	Smoking profile patterns of smoking (regular vs. irregular) perceived role of smoking (e.g., habit, mood regulation, socializing) dependence levels
3.2.	Barriers to quitting motivation self-efficacy and capability to remain abstinent, manage cravings other concerns, e.g., weight gain
3.3.	Views on, and plans for quitting timing of a quit attempt (near vs. distant future) preferred levels of support (e.g., assisted vs. unassisted) approach to quitting (cutting down vs. abrupt cessation)
3.4.	Prior experience with digital programs and user digital behaviors preferences for features found and enjoyed in other apps extending behaviors with other apps to other apps
3.5.	Prior experience with CO testing